# Dolmar 7900 Carb upgrade?



## plowin-fire (Feb 20, 2013)

Anyone have any reports of using a Walbro 12 HD carb on the 7900? I know a few of you on here have them. Suppose to be a bolt on job. Worth the expense?


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## nstueve (Feb 20, 2013)

Why? the OEM carb feeds the saw just fine... The only reason you'd need to swap a higher flowing carb is if you went full blown porting and even then it feeds it alright... you'd also need a unlimited coil to see a larger gain with the saw. why mess with a proven performer? I could understand if it were a choaked off POS saw to begin with...


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## plowin-fire (Feb 20, 2013)

The saw was just ported and MMed.


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## nstueve (Feb 20, 2013)

You can't get it to 4stroke with the OEM carb? the porting i've read about hasn't needed a higher flowing carb...


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## plowin-fire (Feb 20, 2013)

Still at the port shop. Builder recommended different carb...


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## FATGUY (Feb 20, 2013)

nstueve said:


> You can't get it to 4stroke with the OEM carb? the porting i've read about hasn't needed a higher flowing carb...



maybe I'm wrong here, but as I understand it, if you fed the engine more air, you could feed it more gas too.


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## nstueve (Feb 20, 2013)

FATGUY said:


> maybe I'm wrong here, but as I understand it, if you fed the engine more air, you could feed it more gas too.



correct me if i'm wrong here but if your 4-stroking with the current carb it means you are already feeding it enough of both... Plus the saw is still going to be handy-capped by the limited coil...

If it's a race saw go for it... if it's a solid work saw I'd see if the stock carb would make it 4stroke... JMO...


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## FATGUY (Feb 20, 2013)

nstueve said:


> correct me if i'm wrong here but if your 4-stroking with the current carb it means you are already feeding it enough of both... Plus the saw is still going to be handy-capped by the limited coil...
> 
> If it's a race saw go for it... if it's a solid work saw I'd see if the stock carb would make it 4stroke... JMO...



I hope someone smarter than me chimes in, but as I understand it, you're four stroking if you're giving it too much gas for the amount of air that's in the engine at that time, feed it more air, you can feed it more gas. I really am not trying to be argumentative, I just have a vested interest in this question.


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## steve316 (Feb 20, 2013)

I may be wrong, four stroking just means it is rich enough.It dosen't mean a larger carb wouldn't make the saw performe better. Steve


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## redoakneck (Feb 20, 2013)

Just change the jets in the current carb, should be simple and cheaper. If the dude porting advised it, doesn't he have an idea of what other carb???

Not real familiar with the 7900 carb, although I have a 7900 and run the frig out of it. I did change the jets in my 660 after a mild porting and muff mod..


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## nstueve (Feb 20, 2013)

I was always under the impression you had a carb big enough when you could 4stroke at WOT after porting. Lets keep in mind that once you put a larger carb on there is only so much fuel you can flow before you start blowing the extra out the exhaust. The blow down period can only be so long to burn "X" amount of fuel. The initial energy generated from a denser charge of fuel and air might be greater but at what cost... Like i said pros and cons... Work saw porting vs race saw porting. The multiple 7900 build threads that I have read always left the carb alone besides tuning. 

Eager to hear from builders like Stumpy, Randy, Brad (maybe, i know he hasn't done a lot of dollys), TreeMonkey... etc..


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## mdavlee (Feb 20, 2013)

The ported 372s like a larger carb on them from a 385/390. I'm sure a 7900 would respond the same way.


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## nstueve (Feb 20, 2013)

redoakneck said:


> Just change the jets in the current carb, should be simple and cheaper. If the dude porting advised it, doesn't he have an idea of what other carb???
> 
> Not real familiar with the 7900 carb, although I have a 7900 and run the frig out of it. I did change the jets in my 660 after a mild porting and muff mod..



Jets only deliver more fuel which may or may not be the case of what he needs here... You need a larger venturi to allow more air so that when mixed with more fuel (from either opening the "H" or larger jet) you still get the proper fuel air mix only in a higher volume.

The real question is above... Will a higher volume matter on a 7900? keep in mind you also have to contend with tuning it with a rev limited ign,...


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## steve316 (Feb 20, 2013)

the carb on 6400.7300 and 7900 are all the same. I have a 7900 and I believe it could use a larger carb; ported or not. Steve


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## redoakneck (Feb 20, 2013)

Just looked at the 7900 specs for carb for 7910, doesn't look like you can change the jets.


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## mitch95100 (Feb 20, 2013)

So my ms170 Carb wont fit on my ported 660?? 

Sent from my USCCADR3305 using Tapatalk 2


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## Andyshine77 (Feb 20, 2013)

FATGUY said:


> I hope someone smarter than me chimes in, but as I understand it, you're four stroking if you're giving it too much gas for the amount of air that's in the engine at that time, feed it more air, you can feed it more gas. I really am not trying to be argumentative, I just have a vested interest in this question.



Nik I'm not smarter than you, but you are correct. As long as the carb can provide the engine with the proper air to fuel ratio, the engine can run rich. This is actually quite simple if you think about it. The carb, do to it's venturi size can only allow in so much air/fuel during a given time, make the venturi larger and it will allow more air/fuel in, during the same duration of time. Any time you open up an engine, or change timing even just a little, you're going to flow more air. You can also run into issues like turning out the high speed jet too far. When the needle moves farther away from the seat, a non-linear change in the mixture is a result. I forget who taught me that, I think Tzed250. 


.


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## TK (Feb 20, 2013)

Bigger is better :msp_thumbup:


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## FATGUY (Feb 20, 2013)

mdavlee said:


> The ported 372s like a larger carb on them from a 385/390. I'm sure a 7900 would respond the same way.



so if a 372 carb fits on a 7900, and a 385 carb fits on a 372, doesn't it stand to reason that a 385 carb fits on a 7900? :msp_thumbup:


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## VINIFIREWOOD (Feb 20, 2013)

I'm not sure how much if any of what works on a 4 stroke engine crosses over but. I remember back in my days of playing with small block Chevy's. Your average stock to mildly built 350 ran best with a 650 cfm carb. It would run ok with a 750 but was happiest with the smaller carb. Only when you went to a really wildly built engine did you see any benefit fron the bigger carb.
So judging from that I would say there comes a point where you have too much carb and you will start sacrificing throttle response etc.
But truly only one way to find out and that's to bolt it on.


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## TK (Feb 20, 2013)

FATGUY said:


> so if a 372 carb fits on a 7900, and a 385 carb fits on a 372, doesn't it stand to reason that a 385 carb fits on a 7900? :msp_thumbup:



It takes a different intake boot to get the bigger carb to be user friendly because of the offset screw pattern. I'm not familiar with the 7900 carb/intake setup but it's possible.


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## mdavlee (Feb 20, 2013)

The 7900 intake setup is sort of like the 372. I've seen it done by some one on another site. I think it was a 385 carb that was swapped onto it.


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## w8ye (Feb 20, 2013)

A 385 carb has the mounting screws one higher than the other and will not fit on the 6400/7900 boot.


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## plowin-fire (Feb 21, 2013)

Going to have the Walbro put on. Talked to a few people that have them and they were impressed with the results.should get it back next week.


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## nstueve (Feb 21, 2013)

Andyshine77 said:


> The carb, do to it's venturi size can only allow in so much air/fuel during a given time, make the venturi larger and it will allow more air/fuel in, during the same duration of time. Any time you open up an engine, or change timing even just a little, you're going to flow more air. You can also run into issues like turning out the high speed jet too far. When the needle moves farther away from the seat, a non-linear change in the mixture is a result. I forget who taught me that, I think Tzed250.



I'd think that since the jet is after the initial venturi opening... The venturi determines air flow and the jet (or H screw setting) determines how much fuel enters the air stream...??? 

interested to see if we find a larger venturi carb that fits the 7900 with minimal mods...


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## importjunk (Feb 21, 2013)

Subscribed


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## cgraham1 (Jun 28, 2014)

nstueve said:


> Interested to see if we find a larger venturi carb that fits the 7900 with minimal mods...





plowin-fire said:


> Going to have the Walbro put on. Talked to a few people that have them and they were impressed with the results.should get it back next week.


Well? What happened? Did the 372 carb help on a ported 7900 or not?


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## Terry Syd (Jun 28, 2014)

There's a lot more to carbs than just having enough fuel flow at WOT. Many of the carbs have been designed to get around EPA guidelines. Walbro even refers to them on their diagrams as 'emissions carburettor'. The biggest issue is in the low speed circuit where the carbs have been leaned out. The carbs still flow enough at WOT, but loose torque down low.

Then there is the issue of throttle response, again not an issue at WOT, but most people would prefer quick throttle response rather than an EPA approved response.

As far as venturi sizes, the Zama carb appears to have a smaller venturi because of the additional material added next to the main discharge nozzle. However, the venturi appears to have been designed on a flow bench to increase the flow. Many times in porting, it is not what you take out, it is what you put in that is important. When two air streams join together the velocity increases (like at the junction of the nozzle and the venturi curvature). When the velocity in that area increases faster than the velocity through the venturi, an instability of the flow occurs. When it gets bad enough eddies start to form, which means some of the flow is actually going BACKWARDS in the carb. It looks like Zama cleaned up that area and added some material downstream to get the carb to flow better.


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## cgraham1 (Jun 28, 2014)

Terry Syd said:


> There's a lot more to carbs than just having enough fuel flow at WOT. Many of the carbs have been designed to get around EPA guidelines. Walbro even refers to them on their diagrams as 'emissions carburettor'. The biggest issue is in the low speed circuit where the carbs have been leaned out. The carbs still flow enough at WOT, but loose torque down low.
> 
> Then there is the issue of throttle response, again not an issue at WOT, but most people would prefer quick throttle response rather than an EPA approved response.
> 
> As far as venturi sizes, the Zama carb appears to have a smaller venturi because of the additional material added next to the main discharge nozzle. However, the venturi appears to have been designed on a flow bench to increase the flow. Many times in porting, it is not what you take out, it is what you put in that is important. When two air streams join together the velocity increases (like at the junction of the nozzle and the venturi curvature). When the velocity in that area increases faster than the velocity through the venturi, an instability of the flow occurs. When it gets bad enough eddies start to form, which means some of the flow is actually going BACKWARDS in the carb. It looks like Zama cleaned up that area and added some material downstream to get the carb to flow better.


Soooo... I'm a little slow. What does all this mean? Is the Zama the perfect carb for the 7900 or not? 

Thanks!


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## Terry Syd (Jun 28, 2014)

Not stock it isn't. You have to drill out at least the idle jet, probably something around .40-.45mm. Then you have to add a different metering spring to get quick throttle response, probably a ZA 0019013 metering spring.


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## nstueve (Jun 29, 2014)

Is there a set of carb drill bits that you can select a fractional mm size drill bit???


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## Terry Syd (Jun 29, 2014)

I use some 'micro drills', you need a pin vise in order to hold them. Something like this - http://www.ebay.com.au/itm/20PC-Mic...45?pt=LH_DefaultDomain_15&hash=item56658f679d

I've got two sets of drills, one is metric and the other SAE. The combination of the two sets gives me more options to the size of hole I can drill.

The smallest drills aren't drills, they are reamers. You have to take it easy and go slow. I remove the pump section of the carb so the carb sits flat on my desk. I'm able to rest my elbows on the elbow rests of my chair to help steady my hands.


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## Terry Syd (Jun 29, 2014)

If I was going to drill out a Zama for a 7900, I'd start by taking the idle jet out to .35mm and see how it ran in conjunction with the stock auxiliary jet (.25mm jet). I'd then check to see that I had sufficient idle adjustment on the idle screw. Although the auxiliary jet sits further forward of the throttle plate and opens at 1/4 throttle, it also affects the idle even though it is sitting well forward of the throttle plate.

If the idle adjustment is getting down around 1/4 turn, then you will have to block off the auxiliary jet with some epoxy and just use the idle jet. (putting in a slower taper idle screw would be a quick fix, unfortunately, different idle screws are not available) If the engine needs more fuel in the low speed circuit, then drill out the idle jet to .40mm and see how it runs. I'm running a .45mm idle jet on my 77cc saw, so you just need to see what works for you in your conditions.

Increasing the flow in the low speed circuit should show an increase in low speed torque and resistance to bogging. Since you are increasing the fuel flow in the low speed circuit, you have to lean out the high speed circuit each time you drill the idle jet.

Throttle response on a big saw engine isn't as important as a limbing saw, so you might get by with the stock metering spring if you were willing to have slow throttle response. However, if you wanted to tune the throttle response you could get a couple of extra metering springs. The ZA 0019011 is the same spring rate, but is shorter than the ZA 0019013. You can also add a couple of shims under the spring to vary the POP.


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## nstueve (Jun 30, 2014)

Terry Syd said:


> If I was going to drill out a Zama for a 7900, I'd start by taking the idle jet out to .35mm and see how it ran in conjunction with the stock auxiliary jet (.25mm jet). I'd then check to see that I had sufficient idle adjustment on the idle screw. Although the auxiliary jet sits further forward of the throttle plate and opens at 1/4 throttle, it also affects the idle even though it is sitting well forward of the throttle plate.
> 
> If the idle adjustment is getting down around 1/4 turn, then you will have to block off the auxiliary jet with some epoxy and just use the idle jet. (putting in a slower taper idle screw would be a quick fix, unfortunately, different idle screws are not available) If the engine needs more fuel in the low speed circuit, then drill out the idle jet to .40mm and see how it runs. I'm running a .45mm idle jet on my 77cc saw, so you just need to see what works for you in your conditions.
> 
> ...



What do you use for shims? Since I'm into the dollys I might learn how to dig into these Zama's. It would be simpler if they didn't have the aux jet and was just a H&L jets to contend with. It would be nice to find a Walbro replacement that would adjust out right after some modifications... Most guys wouldn't go to the trouble of buying the special tools to do this work but I could see myself using these tools. 

Could you also explain POP (pop off pressure) for those that are still learning? How higher and lower POP effects the saw? I myself have never tested POP on a carb before but believe I can do it with my mityvac right? Is there a good POP to have on certain size chainsaws? I think I have a pretty good handle on what POP is, but it is nice to have all this information in one location. Not like I couldn't find the information elsewhere too.... but still nice to have this info here so that when I go to modifying a 7900/7901 Zama carb I can refer back here for guidance, and to rethink through stuff if I get stuck.

Thanks!
Nate


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## Chris-PA (Jun 30, 2014)

Terry Syd said:


> If I was going to drill out a Zama for a 7900, I'd start by taking the idle jet out to .35mm and see how it ran in conjunction with the stock auxiliary jet (.25mm jet). I'd then check to see that I had sufficient idle adjustment on the idle screw. Although the auxiliary jet sits further forward of the throttle plate and opens at 1/4 throttle, it also affects the idle even though it is sitting well forward of the throttle plate.
> 
> If the idle adjustment is getting down around 1/4 turn, then you will have to block off the auxiliary jet with some epoxy and just use the idle jet. (putting in a slower taper idle screw would be a quick fix, unfortunately, different idle screws are not available) If the engine needs more fuel in the low speed circuit, then drill out the idle jet to .40mm and see how it runs. I'm running a .45mm idle jet on my 77cc saw, so you just need to see what works for you in your conditions.
> 
> ...


If the auxiliary/transfer jets are flowing too much at idle, then I would look for a bigger venturi carb. If that were not available or practical, maybe adding a small hole in the throttle plate opposite to the edge near the idle jet would help? That would require the throttle plate to be more closed for the same idle rpm, moving the edge farther away from the auxiliary jets, plus the air flow through the hole would be on the other side of the throttle bore farther away from the auxiliary jets. It should reduce the pressure drop at the auxiliary fuel ports.


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## Terry Syd (Jun 30, 2014)

I wrote up a lot of the information in this thread - http://www.arboristsite.com/community/threads/chinese-carburetor.246510/

I was using an old aluminium beer can for shims. Recently, I went to try and find another thick aluminium can and couldn't find any, just the paper thin cans, so I'm going to order a little bag of the washers that I mentioned in the thread.

You can block off the auxiliary jet with some GB Weld and just use the idle jet. I mentioned using the auxiliary jet on the 7900 because the engine would probably benefit from the extra flow at that point in the throttle position. You can try it and then if the mixture goes too rich at that point, block it off and drill out the idle jet some more (but you will have to use a stronger metering spring to keep the throttle response). I think you might be able to use the auxiliary jet and not have to go to another spring, perhaps just shim it, if necessary.

The term POP is just a measurement that you can go back to for tuning, it is a measurement that the carb never sees during operation because the needle is pressurised for testing (that pressurising does not happen on the saw during operation). - For example, two carbs may have the same metering spring, but one will have a smaller metering orifice for the needle to sit on. Because there is less area under the needle, that carb will have a higher POP than the other carb, however both carbs will lift the needle off the seat at the same time during normal operation because of the operation of the diaphragm.

If you are wondering why different carbs have different size metering orifices, it is because a different size orifice will give a different RAMP up of the fuel supply. The smaller the orifice the longer the ramp.

There are different size metering orifices available from both Walbro and Zama, but I haven't tried changing the orifice. If I was racing a kart or boat, then the ramp would become more critical and I would be swapping metering seats around to see what worked best (and changing the seats looks like a hassle). However, with a saw you are just hitting the trigger to WOT. I've been able to get the throttle response I needed by just using different springs and shims.

I don't know what any of the POP figures are on my saws. I just tune the spring to get the best throttle response. Kinda like me tuning the ignition timing, I just keep tweaking the timing until I have the best power.

The best way to tune the POP would be to have a few of the springs I already mentioned. Then you can try them and see how they affect the throttle response. You may find one that you like, but there is no rule that you have to use the spring as it is. IE: if you needed a certain pre-load, the shorter spring can be shimmed a bit, or the longer spring can be shortened a bit.


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## Terry Syd (Jun 30, 2014)

The auxiliary jet I'm referring to is a second jet that comes in after the transition holes. If you take a look at the Chinese Carburetor thread it will show a picture of the inside of the twin jet Zama.


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## Chris-PA (Jun 30, 2014)

Terry Syd said:


> The auxiliary jet I'm referring to is a second jet that comes in after the transition holes. If you take a look at the Chinese Carburetor thread it will show a picture of the inside of the twin jet Zama.


Thanks Terry, that looks like an interesting thread I had not seen - it looks like it was right before I returned. You did some interesting stuff, including the air corrector jet to linearize the mixture vs. air velocity. I will have to read it carefully when I get the chance. On first look I did not see the auxiliary jet picture but I will look again. Do you know what models of carb have these? Hoping to find a diagram.


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## Terry Syd (Jun 30, 2014)

I don't know what models have the later twin jet Zama, but they became common on saws when there was some sort of EPA change. The carbs are essentially the same for the Husky 365-372 and Dolmar 6400-7900. The EPA change also affected the Walbros, the HD-5 I had on the 029 has a restricted idle circuit with the same size .25mm jet as the Zama.

It appears that the changes were to lean out the low end of the powerband. The lean low speed circuit forces an operator to keep the engine up in the powerband where there is less time for mixture to escape out the exhaust port.

I expect that someone who has a 7900 and modds the carb will be shocked that there was that much torque sitting in the engine not being realised.


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## Chris-PA (Jun 30, 2014)

Terry Syd said:


> I don't know what models have the later twin jet Zama, but they became common on saws when there was some sort of EPA change. The carbs are essentially the same for the Husky 365-372 and Dolmar 6400-7900. The EPA change also affected the Walbros, the HD-5 I had on the 029 has a restricted idle circuit with the same size .25mm jet as the Zama.
> 
> It appears that the changes were to lean out the low end of the powerband. The lean low speed circuit forces an operator to keep the engine up in the powerband where there is less time for mixture to escape out the exhaust port.
> 
> I expect that someone who has a 7900 and modds the carb will be shocked that there was that much torque sitting in the engine not being realised.


I searched quite a bit and cannot find any diagrams/schematics for a twin jet, or any description of it at all. I can understand the concept but would like to see their explanation - especially as I don't have one to look at.


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## Terry Syd (Jul 1, 2014)

The 'twin jet' is a name I gave it to distinguish it from the early Zamas. The early Zamas did not have any jets, they had a configuration like a Walbro.

The twin jet has an idle circuit that is separate from the transition circuit. You can change the idle mixture without changing the low speed fuel mixture (unlike the Walbro). It also has the auxiliary jet that straight feeds from the metering chamber. The auxiliary jet comes in after the transition holes. It also has a thin channel under the welsh plug to string out the bubbles of the emulsion mixture so that the idle mixture is more uniform and is less susceptible to changes in idle mixture when the saw is rotated.

I don't know what the optimum jet sizes are for various displacements, let alone for different models. I ran a 35/35 combination on the 77cc saw that worked well with the stock spring, however it only had 1/4 turn of adjustment on the idle screw. A slower taper idle screw would be the quick fix, but unfortunately they are not available. If a person tweaks the idle screw carefully, then they should be able to use the 35/35 combination on the 7900. 

I'm trying a different approach now by blocking off the auxiliary jet and opening up the idle jet to compensate. I'm presently trying a .45mm idle jet and with the 19013 spring and I have just a tad over 1/2 turn of idle adjustment.

Another approach to getting more adjustment on the idle screw would be to pull the idle jet out of the carb and re-drill the transition holes that are positioned underneath the jet. The extra air flow through the larger holes would lean out the idle mixture and wouldn't affect the fuel flow of the low speed circuit.

I wish I could give specific advice as to what changes to make on the carburettor for a given application, but I don't have the resources. If I had a dyno, a bucket of carburettors, a box of springs, etc. I could test one carb, make a small change and then test it again. Eventually, I could get the right combination for the best powerband. After I had the jetting right, I could then go back and tweak the throttle response to make it hit like a light switch.

Maybe some bright entrepreneur will set himself up to mod carbs.


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## Andyshine77 (Jul 1, 2014)

Excuse my ignorance, but how exactly do you differentiate between the older and newer Zama carbs on the 7900??


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## Terry Syd (Jul 1, 2014)

I don't think the earlier Zamas may have been on the 7900, but I could be wrong. I do know the early Zama (Walbro circuits) was used on the 044.

The easiest way to tell them apart is to open up the metering chamber and see if the carb has two brass jets (twin jet) on either side of the welsh plug.


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## Andyshine77 (Jul 1, 2014)

Okay got it.


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## Chris-PA (Jul 1, 2014)

Terry Syd said:


> The 'twin jet' is a name I gave it to distinguish it from the early Zamas. The early Zamas did not have any jets, they had a configuration like a Walbro.
> 
> The twin jet has an idle circuit that is separate from the transition circuit. You can change the idle mixture without changing the low speed fuel mixture (unlike the Walbro). It also has the auxiliary jet that straight feeds from the metering chamber. The auxiliary jet comes in after the transition holes. It also has a thin channel under the welsh plug to string out the bubbles of the emulsion mixture so that the idle mixture is more uniform and is less susceptible to changes in idle mixture when the saw is rotated.
> 
> ...


OK, I hate to be pedantic but the use of the word jet for both the calibrated orifice and the outlet port was a little confusing, but I think I understand the positioning of the outlet.

The Walbros do run the idle transfer ports from after the idle screw/jet. They appear to show the transfer ports as acting like air correctors for the idle circuit when the throttle plate is at idle:


In the part of your text I highlighted in red above, where does the air for the emulsion tube effect come from? There is no air bleed on the high speed circuit as far as I know. I was wondering if the aux jet/port you were talking about was actually an air corrector jet, but if it is located between the transfer ports and the main outlet this does not seem possible.

If I were designing a carb to better meet emissions I would want add an air corrector jet to prevent the enrichment of the mixture with increasing air velocity ("4-stroking"). The problem with this is that people would have no way to set the mixture without listening for the "4-stroking" misfire. And as you stated in the other thread you'd need a limiting ignition to prevent over-revving.


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## Terry Syd (Jul 1, 2014)

Take a look back at the Chinese carburettor thread - http://www.arboristsite.com/community/threads/chinese-carburetor.246510/page-3 if you look at post 56 you will see the inside of the metering chamber. There are two brass jets on either side of the welsh plug. The one on the left of the plug is for the idle/transition circuit, the one on the right is the auxiliary jet. The auxiliary jet directly feeds fuel into the carb, the stock jet is .25mm and the hole in the carb is .5mm. The auxiliary jet is uncovered by the throttle plate just after it passes the two transition ports.

In regards to your question about the emulsion tube - the fuel goes through the idle jet (the one on the left), just underneath the idle jet are the two transition ports in the body of the carburettor, they provide the air for the emulsion when the throttle plate is downstream of the transition holes (just like you see in the first Walbro drawing). The quantity of the emulsion is then regulated by the idle screw (either more or less) and then enters under the welsh plug in the centre of the carburettor. The channel/tube for the emulsion then travels down the length of the welsh plug to the idle port behind the throttle plate at idle.

When the throttle plate passes the transition ports (Walbro drawing number 2), the fuel is drawn straight up through the jet and through the transition ports.

If you had one of the carbs in your hands with the jets and welsh plug out, it would be a lot easier to see how it works.

A better drawing for the idle circuit on the Zama would be the Mikuni drawing I posted. The idle needle adjusts the quantity of the emulsion, not the quantity of fuel. Look at the drawing in post #34 in the thread - http://www.arboristsite.com/community/threads/chinese-carburetor.246510/page-2


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## Chris-PA (Jul 1, 2014)

Terry Syd said:


> Take a look back at the Chinese carburettor thread - http://www.arboristsite.com/community/threads/chinese-carburetor.246510/page-3 if you look at post 56 you will see the inside of the metering chamber. There are two brass jets on either side of the welsh plug. The one on the left of the plug is for the idle/transition circuit, the one on the right is the auxiliary jet. The auxiliary jet directly feeds fuel into the carb, the stock jet is .25mm and the hole in the carb is .5mm. The auxiliary jet is uncovered by the throttle plate just after it passes the two transition ports.
> 
> In regards to your question about the emulsion tube - the fuel goes through the idle jet (the one on the left), just underneath the idle jet are the two transition ports in the body of the carburettor, they provide the air for the emulsion when the throttle plate is downstream of the transition holes (just like you see in the first Walbro drawing). The quantity of the emulsion is then regulated by the idle screw (either more or less) and then enters under the welsh plug in the centre of the carburettor. The channel/tube for the emulsion then travels down the length of the welsh plug to the idle port behind the throttle plate at idle.
> 
> ...


OK, I misunderstood and thought you were discussing an emulsion system for the auxiliary circuit, not for the idle & idle transfer circuits (I get where the air comes from for those).

Just to verify: If the auxiliary outlet is just a port (hole) located at the perimeter between the main nozzle and the transfer outlets, then it would have to be in a spot that is of larger diameter than the main nozzle. Does the edge of the throttle plate get close enough to it (at say 3/4 throttle) to give it a stronger signal than the main venturi?

Having multiple venturis for various air flow levels is pretty much standard fare in other types of carbs, and allows one to tune the different ranges independently to avoid flat spots and the like. It would be a nice feature, even if it used a fixed jet.


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## makguy (Jul 4, 2014)

All this talk of carbs, and the one limiting factor on the 7900 would still be the coil. Ultimately pumping more fuel and air into the combustion chamber would allow for higher rpms on a modified saw, but how do you get around the limiter built into the coil? Is there a non limiter coil available for a modified 7900?


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## Terry Syd (Jul 4, 2014)

We're discussing dialling in the fuel curve of the carb for the existing air delivery curve of the engine (which then gives the power curve of the engine)

It appears you have confused the concept of porting a saw to 'pump more fuel and air into the combustion chamber', with the concept of getting the proper fuel/air ratio for the air that does get pumped. I'll see if I can find a diagram on fuel mixtures and power, it may help explain it.

Here's a link to a webpage that has a couple of good diagrams to explain the power characteristics of rich and lean mixtures. - http://www.mummbrothers.com/SRF_Stuff/Secrets/Driveline/Air_Fuel.htm


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## makguy (Jul 4, 2014)

I understand. 

My question is that even if you get the carb dialed in just right for the modified 7900, isn't the saw ultimately limited by the stock coil? Or is it not an issue of a higher RPM, but more grunt at the normal operating RPM range, within the coil's capability? 
I.E is the intent a full race saw, or is it just a more powerful saw within the normal operating range?
Either way, I agree that the carb is too small for optimum power on a modded saw.
Also just curious is there is a non linter coil available for the saw?


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## Gravedigger (Jul 5, 2014)

makguy said:


> I understand.
> 
> My question is that even if you get the carb dialed in just right for the modified 7900, isn't the saw ultimately limited by the stock coil? Or is it not an issue of a higher RPM, but more grunt at the normal operating RPM range, within the coil's capability?
> I.E is the intent a full race saw, or is it just a more powerful saw within the normal operating range?
> ...




Here's a link to put an unlimited coil on 

http://www.arboristsite.com/communi...ay-to-make-your-dolmar-solo-unlimited.164924/


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## makguy (Jul 5, 2014)

Thanks!


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## Poleman (Jul 5, 2014)

Well...I've been following this for a while and very interested in the carb swap. I just got my hands on a really nice BB Makita and was having some carb issues.

It didn't want to start out of the box but once I got it started it dialed right in. Today I went and cut with it and same thing....didn't want to start. I did get it started and got half a tank threw it before it acted up again and couldn't get it restarted

I remembered this thread and came home and pulled carb off (Zama C3 M). Took it apart and looked like it could use a kit. Put the parts in the ultrasonic and went to my carb box and dug out a good Walbro HD 12 for comparison.....it would work...bolt on. I installed this carb and went to see how it would work....needless to say it worked great!!!! Dialed right in, idled good, great throttle responce.!!!! I think this saw is now at its potential with the BB Kit. Only other thing would be to port it to possibly make it stronger.

Not worried about the coil because it would have to turn 13,500 in the cut.....plenty of rpm to play with.

I feel this is a VERY good swap!!! Slightly larger Venturi's to flow more.....and the best part its a direct swap.

Just my 2 cents.....worked for me. I like strong work saws and not into cookie cutters.....well not a lot!!


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## Terry Syd (Jul 5, 2014)

Poleman, great news on the carb swap, good carburetion makes a saw a pleasure to use. Here's the thing - in my experience, the C3M is jetted for an engine around 65cc, the HD-12 is jetted for an engine around 71cc - you have an engine that is 84cc.

You have a C3M that needs a carb kit, while you have it apart you could ream out the idle jet to .30mm and the auxiliary jet to .35mm. That would be a conservative drilling and you would still have reasonable adjustment on the idle screw.

You say you like STRONG work saws, so do I. Perhaps your saw can be made even stronger, more torque and more resistance to bogging. For around $10-15 you can get the micro drills and the pin vise to do the mod while you're rebuilding the carb.

You are then in a position to test the HD-12 against a modded C3M - I'm sure a lot of guys on this forum would like to hear the results.


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## Poleman (Jul 6, 2014)

That would be a fun project...I would love to contribute!!

Any advice where to obtain micro drills and pin vice??

What other numbers on drill sizes and how would sizes affect performance? The numbers you gave would be a start....how would larger affect performance? Would be interested in knowing the limit before performance falls off.

Thanks Terry


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## Terry Syd (Jul 6, 2014)

Great stuff, welcome to the leading edge of chainsaw carburettor modifications. Here's a link I just pulled up on Ebay - http://www.ebay.com.au/itm/21pcs-Ti...519485792?pt=Model_Kit_US&hash=item3ce3c6ab60

If you build a better carb than the HD-12 for your saw for less than $10, I'd consider that a win.

As far as the numbers, I wish I had something definitive, but I'm still hacking away at this stuff myself. What I have found is that it is better to go too rich than to be on the lean side of the curve. I've used a 35/35 combination in the Zama on a 77cc saw and it worked great, however the idle adjustment is only 1/4 of a turn. The 30/35 combination could be better for power (and adjustment), I don't know, but it is a good place to start. If the 30/35 combination won't pull better down low than the HD-12, then I would go up a jet size until it does.

I mentioned how to ream out the jets in the Chinese Carburettor thread, I'll get you a link.

EDIT: Here's a link to the Chinese Carburettor thread. In post #55 I mention which jet is the idle jet (left side of the welsh plug) and the auxiliary (or Torque jet as I called it) on the right side of the welsh plug.

In post #77 I explain how I set myself up to ream out the jets.

http://www.arboristsite.com/community/threads/chinese-carburetor.246510/page-3


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## CR888 (Jul 6, 2014)

l find Terry's posting on carburation very interesting/informative. opcorn:


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## Poleman (Jul 6, 2014)

Great stuff!!! Ordering what I need and will post results!!

Great to have a teach.....thanks again Terry for you help and encouragement!!!


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## Gravedigger (Jul 8, 2014)

Think I need new carb. Should I get HD12 or original? Weeedeaterman BB kit


Sent from my iPad using Tapatalk


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## Terry Syd (Jul 8, 2014)

Why don't you wait until Poleman has done some modding to the C3M. He can compare it to the HD-12 and let you know which is a better carb to be using.


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## Poleman (Jul 11, 2014)

Hi, all...just a little update on the funtionality of the Walbo change. I've used the saw all week without a glitch. Startes, idles and runs AWSOME!! It had good power, excellant throttle responce everything that I look for. It will be hard to find a better carb combo for this saw.

Still waiting on the micro bits so I can MOD the Zama. Terry said its a good Mod and excited to give it a whirl. I find the advice here on AS to be GREAT!!

Terry, was wondering if when drilling out the jets and only having 1/4 to 1/2 turn of idel adjustment left, what would happen if one would drill a hole or grove the throttle plate to give it more air. I had to do this on an Echo 8000 for idle adjustment after porting the saw and adjusting. It had little or no idle adjustment left so I did alter the throttle plate to draw more air at idle...solved the issue I had. Now saw tunes and seems to have all stock type settings back to normal...just has a VERY nasty disposition toward wood now. Just thinking!!??

Also Terry, you say you can alter throttle responce with the springs? Been thinking about this and trying to digest it....stronger spring faster responce??? Thus the spacer washers and/or differant springs??

Any way just wanted to update and tell my findings thus far.


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## Terry Syd (Jul 11, 2014)

Yep, you can take a little off the throttle plate (on the side opposite to the idle cutaway) to lean out the mixture, just make sure you have sufficient idle screw adjustment to turn the idle back down after you trim the plate. If you go too far, just solder the edge back up and start again.

Throttle response is a combination of a few things; spring pre-load, spring rate, size of the metering orifice and the low speed jets. For all practical purposes, we just use the spring pre-load (otherwise known as POP) to tune the throttle response.

If the metering system wasn't in the carb, then as soon as we started to open the throttle we would get a full hit of fuel from the jet/s. So the metering system is used to 'ramp' in the fuel flow. For a saw carb, that ramp mostly occurs in the first 1/4 of throttle movement (unless you are using a really strong spring).

If you slowly open the throttle you can feel how the engine is responding to the fuel flow. If it is bogging with extra fuel, then an increase in spring pre-load will delay the ramp a bit and make it leaner. If you need more fuel, then a decrease in spring pre-load will start ramping in the fuel sooner.

A higher spring pre-load will also lean the idle mixture a bit, so that is another method you may be able to use to get a little more idle adjustment.

If the modded Zama turns out to be a stronger running carb on your saw, then I'll work out a further mod to the transition holes so that the carb has at least one turn of adjustment for the idle screw. It would be easier if slower taper idle screws were available, but they aren't (I waited 6 months for Zama to fill a back order and finally gave up). If you have a 044 carb handy, that idle screw has a slower taper and will double the adjustment.


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## Poleman (Jul 11, 2014)

Terry,

Thanks for all the GREAT information. I do have a 44 carb handy if need be. 

Really interested to see how the Zama acts with mods and get further instructions and input from you.

Thanks


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## Poleman (Jul 12, 2014)

Well got the Zama jets drilled out to .30 and .35. Still a little cross eyed from the procedure.....wow those drills a small. Carb has been rebuilt and a stihl 044 slow ramp idle screw installed.

Will head out in the morning and cut some larger wood (40+ dbh) to get an idea of Walbro bottom pull. It has been great in up to 24" pine. I will make it work then do a field carb change and try the Zama. Possible further mod's may follow. I am starting out conservative and can go from there, per Terrys advice. 

Learned so much so far and was intimidated by the drill work.....not that bad once I got into it. It turned out to be easier than expected. Terry advised to measure the drills before drilling.....VERY good advice as my drills were mixed up a little upon receipt of them....besides it's just makes good sense to take your time and do it right the first time. Patients Grasshopper!!! Lol

I may take a 28" bar along and run that also since my normal bar I use is a 24. Little more pull in the wood.

I'll report back after testing and may request more thoughts on further mods after describing results. Terry can analyze and give me more tuning tips.


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## MCW (Jul 13, 2014)

Interesting thread. The rev limited coil is not an issue as mentioned. Even methanol piped 3120's generally won't be pulling over 13,500rpm in the cut. Unlimited coils can help with tuning but proper tuning should be done in the cut anyway. 
Anybody who doesn't think a stock saw can use more fuel and air really needs to see a properly modified and bored out carby on an actual stock saw before and after the mod 
I've seen it and was absolutely blown away that it was the same saw.


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## Poleman (Jul 13, 2014)

k..here it is.

I cut another tank with the Walbro in big wood. It pulled very well, had good throttle responce as noted before. A very good carb for this saw, very sensible, easy to tune. I would probably leave this carb on and would be very happy with it if I didn't have othere options. I used it and tryed it because I had one and becaus of this thread. The Zama that was on this saw was running irattic and wouldn't stay in tune, it ran good when it did...very conparable in performance and responce as Walbo...but it had problems.

Now for the Zama... after drilling out per Terrys instructions and setting everything to factory spec, new kit etc. I changed carbs on site after one tank was run through with Walbro. Had H & L 1 turn out to start with and idle screw just touching. Had to do quite a bit of tinkering to get it to fire and run. I found I had to totally screw in idle screw to get it to stay running.....making progress. Finally got the low somewhat dialed in and able to back off idle just a bit...less than 1/4 turn. Getting it to the point of tune. It would stay running but seemed to load up and slow down a little at idle.

Now for the GOOD news...the throttle responce is REMARKABLE....it actually blew me away. It was lighting fast from idle to top!!! I could stop the saw in a cut let it idle and just pull the trigger and it would just flat GO!!!!! No bog or sputter just flat start cutting with the bar buryed....my kinda saw!!!

Like I said the Walbro is GOOD by this Zama just makes this saw scream with its mod's. I think one thing I will do is put a grove in the throttle plate to give it more air and try to regain some idle adjustment. It was starting and acting good by the time i quit but earlier the only way I could get it started was to pull the chock out to set fast idle then start....reasoning to think it needs more air.

Power wish I'm not sure there was a differance. The Walbro was turning 10,500 to 11,300 in the cut and the Zama about 10,800. I was pushing both pretty hard and almost feel the Zama wouldn't bog as much dogging in. I may not have been turning as fast but I feel it pulled better.

What I REALLY like about the Zama is the throttle responce. It still has some bugs but it's close. I don't know about needing any more size in the jets. I'm at 30/35 now per Terry and would hate to loose throttle responce. I am so astounded by it it's unreal...I'ts like a ported saw.

The walbow again is really good, after the swap back to the Zama I would full heartedly recommend this mod!! The only difficult part is to get the saw started to tune in once you get it to run. The drilling process wasn't hard, but get a pair of those magnifying glasses...you'll need them. It makes it much easier to see the tiny drill bits your using

Saw had a 24 inc bar and Oregon 72L chain with a 7 tooth sprocket. I was running Stihl ultra at 40/1 with straight 91 non ethanol. I forgot the 28 inch bar in my hast this morning...oooppsys.

I hope this wasn't to long winded but I'm rather anal about things. I hope to get more tuning tips and recommondations.


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## makguy (Jul 13, 2014)

Just to clarify, these carb mods were on an otherwise stock 7900?


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## cgraham1 (Jul 13, 2014)

makguy said:


> Just to clarify, these carb mods were on an otherwise stock 7900?


I'm pretty sure it's an 84cc big bore on a 6401 Makita.


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## Poleman (Jul 13, 2014)

No. They where on saw pictured in Clints post. I purchased it a couple weeks back. It's a 6401 With I think a Baileys 84cc Big Bore. Im going to get the carb nailed down then pull the base gasket and possibly port. Then go from there.

I did put a notch in the throttle plate this afternoon a little smaller than on the jet side. I know have TOTALL adjustment. I put orininal idle screw back in and had to keed backing off to just touching to make it idle where I wanted. I was then able to richen the L to where I wanted it now because of low idel issues before. I sure the little change will increase low end tourqe......it was good before. It also retained the lightning fast throttle!!!!

Sorry for any confusion I may have caused.


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## Terry Syd (Jul 13, 2014)

Poleman, good write up. I agree about nipping a bit off the throttle plate to try and get it to further lean out the idle, I did it to my carb and it worked to give me 1/4 idle adjustment with the stock needle. If you are only getting 1/4 now with the 044 needle, then reworking the throttle plate should give you more adjustment.

What you call throttle response in the cut is actually the torque off the bottom of the power band. I've found that kind of effect in the carbs I've done.

The 30/35 jetting may be the hot set-up or it may be that a 30/40 combination would give you a bit more off the bottom. As far as top-end power, you may be able to use the 30/40 and then lean out the high speed to give a flatter fuel curve and get more speed in smaller wood.

If you had a second Zama to work with you could drill it to 30/40 and compare it to the 30/35 carb. That would be the ideal way to determine which jetting works best.

The problem with the idle adjustment can be fixed with drilling a larger transition hole/s. I'll have to get my butt in gear and mod a carb. Pulling the idle jet out of the carb to get to the transition ports will be the biggest hassle, but do-able - I just wish Zama made screw-in jets.

Edit: I see you posted while I was writing, great to hear the throttle plate mod worked.


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## Poleman (Jul 13, 2014)

Yep....I see a second Zama in my very near future!!!!!!!!!!


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## MCW (Jul 13, 2014)

Hi Terry. 
Are your carby mods still on a stock 372XP () BB end or have you ported it by now? The improvement I saw on Neil's saw (same as yours) with a simple muffler mod yet stock top end were huge. That was a bored out and modified 395XP carby - it showed me just how much fuel and air a stock saw can utilise when done properly.


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## Terry Syd (Jul 13, 2014)

My 365/372 BB is essentially stock timing. I took the intake timing out to 160 duration, but exhaust and transfer are stock. Neil gave me a 395 carb to try on my saw, but there were issues with the mounting and choke lever. Since this is just a cheap Chinese bucking saw that is only running a 24" bar with 8-pin, I still have the 395 carb sitting on the shelf. The modded Zama carb gives me all the torque and power I need.

As I was working out the mods, I tried a 35/35 combination. The .35mm idle jet causes more issues with the idle adjustment, that is why I recommended that Poleman start with the 30/35 combination. I'd suggest that if someone was modding a Zama for a stock 372 or other saw that they don't go larger than .30 on the idle jet.


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## Terry Syd (Jul 13, 2014)

Poleman said:


> Yep....I see a second Zama in my very near future!!!!!!!!!!



LOL, Now you're hooked! Not bad for a $10 investment in tools. You may even start a business of 'Poleman's Carbs' - for the discerning wood cutter.


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## cgraham1 (Jul 13, 2014)

Poleman said:


> No. They were on saw pictured in Clint's post. I purchased it a couple weeks back. It's a 6401 with I think a Baileys 84cc Big Bore. Im going to get the carb nailed down then pull the base gasket and possibly port.


It is an NWP kit from Bailey's. I found a Dolmar 7900 while I was messing with this saw, that's the only reason I sold the Makita to Poleman.


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## Poleman (Jul 13, 2014)

Yep....I'm addicted!!!!

I've learned so much and have put you research to an applied aplication.....with good results!!!!

I don't know about that but I do now how to mod the Zama. I will get another carb and go another step and see what happens the way it is then maybe port this saw and again experiment. 

Terry, Thanks for sharing all your finding on what you've already done.....I couldnt have done any of this without our help.

Big THANKS, Rich


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## one.man.band (Jul 19, 2014)

2 cents: drilling jet holes/butterfly throws initial screw settings out the window, main nozzle check valve (unadjustable) becomes an issue/limiting factor

needed this adjusted: see attachment photo


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## Poleman (Jul 19, 2014)

one.man.band said:


> 2 cents: drilling jet holes/butterfly throws initial screw settings out the window, main nozzle check valve (unadjustable) becomes an issue/limiting factor
> 
> needed this adjusted: see attachment photo


Your absolutely right....stock setting are out the window since it is no longer stock. With any modifications that is an understanding. To improve is to change. Let's face it here on AS we're all looking to improve the cutting ability of our saws.....and who like stock anyway??? Once bitten it becomes habit forming to change and modify.....CAD's is the results.

Have some more carbs coming in to test. Will try the 30/40 and see how that works and make a judgment as to which works better on the BB kit. I think the 30/35 would be killer on a 7900!! 

One word of caution....I played with the notch on my throttle plate some more and it seems I'd you go to much you will gain more adjustment but loose some of the throttle snap off idle. It still runs and cuts the same just loose a bit of crispness.

I may try to play with some on the washer Terry had mentioned in another thread to see if that can be gained back what I lost.


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## Chris-PA (Jul 19, 2014)

one.man.band said:


> 2 cents: drilling jet holes/butterfly throws initial screw settings out the window, main nozzle check valve (unadjustable) becomes an issue/limiting factor
> 
> needed this adjusted: see attachment photo


I think Terry's mods make sense for the particular Zama model he's working on. The idle and idle transition (part throttle) circuits of the various carbs work differently, and he's using those particular Zama carbs because they allow him to modify the operation at part throttle the way he wants to. 

That's a complicated portion of the carb function, as it must provide a correct steady-state mixture, but also must provide a properly enriched mixture under dynamic conditions (acceleration/throttle response). The Walbro and Zama designs are different, and even the various models of a given brand are different. Carbs with accelerator pumps have an even more different approach (which is my preferred method), supplying the fuel for acceleration from a totally separate system rather than requiring the fixed jets to do it. 

I find carbs to be quite interesting and enjoy reading about the mods by people who really understand them. Sometimes they take different approaches to how I would do it, but that doesn't mean they don't work. 

I'm not sure why the check valve should be a problem? Ultimately the flow through the main nozzle cannot cannot be changed too much or the main mixture would be grossly rich, and I would assume the check valves have at least a little overhead? The purpose of drilling/filing the throttle plate is to move the mixture screw so that it is more centered on the taper - if you are at the ends (either just off full closed or where the point is withdrawing from the hole) then you have very poor control.


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## Terry Syd (Jul 19, 2014)

Poleman said:


> Your absolutely right....stock setting are out the window since it is no longer stock. With any modifications that is an understanding. To improve is to change. Let's face it here on AS we're all looking to improve the cutting ability of our saws.....and who like stock anyway??? Once bitten it becomes habit forming to change and modify.....CAD's is the results.
> 
> Have some more carbs coming in to test. Will try the 30/40 and see how that works and make a judgment as to which works better on the BB kit. I think the 30/35 would be killer on a 7900!!
> 
> ...



In regards to the 'snap off idle', perhaps you can move the throttle a few times and see if the 'off idle' transition is going 'rich' or 'lean'. If it is going too 'lean' you may want to try a shorter metering spring. The shorter spring will allow the 'ramp' (slope) of the fuel to start sooner. Don't cut the spring, see if you have another spring from another carb you can try.

After you try the 30/40 combination, there's more you can do to the Zama. Although the designer of the carb flowed the venturi on a bench, the freaking middle management probably save $.23 production costs on every carb by using two halves in the mould that come together at the SMALLEST POINT IN THE VENTURI. Crikey, the area of fastest flow through the carb and they put a line of casting flash 360 degrees around the inside of the carb, brilliant. The designer of the carb was probably physically ill after he saw what the management wonks had done. Anyway, you can clean up that line of casting flash and smooth out the flow.

AND, as soon as you have got the Zama sorted out for your saw, you may want to try modding the Walbro HD. What if, like, what if a modded Walbro could be made to work better than the Zama????


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## Poleman (Jul 19, 2014)

I'm rirht with you Terry.....I been thinking along those lines also.

I do have more springs I can try and will also look at cleaning up any casting marks. 

So a shorter spring will help snap? What will shimming the spring do other than increase spring pressure....still sorting these mods out.


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## Terry Syd (Jul 19, 2014)

Less pre-load, that is decreasing spring pressure, makes it richer off idle.

More pre-load, that is, increasing spring pressure, makes it leaner off idle.

You have to determine if it is going lean or rich off of idle, then you can adjust as appropriate. - If you have difficulty determining if it is rich or lean, then try a shorter spring to see what happens. If a shorter spring helps, then that is the direction you need to go and you can try an even shorter spring.


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## one.man.band (Jul 19, 2014)

check valve opens the main at a set vacuum depression, (ie. air velocity) through the venturi. the 3rd transition hole seems to affect it a small amount as well.

there is only so far you can go w/the butterfly before you reach clutch engagement rpm.

screwdriver points to something that needs a tweak, when putting notches in things, imo.

hope it helps.

-omb





folks have different goals here for the carb. mine is more adjustability.


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## Terry Syd (Jul 19, 2014)

OMB, perhaps if you were less cryptic we would understand what you are referring to. So WHAT is the screwdriver pointing at and WHY is it important.

The check valve in the main nozzle is just a flap that keeps the air in the venturi from going back into the metering chamber when the engine is idling. Without the flap/check valve the engine won't run at idle


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## one.man.band (Jul 20, 2014)

Terry Syd said:


> OMB, perhaps if you were less cryptic we would understand what you are referring to. So WHAT is the screwdriver pointing at and WHY is it important.
> 
> The check valve in the main nozzle is just a flap that keeps the air in the venturi from going back into the metering chamber when the engine is idling. Without the flap/check valve the engine won't run at idle



yes.

photo: walbro hd

looking at diagrams on page 9 of 'walbro service manual'.......

this is what i see.

idle circuit works on vacuum.

main works on pressure.

note position of check valve between the two diagrams.

idle circuit has no air bleed to control the backflow.
as pressure builds, main check valve is in control of
the transition from vac to pressure, since they
share the same metering chamber. (referencing
the diagram, metering chamber is below the screws).

the idle circuit stops contributing fuel past part throttle.
(again.. reference the diagram to verify this).

took me about 12 hours of yanking on the cord, mounting
and dismounting the carb, drilling, notching, turning jets
over the course of 4 days to find out if the following
is true:

"if notching or drilling plate extends the range of the
idle circuit?" works better when plate is completely closed,
before doing any of this.

to enlarge photo....hit ctrl + on the keyboard
screwdriver points to the "throttle stop"

thought photo would only help to identify this part.

-omb


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## Terry Syd (Jul 22, 2014)

Poleman, if dropping the pre-load didn't help that much, then the throttle plate is probably sitting a bit too far back from the transition ports.

I know I can fix the idle issue if I pull the idle jet and re-drill the transition/air bleed holes, but it would be nice to find a 'quick fix' that makes the mod easy (like the non-existent slow taper idle screws). I was looking at the difference between the idle and high speed needles and there's a possibility that a high speed needle (with its slower taper) could be used in the idle circuit. The needle shaft would have to be narrowed so it was like the idle needle shaft - and THEN there is the issue of the different thread pitches. It maybe able to be screwed in (I'll check tomorrow) with a bit of force, or maybe run a tap on the screw thread to change the pitch.

Another alternative might be to flip the throttle plate over and put the idle notch on top, then cut a very small notch on the bottom to provide some airflow over the idle port.

I think the next Zama carb I do I'll just pull the idle jet (I'm sure that sounds easier than it is) and find an air bleed hole that gives us the adjustment.


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## LegDeLimber (Jul 22, 2014)

Terry Syd , Have you given any thought to chucking the screw in a drill press or drill
and then a bit of gentle useage of a small file?
Something like a diamond grit nail file (you'll need to eyeball the flatness)
or a couple of grit sizes of sandpaper lightly glued to some flat stock to some small,strait flat stock.
Prior to finding non-corn fuel, I was fighting with swollen needle seats in my B&S mower carbs.
The swelling pushed the float lower, etc and created a lean condition.
I ended up chucking the float needle in a drill and using a small flat file to dress
off enough of the head end of the needle to compensate for the thickened seat.
A diamond nail file was great for getting the finish of the needle back to a nice, slick
surface that wouldn't wear into the plastic float and slowly alter the fuel level.


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## Terry Syd (Jul 22, 2014)

What I had thought of doing was making up an idle screw using the original screw, but heating up the taper and putting a thick layer of solder on the tip. Then trim it down - good idea about chucking it up in a cheap drill press (I have one) and turning down the taper.

Hmm, got my interests up now. With the screw hanging from the chuck of the drill press I could heat up the tip of the screw and apply the solder (or brass) on just the tip. Then I could start the drill press and turn the tip down to the taper I wanted. Yep, sounds do-able.


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## Terry Syd (Jul 22, 2014)

A couple of days ago I got the package with the carb parts. I didn't open it as I was frustrated that they didn't have the idle needles and stuffed me around for 6 months. Well, I opened it today and stood there dumbstruck as there were 3 needles in a package. I compared them to the 044 needle - and they were even a slower taper than the 044.

I don't know what happened, maybe part of the order was completed and they were waiting for the rest of the parts. In any case, I think the idle problem is solved. The part number for these slow taper needles is 0031023.


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## Poleman (Jul 23, 2014)

So Terry, You would prefer to use a long taper idel screw instesd of doing throttle plate alterations??

I recieved more carbs and will test the 30/40 combo on my 7900BB probably tonight. I will update and give results as I gather them. I may do more tweeking this time since I got some big rounds at the house and don't have to go in the field to test and have more tools, parts, etc. avalable to make changes. I have extra sizes of springs and a bag of the small hobby washers to be able to try some different combos.

I will be very interested as to the potential of this combo. Yo said you are using it on a saw of yours that is smaller cc's. This SHOULD be the HOT setup for the BB Dolmar!! I'm hoping I have a grasp of how to change throttle responce once/if I get this combo sorted out.

Fast throttle responce and killer bottom end is what end goal is....top end will take care of itself I feel.


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## RyanTowry_81 (Jul 23, 2014)

Very nice thread guys!!! Keep up the good work! 

I need to find some of these zama carbs to play with for my 365bb build!

Terry, you said you were using the zama c3m but is their a particular model saw they came on so I know what sub model c3m to try and find?


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## Terry Syd (Jul 23, 2014)

Poleman said:


> So Terry, You would prefer to use a long taper idel screw instesd of doing throttle plate alterations??
> 
> I recieved more carbs and will test the 30/40 combo on my 7900BB probably tonight. I will update and give results as I gather them. I may do more tweeking this time since I got some big rounds at the house and don't have to go in the field to test and have more tools, parts, etc. avalable to make changes. I have extra sizes of springs and a bag of the small hobby washers to be able to try some different combos.
> 
> ...



I think the slow taper idle screw make the most sense for most of the guys. They probably already find it a bit intimidating to sit there with little tiny drills and drill out the jets. Throw in trimming the throttle plate when they don't know how much to trim would probably push the limits of their comfort zone - sticking in a different needle is a quick fix. A person could do both, in fact, I may pull the idle jet and drill the air bleed holes just to see how it affects the idle. Heck, maybe it could give an even steadier idle (uh oh, something else to try)

The hot setup for each saw will vary to location. I am cutting between 2,000-2,500 ft. Someone else may be cutting at sea level and another at 7,000 ft. Ya just gotta try and find what works. Once you start tweaking the metering spring, the throttle response concept becomes easier to understand. You'll quickly figure out which way to go and when you go too far. THEN, because you now can FEEL the difference, when you pick up one of your other saws you'll notice it - like, how could I have used this saw for 10 years and put up with that throttle response - so you mod that one too, and then another.

Yeah, the top end power is easy to tune for, just tweak the high speed needle to give the appropriate amount of fuel for the air flow at peak power. Chainsaw racers are probably a good example, they run the engines in a very small portion of the powerband, that is, where the engine will do the most work. However, once you move away from cookie cutting to production work, a broad powerband makes the saw a lot easier to use.


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## Poleman (Jul 23, 2014)

OK, modded the first carb to 30/40, notched the throttle plate, set everything else to spec. 1 turn L & H, ideal screw 1 turn in from just touching.

Fired first pull. Increased ideal and agjusted low...still rich. I then remembered Terry saying you may have to lean out the H....that was it. Good throttle responds and amazing torque in the cut. I'm using 28" bar, 72LG chain, 7 tooth rim and at about4000 ft in elevation. I was cutting 30" green pine.....the saw just laughed....

Modded second carb and set everything the same as first. This carb received a kit and a bath before being put together. Again everything set to factory spec.

Started relatively easy but not like first carb but dialed right in with plenty of adjustment on idle screw. Again H almost leaned out.

At this point I figured this is cake....well my head got to big. Next carb was going to get a kit and bath also....upon disassembly I noticed the idle screw almost all the way in...note to self on that because that seemed unusual. Kitted and ultrasoniced carb 3 and assembled like others. 

Saw did not want to start and when it did it just wanted to blubber and wouldn't take throttle without during. Idle adjustment all the way in. Notched throttle plate more and tryed again.....better but not right. Removed carb again notched more and put two shims under spring. Better


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## Poleman (Jul 24, 2014)

Better but idle screw still all the way in. Saw runs good but I don't like the idle adjustment being like it is.

I will think about this tonight and tomorrow and try to figure this carb out. It may have a previous problem that is carrying over into this mod. The giveaway was what I seen on disassembly. Apperntly for some reason this carb wants a lot of air.

Just not happy with this one as it seems to be way rich. It is more difficult to start and actually like to start on high idle on a restart. I'm getting there with this one but I'm not where I want to be. 

Till tomorrow......any thoughts why this one carb is acting like it is?? Solutions, ideas????


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## Terry Syd (Jul 24, 2014)

First things first - did you prefer the 30/40 combination over the 30/35?

As far as that idle problem in the second carb - sounds like there may be something clogging one of the transition/air bleed holes. If one of the air bleed holes is clogged, the idle is going to be pig rich. Try taking a piece of wire, bending the tip 90 degrees and sticking it through the air bleed holes over the idle jet, if something is there, maybe you can clear it.


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## Poleman (Jul 24, 2014)

I guess I never answered the million dollar question did I.

The Mod that has more power is the 30/40 combination. But at this point I like the adjustability more on the 30/35 combo. I'm still trying to get a grasp on the affects and reactions of the change. Terry I couldn't believe how lean I had to adjust the high...you were spot on on that one. The saw is getting so much fuel down low and midrange it carry's over to the hi. This saw is an absolute torque monster now!!!!! I wish I had access to a 42" bar....I'd love to try it. I actually think the oiler could keep up also as this saw oils really really well.

I will try cleaning the passage and maybe ultrasonic it again...something is deffinatly going on with this particular carb because it acts totally different than the first two. But I had my suspicions on teardown seeing the idle screw like it was.


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## Terry Syd (Jul 24, 2014)

As far as the adjustability of the 30/40 combination, have you tried it with the slower taper 044 needle? By the way, those slower taper needles I bought only cost $2.50 a piece.

I know you have been concentrating on the torque, but did you notice if the engine picked up some revs in the cut above peak torque? Perhaps it might be more noticeable in smaller wood.

EDIT: As far as that wonky carb, there is one other thing that may be causing an excessive rich mixture at idle - and that is a leaking welsh plug.


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## Poleman (Jul 24, 2014)

Terry, I haven't tried the slower taper screw yet. The other carbs all acted and responded to the changes I have made to them in a reliable manner and didn't need the slow taper screw.

Now the carb in question does not respond in a like manner. To a point it does but not all the way. This carb also had a leakon my pressure ggauge after I put it all together after mods and rebuild.

It didn't occur to me to question the Welch plug....I will reseal and/or put in a new one first thing to rule that out. I may tank & pressure the carb to see if that's what it is. I ran out of time last night to do more tests. 

Terry, the saw with the 30/35 jets was awesome but the 30/40 is definitely stronger yet. I appolagize for not getting any tack readings in the cut I was concentrating on getting carbs dialed in. I can tell you just by feel it is faster in the cut. The 30" wood I have in no way challenging to this saw. I may have to make a run to get it into some larger hardwood. I just want to get this 3rd carb sorted out and you have sure helped with that.


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## Terry Syd (Jul 24, 2014)

Rather than make a run to get into some larger hardwood, why not put an 8-pin on the 28" bar and try it. The carb mods give the engine a greater resistance to bogging/stalling, so the 8-pin may work just fine. You may be able to fine tune the engine a bit better with the greater loading.


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## Poleman (Jul 24, 2014)

Thats an idea. I've never been a fan of an 8 pin sprocket but this saw procuced so much torque I dont think it would hurt. Bog in the wood...right now with what I've been cutting thats seems almost impossible. LOL.


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## Terry Syd (Jul 24, 2014)

If you want to see a visual representation of what I believe you did, take a look at this thread with Chadihman's dyno. There are several power curves there that have a 'dip' prior to peak power. My post at 519 is my take on the dip.

http://www.arboristsite.com/communi...lie-ms-461-is-king-over-ms-660.258040/page-26

It looks like you have filled in your 'dip' with the extra fuel flow in the low speed circuit and may have extended the power band after peak torque when you leaned out the High speed needle and flattened the fuel curve.

EDIT: I don't think you can improve much on the 30/40 combination and there is a bigger risk of going too far at this point. When you had to back off the High speed needle that much, I expect that was a strong signal to look for when doing this mod.


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## Poleman (Jul 24, 2014)

Terry, I think your absolutely right!! The extra fuel give the saw a VERY WIDE power band. It accelerates like a rocket and pulls like a train is about the best way to describe how it runs.

I also think the 30/40 is the limit for this saw and any more would cause more issues. The only other thought would be for a ported saw with squish cut to get around 200lb max compression. I think things could be opened up to the max because the carb could feed it. I understand that the carb issues or lack of performance from carb limited the porting of the 7900. This may change all this and could be a big boom on ported Dollars in the future.

I know I sure like how these changes have made mine perform!!! Still have porting and dropping base gasket in mind and possible timing advance in the future.

The second carb I spoke about earlier did turn out to be a leaky Welch plug. Pulled the cover, pressurized and added a squit of leak finder(kids blow bubbles) and sure enough it was leaking bad all the way around it. Applied sealer, let it dry then tryed on saw again....whala...a whole different acting carb. One more little change to make, and retest. I gained more adjustment but this carb is still on the hi side of the idle screw. I wish I had some of the slow taper screws to work with so I wouldn't have to massage the throttle plate anymore.

Was another good night in the garage and things are looking good!!!


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## Mastermind (Jul 25, 2014)

You guys must have loads of patience.


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## Terry Syd (Jul 25, 2014)

Here's my opinion of where to go next. I look at cost effective solutions, doing the carb was a cheap mod, why not follow the same philosophy for the rest of the tuning.

To put your carb in perspective, I have a 50cc saw with the equivalent of a 19mm venturi, you have a 84cc engine with a 17mm venturi. You just aren't going to be able to feed that cylinder if you raise the cutting speed with radical port work. It will likely make the engine less efficient. If you want more cutting speed, then using the torque and going to an 8-pin seems more logical.

Removing the base gasket will bump the compression and give more torque. Increasing the intake timing to 160 degrees will help feed the existing port configuration with the small carb. I'd leave the exhaust and transfer ports alone (except for cleaning them up).

Your saw is based upon the 64cc Dolmar, the mounting location for the coil is based upon the 64cc cylinder. The 84cc cylinder has a lot wider combustion chamber and may need the timing reset to handle the extra distance for the flame to travel.

If you did those three things you may decide you didn't need anymore - and those three things won't cost you any more money, just your time and effort.


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## Poleman (Jul 25, 2014)

Lol...and you don't Randy????

What do you think about this? Do you think this could make a BIG difference on feeding a ported saw and make it perform optimally? It would sure give a thirsty saw what it needs!!


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## cgraham1 (Jul 25, 2014)

Well there's only one way to find out, right?


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## Terry Syd (Jul 25, 2014)

Mastermind said:


> You guys must have loads of patience.



I don't know if it is patience or stubbornness. Crikey, I spent some time trying to figure out how these freaking carbs worked. In fact, One Man Band just sent me a Kart article on carbs that was pure BS - just typical of the crap out there you have to weed through. I'll post a link to the article and you can see what I mean.

If someone can come up with the schematics on the electronic carbs, I'll figure out some mods for those (I spent 7 years as an electronic technician and have modified electronic fuel injection systems).

I've been modding for 50 years now, its become a sick habit that I can't break.

EDIT: Here's the link to the Kart article, see if you can pick all the BS in it - it is chock full of it.

http://nkn.com/nationalkartnews/articles/AnatomyOfWalbro0708.pdf


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## Poleman (Jul 25, 2014)

Never thought of it that way Terry...
Going to test the 8 pin sprocket this weekend and see what I think. Still weighing other options. Always want to improve and move forward...hate to ruin an already good thing...


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## Mastermind (Jul 25, 2014)

I'm playing chess right now........I'll need to read through carefully......or get a cliff notes version.


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## Poleman (Jul 25, 2014)

Clint.....that's what I'm thinking too!!!!


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## Poleman (Jul 25, 2014)

Randy, here's the cliff note version.....

You port saws.......a Dolmar 7900 is scheduled to arrive in September......I send carb for you to test against stock and see if this is a viable solution to feeding a MM ported Dolmar. You do your as usual, test stock then test modded carb to see difference...report results.


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## Mastermind (Jul 25, 2014)

Poleman said:


> Randy, here's the cliff note version.....
> 
> You port saws.......a Dolmar 7900 is scheduled to arrive in September......I send carb for you to test against stock and see if this is a viable solution to feeding a MM ported Dolmar. You do your as usual, test stock then test modded carb to see difference...report results.



Cool.


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## Terry Syd (Jul 25, 2014)

OK Poleman, how much will you charge to mod someone else's carb. You have spent some money on the appropriate tools, picked up a fair bit of knowledge, experience and most importantly the 'feel' for the mods. Other guys don't have that, so you're sitting on a gold mine.

Someone sends you a carb that you rebuild and mod and then send back to them - I wonder what the market will bear for that kind of work.


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## Poleman (Jul 25, 2014)

Good question Terry. I lack the knowledge you have about tuning and altering these carbs. You gave me your specs and knowledge of what to do and was open and VERY helpful to get me where I am with all this. So what do I owe you??

I just like saws and making them perform to their full potential. I like this site because of people like you and many, many others. I've learned a lot about porting, cylinder clean up muffler mods from guys like Mastermind, Terry Landrom, Brad, Stumpy and whomever shared their knowledge over the years. I just want to contribute and repay what I have learned.

What do I charge???? I want to be fair, I want to help others because so many of you helped me with sharing your knowledge. I would use what you have found and tryed in my own applications. Besides it's something I like doing!! A job you like doing is like a paycheck. This is my fun and hobby, and for the most part keeps me out of trouble.

It's a very good question and I don't have an answer.


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## Mastermind (Jul 25, 2014)

I'll be the first to admit that modding carbs is an untapped area for me. I'm sure there are good gains there. 

I'm all about seeing someone make money by doing something they enjoy. 

Zama carbs have frustrated me so many times........


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## Terry Syd (Jul 25, 2014)

I can't charge anything for what I've contributed, I've already gotten enough back from other members to more than compensate me for my ramblings. If it wasn't for that spirit of contributing about what each of us knows, I wouldn't be here.

Although... to bad there have been some azzholes that have driven away some of the old timers that probably already knew what I figured out about the carbs. Heck, you don't think some of the ol' timers figured this stuff out when the 'emission' carbs started coming in, they have probably been sitting on this information for years.


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## Mastermind (Jul 25, 2014)

I've done some drilling on those Zamas........but never had enough time to really get it down to knowing how much was just enough. 

Normally I find a Walbro or a Tilly and throw the Zama in the can. 

I'm all ears here.......believe that Terry.


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## Terry Syd (Jul 25, 2014)

OK Poleman, now that you've got your head around modding carburettors, ya wanna try modding that HD-12 and see if it is possible to get it even better than the Zama?

It's basically the same concepts, but a different approach to get the results. You'll need a fist full of welsh plugs for that puppy.

EDIT: I got a better idea. Let's find the crappiest EPA approved HD and turn it into a HD-12 for the guys that really like that carb. Then we'll see if we can mod it further to compete with the modded Zama.


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## Poleman (Jul 25, 2014)

Lol....your pushy arnt you!!! Lol

Been thinking of the same thing but have been concebtrating on the Zama. Haven't studied a HD diagram to get any ideas yet. I'm sure it can be done


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## Gravedigger (Jul 25, 2014)

I just got my new Walbro HD12-1 and bolted right in my Makita 6401 Weedeater big bore. Started first pull set on fast idle. Won't idle off fast idle. Need help adjusting carbs. I'll travel for lesson.


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## Poleman (Jul 25, 2014)

Is this a known good carb? Sounds rich to me...

Turn the L (low) in all the way and back it out one turn. That should get you in ball park. Increase idle as needed. I'm assuming the limiters are off on the carb or you will need to remove/trim them to get full adjustment. You will probaly need it with the BIG change you made.

The one I tryed on mine a couple weeks ago dialed in in about 5 minutes. Mine was easy and I knew the carb and that it was good.

Hope that helps!!


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## Gravedigger (Jul 25, 2014)

New out of the box Walbro carburetor. Original Zama would just sputter, rebuilt no help. Saw was run lean and seized when I got it. Early in this thread is when I ordered new carb. How are the limiters removed. I tried pulling off no go, should I just break or cut off.


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## RyanTowry_81 (Jul 25, 2014)

Gravedigger said:


> New out of the box Walbro carburetor. Original Zama would just sputter, rebuilt no help. Saw was run lean and seized when I got it. Early in this thread is when I ordered new carb. How are the limiters removed. I tried pulling off no go, should I just break or cut off.


I pulled the limiters off the HD12 I have on my husky 365, it just takes some wiggling with some needle nose. I wanted to just trim them off until I noticed the plastic was stripped out on the high so I just pulled them off.


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## Terry Syd (Jul 25, 2014)

Poleman said:


> Lol....your pushy arnt you!!! Lol
> 
> Been thinking of the same thing but have been concebtrating on the Zama. Haven't studied a HD diagram to get any ideas yet. I'm sure it can be done



Yeah, it can be done, but the Zama is so much easier to work on. If you want to get some of the slow taper idle screws, here's a link - http://www.m-and-d.com/ZA-0031023.html


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## Poleman (Jul 25, 2014)

Thanks Terry, I was giing to go back and look at your post were you gave the part number and see if you put a link to it......you were reading my mind.
I think the slow taper screw will really help in the tuning ability.


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## SawTroll (Jul 25, 2014)

This likely has been said before, but the 17mm venturi of the 7900 Zama carb and the 17.5mm of the HD12 both are obviously too small. The saw brands are sort of pulling "games" on us, regarding where in the design the worst restriction is - carb, engine porting or muffler. What they choose to do has an impact on sound level and emissions + the obvious.

As an exemple, the 385xp/2186 was almost ideally ported - but was extremely restricted at both ends (carb and muffler).


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## Gravedigger (Jul 26, 2014)

Poleman said:


> Is this a known good carb? Sounds rich to me...
> 
> Turn the L (low) in all the way and back it out one turn. That should get you in ball park. Increase idle as needed. I'm assuming the limiters are off on the carb or you will need to remove/trim them to get full adjustment. You will probaly need it with the BIG change you made.
> 
> ...


Popped the limiters off ( destoyed ) Readjusted L to one turn out and she idles. Needs fine tuning. Poleman was right way rich. Thanks.


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## RyanTowry_81 (Jul 26, 2014)

Terry Syd said:


> Yeah, it can be done, but the Zama is so much easier to work on. If you want to get some of the slow taper idle screws, here's a link - http://www.m-and-d.com/ZA-0031023.html


I was just looking back for the part number also to write it down, might just have to order a couple. Looks like it has the bigger adjustment end also, not like the tiny slot on my hd12 on my 365 after I pulled the limiters off. Those little slots are hard to hit and tune while the saw is running and vibrating around.


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## Chris-PA (Jul 26, 2014)

Terry Syd said:


> I don't know if it is patience or stubbornness. Crikey, I spent some time trying to figure out how these freaking carbs worked. In fact, One Man Band just sent me a Kart article on carbs that was pure BS - just typical of the crap out there you have to weed through. I'll post a link to the article and you can see what I mean.
> 
> If someone can come up with the schematics on the electronic carbs, I'll figure out some mods for those (I spent 7 years as an electronic technician and have modified electronic fuel injection systems).
> 
> ...



I read through that article and find it wanting too. I'm curious what parts you didn't like, though I suspect I know. To me the biggest problem stems from his statement in Section C, pg 6:

_For our purposes, it is enough to understand that the average pressures in the venturi are proportional to the square of the average intake air velocity (or, effectively, the square of the volume flow rate into the engine). In simple terms, average pressures in the venturi are typically less than atmospheric, and will decrease by a factor of four as the intake flow rate doubles. A complementary, and equally important, relationship governs the metered fuel flow rate. This is based on the same physical laws governing the venturi pressures above. The fuel flow rate is determined by the effective area in the metering circuit, and the square root of the pressure across that circuit. The effective area is the combination of all the restrictions in the flow path, like jet diameters, needle valve position, and cross sections in the flow passages. If the Walbro pressure regulator is doing its job, the pressure across the metering circuit will be held nearly equal to the pressure in the venturi (less the slightly negative pressure maintained in the metering cavity). Therefore, when the intake airflow rate doubles, the fuel flow rate doubles in step. This makes sense since the venturi pressure increases by a factor of four as the airflow rate doubles. The square root of four (relative change in the venturi pressure) is two, so the fuel flow rate doubles. Sounds complicated, but it isn’t if you pay close attention to what is being said here. *The beauty of these physical relationships is that the resulting air-fuel ratio remains a constant. This is true despite wide variations in engine speed or throttle position.*_​
This is incorrect, and does not match the known behavior of these carbs – we know that they provide a drastically richer mixture with just a small increase in air velocity (4-stroking), and most carbs in other application have air corrector jets to linearize the mixture.

I can see how he came to this conclusion. The Bernoulli equations show that the pressure at the outlet port is the square of the air velocity. But the characteristics of the flow of the incompressible liquid fuel through the jet is not the inverse of the compressible air through the venturi. This article ( http://en.wikipedia.org/wiki/Carburetor ) contains the following description:

_The main disadvantage of basing a carburetor's operation on Bernoulli's Principle is that, being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake air speed. The fuel jets are much smaller and limited mainly by viscosity, so that the fuel flow tends to be proportional to the pressure difference. So jets sized for full power tend to starve the engine at lower speed and part throttle._

Later in the article he goes on to discuss how he didn't understand why some cart tuners were using the low speed circuit and deactivating the H circuit – it's because the L circuit on that carb actually has an air corrector jet and so can be made to meter correctly. They'd be better off adding an air corrector to the H circuit.

He discusses the L circuit air corrector jet, but misunderstands its function like many do. It is not to improve atomization, it is to turn the incompressible fluid in the jet into more of a compressible fluid, and then the flow is more linear with air velocity as he described.

Other than that, he seems a little foggy on the dynamic function (acceleration). The description of the main fuel diaphragm is decent for steady-state function.


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## Terry Syd (Jul 26, 2014)

Take a look at 'Component 1' - he thinks that the pump diaphragm provides ALL the fuel by 'pumping'. It is a common mistake, but once you make that assumption, any analysis goes down hill from there. For example, take a look at what he came up with for 'Component2' - because he thinks the pump diaphragm is pumping the fuel, he draws the wild azz conclusion that the metering diaphragm is a 'pressure regulator' for the pressure provided by the pump diaphragm.

The pump diaphragm can be removed from the carb and the engine will run. Just remove it, choke the carb and start it. As long as you keep the revs up and don't try to idle it, the engine will keep running. The pump diaphragm and metering diaphragm are used in the idle system and the ramping of the fuel flow off the idle jet.

There's a bunch of other crap in the article, but it is clear he doesn't have a clue how these carbs work. - However, the pictures and magazine layout are very professional, it does make the article look very authoritative.

The best document for how these carbs work is the Mikuni document I linked to in the Chinese Carburetor thread.


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## Terry Syd (Jul 26, 2014)

This thread is about building a better carb for a big bore, like the 77cc 365BB or a Dolmar 7900. However, a person should be able to mod a 372 carb to turn it into a 'torque monster'. I don't have any idea what the jet sizes would be as I wasn't working on a 71cc engine.

Perhaps someone might convince Poleman to start modding a 372 and see what he comes up with. If he ends up going too far in the jet sizes for the 372 while he is testing it, no problem, that carb will become a big bore carb.

I would suggest that a person have on hand a 0019012 metering spring in case it is a bit too rich off of idle. The 0019012 spring has a stiffer spring rate to slow down the ramp. If the pre-load is a bit too much, the spring has 6 passive coils that can be clipped off with a nail clipper to decrease the pre-load.

The spring can be ordered from the same link I gave for the slow taper idle screw.


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## RyanTowry_81 (Jul 26, 2014)

Maybe if I get time before I leave I can throw the zama on my 365 before I big bore it. It might not be the perfect example of performance though as it only has about 120psi compression and i believe it is pretty worn.


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## Chris-PA (Jul 26, 2014)

Terry Syd said:


> Take a look at 'Component 1' - he thinks that the pump diaphragm provides ALL the fuel by 'pumping'. It is a common mistake, but once you make that assumption, any analysis goes down hill from there. For example, take a look at what he came up with for 'Component2' - because he thinks the pump diaphragm is pumping the fuel, he draws the wild azz conclusion that the metering diaphragm is a 'pressure regulator' for the pressure provided by the pump diaphragm.
> 
> The pump diaphragm can be removed from the carb and the engine will run. Just remove it, choke the carb and start it. As long as you keep the revs up and don't try to idle it, the engine will keep running. The pump diaphragm and metering diaphragm are used in the idle system and the ramping of the fuel flow off the idle jet.
> 
> ...


Yes, I realize that the pump is not always required above idle. Regardless of that however, I do agree with him that the primary function of the metering diaphragm is as a pressure regulator analogous to the float bowl in a normal carb. This is where my view of how they function differs a bit from yours, probably as I am coming from automotive carbs like Webers, where one doesn't mess with fuel bowl levels much as a means of tuning mixture. I have not really understood your terminology of ramping, and how the metering diaphragm provides this.

My preferred approach would be to have the metering diaphragm do its job of providing a fixed pressure differential (as he described), and the extra fuel for acceleration would ideally be provided by a pump. If the carb has been crippled by the lack of an accelerator pump, then the fixed jets must provide it by being set richer than optimum for steady state performance, especially by the richer mixture provided by the transfer outlets as the throttle plate edge moves toward them. You can reduce the spring pressure some so that a fast acceleration will allow the inlet valve to open faster, and then you don't have to set the idle & transfer jets quite so rich, but you can only push that so far.


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## cgraham1 (Jul 26, 2014)

I am so confused. 



SawTroll said:


> This likely has been said before, but the 17mm venturi of the 7900 Zama carb and the 17.5mm of the HD12 both are obviously too small.


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## Poleman (Jul 27, 2014)

Well Terry....your reading my mind!!!

My next project is to put the 30/35 carb on my Husky 365 BB and see what happens. I also want to try it on my MM ported MAKITA 6401!!

Also want to give more details on carb #2 I was working with. I was trying to adjust out the problem of the Welch plug leak before it was found. Sealing the Welch plug made the carb predictable and I reversed all my changes trying to correct that problem. The carb now tunes and dial in very well.

After modding these carb I find they are very stable and not finiky at all...set and go. That is a BIG plus since no one wants to go cut wood and you have to adjust on your carb for 20 minuted before you can.

Procedure:
Start
Warm up
Cut
SMILE(OR GRIN IF YOU PREFER)

There a lot of power in that trigger finger now!!!!

Rich


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## RyanTowry_81 (Jul 27, 2014)

Don't forget beer after smile. Haha


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## Poleman (Jul 27, 2014)

Dang.....forgot the most important part......

CHEERS!!!!


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## cgraham1 (Jul 27, 2014)

All I want in the end is for my Masterminded 7900 to keep up with my Masterminded 064... is that too much to ask?


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## Poleman (Jul 27, 2014)

I think with the possibility and potential of the mnodded carb it will have a good chance!!! It will be giving up a few cc's but if it has the potential Randy thinks it has and it was limited by a carb limitation, this may change everything on the Dolmar 6400-7900 's

Either way it will be a GREAT SAW!!!!


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## Terry Syd (Jul 27, 2014)

Rich, for your business logo I thought a blue carburettor with a lightning bolt through it. I think the lightning bolt should be yellow or gold, either colour should go well with the blue carb.

Oh yeah, the blue carb should be the same colour as the blue anodizing you use on the carbs. The classic aircraft blue anodizing that is used on racing components would really set the carbs off - all the cool guys would be running the carb.

I feel you should put two stickers with your logo with each modded carb. One can go on the saw and the other could go on the beer fridge.


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## Poleman (Jul 27, 2014)

Lol....I like your Business vision!!!! Do I have your permission to use your concept?? I especially like the lightning bolt and colors.....hell Terry I like it all!!!! Blue anodizing.....COOL!!!!


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## RyanTowry_81 (Jul 27, 2014)

Or a blue like layout die with the lightning bolt etched in that could be cool


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## Terry Syd (Jul 27, 2014)

Mate, if you've got the 'nads to go into business, ya got my permission to use anything I've contributed. If you do it, I'd go with the anodized carbs. One of the problems with most of modding is that it is INSIDE the engine. Guys don't have anything to show off.

Even the builders would probably like the concept as when the owner gets the saw back, he can SEE something that looks 'trick' on the saw. They wouldn't be set up to do that kind of work and purchasing the carbs from you would probably be a cheaper option.

AND THEN - Once you got it all going you could have the Chinese do all the work and you just repackage the carbs they send you, then hire an illegal immigrant to do the packing..... Sounds like a familiar business model in today's world.


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## cgraham1 (Jul 27, 2014)

Poleman said:


> I think... this may change everything on the Dolmar 6400-7900 's


But SawTroll says the venturi is too small. 

But I don't care. I'm excited as hell to see what the modded carb will do on my saw. 

Thanks Poleman for the effort in trying to figure this out, and Terry, thanks for sharing your knowledge!


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## Poleman (Jul 27, 2014)

Lol....Would you be my Business consultant??? You've got this all figured out!!!!


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## Poleman (Jul 27, 2014)

Clint, I'm excited also to see what it does on a quality ported saw and the potential it poses for saws that use this carb or for saws that could. 

Figuring out jetting sizes for each application and mods done. Be like custom ground cams in cars.

Hopefully we will be making strides in saw performace.


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## Terry Syd (Jul 27, 2014)

If you're trying different sizes, you may want to also get another grading system set of drills. It can give you a few more options on the size of jet you drill - http://www.ebay.com.au/itm/HSS-DRIL...Home_Garden_PowerTools_SM&hash=item4d064b3ac8


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## Poleman (Jul 27, 2014)

Thanks Terry...Ive been looking for SAE sizes and anything that would fill in the blanks instead such large jumps. A drill size for every step would be optimal as one could go up 1 size at a time which could lead to ultimate
tuning for each appication.

Rich


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## RyanTowry_81 (Jul 27, 2014)

Even better would be if you could find another small carb that uses thread in jets in similar sizes as needed for the zama c3m and drill and tap the brass jets in the zama to accept a removable jet so you could have a stock pile of drilled and tapped carbs then just put the jets in for the application needed and ship out. Would help you streamline your inventory. As you got bigger you could contract the Chinese like terry said to send you the carbs already drilled and tapped and just out the jets in and done. Would also make your testing for other model saws quicker too


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## Terry Syd (Jul 27, 2014)

The Zama carb was designed by a bloke that knew carbs. I already mentioned the flowed venturi that middle management stuffed up - (Rich, you can clean up that venturi before you get it anodized - actually, after it is anodized you can take a rubber hose with some lapping paste and polish up the edge of the venturi, it would look HOT with the anodizing and the polished aluminium on the venturi) - I would bet you 10 to 1 odds that the bloke that designed that carb had designed it with screw in jets.


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## RyanTowry_81 (Jul 27, 2014)

Wouldn't it be a better world if money wasn't an object and everyone just listened to the engineer....


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## Poleman (Jul 27, 2014)

Well finally got thing pretty much sorted out for the BB Dolmar so I thought I'd try the 30/35 ,carb on my BB 365. 
Easy install as it's the Sam carb. Only difference is the size of the choke hole in the carb for the Husky. Put it on and got it fired up....running waaaaay rich, I had both H & L closed....it did clear out a couple times and ran good but loaded up again. Didn't spend much time on that combo.

Next up I put the same carb on a Ported Mastermind 6401. That dialed in and ran good other than a soft bottom end from being a little rich. I put 2 Spacers under the spring and that cleaned everything up. With a 24" bar in Pine with a 7 tooth sprocket it was turning 11,400 to 12,000 and showed no sigh of bogging in the wood. I could lean on it hard and it just kept going. Would do the same from idle in the cut...just pull trigger and throw chips!!!!!!

Don't know the reason yet to why the Husky didn't like this carb as it has more cc,s than the 6401 and should work on it and be better. I guess that will the next project to work on......


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## Terry Syd (Jul 27, 2014)

Rich, here's something I ran into. On the early carbs the high speed circuit doesn't need any extra fuel as so much is provided by the low circuit. The later carbs had a lean low speed circuit, so they had to add fuel in the high speed circuit to compensate.

On the Zama you will find where the back of the discharge nozzle has been drilled to various jet sizes. On the Walbro I've found a second jet added to the high speed circuit to add the extra fuel.

It may be that you have to block off the jet on the back of the discharge nozzle to get the top end leaned out.

EDIT: Maybe the reason the 64cc engine could use the carb and the 77cc engine couldn't, was because the velocity through the venturi was higher and the additional depression in pressure pulled harder on the high speed jetting. If so, then if you block off the back of the discharge nozzle that should cure the problem.


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## cgraham1 (Jul 27, 2014)

Well, which one runs stronger? Those Makitas sure are nice looking saws. 

Nice muffler mod, btw. I did mine too.


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## Poleman (Jul 27, 2014)

Clint,
Nice MM....did the same exact way on saw I got from you!!!

Well they both run quite well. The BB is definatly a bigger wood saw while the 64cc saw would be a good all around saw. It has the power for a felling saw and the speed for a limber. It does has a little quicker throttle responce than the BB. Size maybe, porting on the smaller saw ....not sure the answer there. The 30/35 combo really woke up the MM saw....it was good before but now it if FANTASTIC!!! I really do think the carb mod will help a ported saw.....especially having bigger displacement!!!! Guess we'll find out!!


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## cgraham1 (Jul 27, 2014)

Poleman said:


> The 30/35 combo really woke up the MM saw....it was good before but now it is FANTASTIC!!! I really do think the carb mod will help a ported saw.....especially having bigger displacement!!!! Guess we'll find out!!


That's good to hear!!


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## Poleman (Jul 28, 2014)

Terry, I noticed that. Its almost like you dont need the H. On the smaller saw the H was just cracked to set at 13,450 top end. The BB saw it was opened slightly more. Your right about the low feeding the high side. That extra fuel down low gives these saws a very wide power band as it getting the fuel it is needing to create this.

On the BB365 it was rich from the get go. I know there is a lot more I could have done to get this better but I will make that chapter two


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## Terry Syd (Jul 28, 2014)

Rich, if you have another carb to try you might try taking the carb up in sizes in increments on the 6400. If you found that a 25/30 worked better for the 64cc engine, then it may give you an indication of what would work on a 71cc engine.

My gut feeling is the 30/40 would work better on the 365BB. Perhaps try that carb off the 84cc Makita and give it a go on the 365BB.

The low speed circuit can be made to be overly rich and still work well, but why use more fuel than you need to. On the other hand, if you get the low speed circuit a bit too rich and it dampens the flame front, you could advance the ignition timing to compensate and probably pick up a bit more power up in the higher revs.


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## LegDeLimber (Jul 28, 2014)

Has anyone spotted a drill bit index set that covers all the decimal, metric and letter sizes?
the seller "gadgetscollection" has a set of numbered and a set of metric sized that looks like a nice range of increments if put together.
http://www.ebay.com/itm/21pc-Micro-...356?pt=LH_DefaultDomain_0&hash=item3f1a93b0f4

http://www.ebay.com/itm/15pc-HSS-Me...321?pt=LH_DefaultDomain_0&hash=item416e1bc2b9

The old gut feeling makes me want that sort of small incremental stepping all the way to about 2.5 mm or so...
I've got some mower carbs that just might benefit from a lil tweaking.


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## Poleman (Jul 28, 2014)

Ahhhh, forgot about using a timing advance to help!!!!
I do need to get more carbs to go up in size in smaller increments. That way I can see what what size/combination would work best on give saw cc size. I know it woun't be fool proof but will establish a baseline so on a given saw size there would be a starting point. 

I guess I dont see how the 30/40 combo would be better on the 365BB....It was so overly rich with the 30/35.


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## Terry Syd (Jul 28, 2014)

I wonder what the jet size that was drilled in the back of 30/35 and the 30/40, that might give you an idea of what was happening. If the 30/35 had a bigger jet size, it could throw off your testing.

Was it overly rich down low, or was it overly rich up on top? Even then, because you have the overlap between the two circuits, the rich high speed circuit could have been feeding lower in the rev range.

Mate, you're probably going to end up with a dozen carbs all drilled differently to see what works best on various saws.LOL


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## LegDeLimber (Jul 28, 2014)

I think you guys brought up threaded jets, earlier.
I'm thinking mini-lathe and some brass filler rod to fab the jets.
Drill 'em and run a die over the O.D. then part it off....
Or if the holes are a bit big, just "crazy glue" the holes then re-ream them with
your smaller choice of bit after the glue dries.

and I've been sleeping in this chair....time for the bed i figure.


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## Terry Syd (Jul 28, 2014)

Never crazy glued them, but I have soldered them up and started over.


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## RyanTowry_81 (Jul 28, 2014)

So is the rpm of the 64cc saw the same at tuning as the 365bb? If it is then the increase I. Velocity should be proportional to the transfer stroke volume right? Maybe the 64cc saw has a more efficient transfer which moves more air per revolution causing it to handle more fuel than the 77cc?

Just a theory. Only way to really know would be to measure the intake velocity somehow. I wonder if you could run the saw at WOT with a small hand held wind meter in the place of an air filter and be accurate enough to know the difference.


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## Terry Syd (Jul 28, 2014)

In reference to different drilled carbs for saws that are the same displacement. It seems to me that if a saw was modded for a higher cutting speed and therefore peak torque would be sitting higher, then the modded saw might need a bit more flow in the low speed circuit so that when the two circuits significantly overlapped they would be overlapping at a higher speed.

Otherwise, you might end up with that 'dip' we saw on the graph.


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## Poleman (Jul 28, 2014)

Terry the said carb was way rich down low and when I did get it to clean up and run it was rich on the top end also. Just can't understand this as the saw runs SUPER with the Walbro HD-12 that was on it. On the other saw the Walbro was good but the modded carbs out performed it and added a lot of torque off idle threw the rpm range up to the top. I still think the Zama is a viable carb for the BB365 and the saw would be helped emencelly by it...I just don't have it there yet.
I know you have a 365 so I know it's possible and can be done.

Yes, and I'm prepared to have the corner on the Zama carb market......LOL


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## Terry Syd (Jul 28, 2014)

Poleman said:


> Terry the said carb was way rich down low and when I did get it to clean up and run it was rich on the top end also. Just can't understand this as the saw runs SUPER with the Walbro HD-12 that was on it. On the other saw the Walbro was good but the modded carbs out performed it and added a lot of torque off idle threw the rpm range up to the top. I still think the Zama is a viable carb for the BB365 and the saw would be helped emencelly by it...I just don't have it there yet.
> I know you have a 365 so I know it's possible and can be done.
> 
> Yes, and I'm prepared to have the corner on the Zama carb market......LOL




Bugger, I hate moments like that. When I was a pilot we called it 'being out of air speed, altitude and ideas at the same time'.

Something ain't right there. I was running my 365BB at 35/35 and it did everything well (except the idle adjustment, but that sent me looking for the slow taper idle screws, so it was a helpful learning experience). Maybe you'll have to sit back and have some 'Zen' time with the carb - (that sounds better than WTF?)

Good thing you are taking on the Zama carb market. The folks at Zama probably dread seeing another of my emails arrive. They probably leave my messages for when the 'new guy' fills in for the day, rationalising that it will be great OJT for the poor bloke. And, the folks at D&G started carrying the slow taper screws (even put a picture of it on the webpage) after all the hassle I gave them. I can't wait to see how you run them around in circles.


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## fordf150 (Jul 28, 2014)

You guys are what makes this site what it is. Thank you. Get this all figured out and I will send you your first paying job modding a carb for my 7900.


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## Poleman (Jul 28, 2014)

Pretty much have it figured out for a ported 6400/6401 and an 84cc BB. Just need a stock 7900 to see where and at what jet size is need for it. I'm thinking it will be close to the BB settings because of its performance!!!

I know where those saws are at but I need more test saws or need to aquire a stock 7900. Of course for testing and research!!!! Honest!!!!!

I think you'd be surprised at the power gains that come from this nodded carb. It starts off idle and carry's through the power band to the top. The saw is smooth and fun to run with such a wide power band. Power pretty much at any speed.


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## fordf150 (Jul 28, 2014)

I have a healthy stock 7900. Complete rebuild with oem parts. Little over 200 psi last I checked it only has about 8 to 12 tanks through it right now. If you get me some part numbers on zama parts I will make a contribution to this research. I am jealous of you guys getting to do this kind of work. I am barely able to keep up with the minor repairs. I've been trying to get to testing sound levels/power on different muffler mods for six months and don't see me getting to it any time soon. Jonsered rep just dropped me off a toy to play with for the next month


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## Poleman (Jul 28, 2014)

Hi Nate,

Ya I want to get a 7900...seen a couple on AS but had bad timing. I did get a nice BB I have been doing the carb mods on. That's been working good.

As far as parts I haven't really put anything in/on them. Mostly drill jets and possibly shim spring to tune off idle response. I have ordered some slow taper idle screws to see if I can get away from notching the throttle plate. I'll have to possibly get some of the spring that Terry mentioned. Other than that there's not a lot that goes into them. 

Any time you want one let me know....I will be happy to do one for you!!!! I really appreciate you getting me and sending the parts I need. Got the fuel lines and carb piece today!!

That's quite a toy!!!!! I like the Jonny's!!! I nodded a 2166 for a cutter and he loves it!!!!! I do too!! Has the best air filtration set up on the market.....air cleaners just doesn't get dirty!!

Rich


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## cgraham1 (Jul 29, 2014)

Poleman said:


> Ya I want to get a 7900...seen a couple on AS but had bad timing. I did get a nice BB I have been doing the carb mods on. That's been working good.
> 
> As far as parts I haven't really put anything in/on them. Mostly drill jets and possibly shim spring to tune off idle response. I have ordered some slow taper idle screws to see if I can get away from notching the throttle plate. I'll have to possibly get some of the spring that Terry mentioned. Other than that there's not a lot that goes into them.
> 
> Rich


Keep up the good work, Rich!


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## Terry Syd (Jul 29, 2014)

Rich, I got thinking about your 365BB and couldn't figure out why the carb didn't work. Then I had a flash, maybe it isn't the carb!

When you set up the 365BB did you relieve the area at the bottom of the transfers so that the transfers could get a good feed from the crankcase? I say that because many Big Bore jugs have transfer tunnel problems. If not, let me know. I've got an old 365BB jug and piston that I modded and I can take a few pictures to give you the idea of what needs to be done.

Maybe that will fix the problem. If not, fork if I know, you're on your own...


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## RyanTowry_81 (Jul 29, 2014)

Terry Syd said:


> Rich, I got thinking about your 365BB and couldn't figure out why the carb didn't work. Then I had a flash, maybe it isn't the carb!
> 
> When you set up the 365BB did you relieve the area at the bottom of the transfers so that the transfers could get a good feed from the crankcase? I say that because many Big Bore jugs have transfer tunnel problems. If not, let me know. I've got an old 365BB jug and piston that I modded and I can take a few pictures to give you the idea of what needs to be done.
> 
> Maybe that will fix the problem. If not, fork if I know, you're on your own...


If you want to take the pictures and not detail this thread you can always post them in mine. My 365BB is in the mail so hopefully porting is the next step after I receive it. Transfer volume was my first idea of why rich's carb was not acting like yours terry. I would assume transfer volume per revolution would be displacement times duration(as a percentage of one whole revolution) right?


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## Terry Syd (Jul 29, 2014)

Transfer volume has nothing to do with it, just like some guys go on about 'velocity' - it's 'flow' that is important. It's dark in Oz and I've had a few drinks of my homemade vodka, I'll post some pictures tomorrow with an explanation.


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## Poleman (Jul 29, 2014)

Please do because I'm clueless as to what the problem was other than jets being to large....

Time to step back and do some Zen contomplation on this......


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## Poleman (Jul 29, 2014)

Going to swing by saw shop and see if I can score another carb and start baby stepping it into usability. I know it will work just not sure why previous try didn't work. I'm thinking of the torque gains through the full rpm range it produced on the Dolmars!!!


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## Terry Syd (Jul 29, 2014)

Rich, here is what I did on the 365BB to make sure the transfers were fed. On the cylinder I took the side of the cylinder down to the level of the base gasket. The piston received a couple of holes on each side where the piston drops below the cylinder at BDC. I also trimmed under the piston and also flattened the side (so the flow doesn't have to go in so far in order to turn up towards the ports).

Max flow of the transfers occurs just before BDC. The BB piston comes down close to the crankshaft at BDC and there isn't much room under it for the mixture to flow.


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## Poleman (Jul 30, 2014)

Interesting Terry!!!! I'd love to run that saw!!!

Well, I was unable to get any more carbs to play with, so I could try some things. Oh and of course the slow taper idle screws are on back order....sure wanted to have that to send to Randy....but there's still a little time. Got my fingers crossed.

I do have one carb left...its a NOS that has never had gas in it. It had a double D hi screw but I changed that so nothing fancy will be needed. BTW...Thanks Makguy, he supplied me with some carb to play with. Hopefully he will receive his back and add some input to this tread.

This carb will be going to Randy (Mastermind) to be put on and tested on a Dolmar 7900 that is scheduled in September to be ported. I really want to see what this will do on a ported saw and how it compares with the stock carb. Using the same saw will be a good determiner I think!! I hope it does what I expect and not a disappointment.

Here are some pictures of what I use (per Terry's help and advice) so you all get ideas of what is being done. 










Here's what's uses... The Micro drill and Pin vice. The jet on the left is the idle jet, .25mm to .30mm. the one on the right is the auxiliary .25mm to .40mm. Just go slow and steady with not a lot of pressure...just feel. Check the needle arm height to make sure that's at factory setting. I would then put on a slow taper idle screw and test. Dial carb is as best as possible and make changes as needed. If rich off idle I would put a couple spacers under carb spring depending how bad it is. I had to this on the ported 6400 to get the quick throttle response back. I don't feel that will be an issue with the ported 7900.

Drilling these jets out feeds the low end off idle and adds to the torque of the saw all the way through the power band. Feeding that much fuel through these jets lessens the amount the Hi is opened. The settings are a little different than stock as the Hi may be almost closed but that's not a problem because the drilled jets are supplying the fuel needed but you can still adjust as need be.

The one thing I did omit was notching the throttle plate just slightly. I wanted to wait for the new screw before I did that to see if that was even required with it installed. If I don't get them I probably will notch it. The saw is getting more fuel at idle and needs more air also to balance things out. With this added fuel though the power increases(torque), the saw pulls better, doesn't bog in the cut. It may not turn any more RPM's but it maintains in the cut. The saw I was testing was turning 11,400 to 12,000 in the cut with a 24" bar buried in wet Pine.

Hope this wasn't to long.....I get kinda windy!!!!


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## Terry Syd (Jul 30, 2014)

Slow taper idle screws on back order.... I wonder is someone else jumped the gun and picked up YOUR idle screws. LOL

Maybe a few people that are going to try these mods should send in their order for the screws as soon as possible. It would be nice to have the screw/s on hand before tweaking the carb.


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## Poleman (Jul 30, 2014)

Thats what I'm hoping for Terry....was dissapointed when I recieved the back order notice. Yep everyones got the bug!!!! Lol

Sorry all about the pictures all over the page....


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## RyanTowry_81 (Jul 30, 2014)

Terry Syd said:


> Slow taper idle screws on back order.... I wonder is someone else jumped the gun and picked up YOUR idle screws. LOL
> 
> Maybe a few people that are going to try these mods should send in their order for the screws as soon as possible. It would be nice to have the screw/s on hand before tweaking the carb.


It wasn't me but now I know those are a long lead item I might just order a couple now so I can get my name on the list. Hope to have my carb by the end of the week


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## Terry Syd (Jul 31, 2014)

Rich, good write up, I think everyone can read that explanation with the pictures and figure it all out.

As long as you are on back order, you might tell them to add a couple of the springs - before they end up on back order!

EDIT: Rich, are these the washers you are using? - http://www.scalehardware.com/produc...-11/1-4-mm-machined-washer-brass-qty-25-p-252


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## Poleman (Jul 31, 2014)

That's a good idea Terry! I will contact them and get those also.

Yes those are the washers I'm using....the ones you suggested and that you use/have!!

Rich


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## fordf150 (Jul 31, 2014)

I ordered the screws yesterday from my distributor. In about 6 months they will show up and I will be able to send Rich my carb for modifications. By then he should have it all figured out


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## Poleman (Jul 31, 2014)

Oh.....don't say 6 months.... I needed them yesterday!!!!! Never heard that before have you??? Lol.

Yep. By that time I will be in mass production....anodized blue carb all polished up and shinny with a lightning bolt across it....chrome Hi & Low screws and also a chrome slow taper screw.....and find forget to get your custom braided fue!l line and impulse line with crime a & n fittings. No one will want to cover this all up behind an air cleaner!!!!! I' m thinking custom K&N's on a voleocity stack for a ram air effect.


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## fordf150 (Jul 31, 2014)

The press in 90 degree fuel fittings take 2 to 3 months to come everytime I order them so 6 months for these seems reasonable.


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## Gravedigger (Jul 31, 2014)

Poleman said:


> Oh.....don't say 6 months.... I needed them yesterday!!!!! Never heard that before have you??? Lol.
> 
> Yep. By that time I will be in mass production....anodized blue carb all polished up and shinny with a lightning bolt across it....chrome Hi & Low screws and also a chrome slow taper screw.....and find forget to get your custom braided fue!l line and impulse line with crime a & n fittings. No one will want to cover this all up behind an air cleaner!!!!! I' m thinking custom K&N's on a voleocity stack for a ram air effect.


I'm in for the works once available


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## Poleman (Jul 31, 2014)

AWSOME GRAVEDIGGER......YOU'LL have some major BLING going on that saw......then you can throw a BIG rooster tail at all your friends with chips from it!!!!! It will run as good as it looks!!!!!


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## cgraham1 (Jul 31, 2014)

3 weeks and my saw is headed to Mastermind. 

I don't need all that bling. I just want my saw to run at it's maximum potential.


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## Terry Syd (Jul 31, 2014)

You'll have to order the 'bare bones' model. No bling, looks like a stock carb - just install and go. It's made for commercial cutters that don't care what their equipment looks like, they just want it to WORK.


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## RyanTowry_81 (Jul 31, 2014)

Bare bones is good for the sleeper look too, especially with the 365 stickers still. Really surprise people what a "65cc" saw can do. Haha


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## Terry Syd (Aug 1, 2014)

You should see what a Chinese 365, otherwise known as the 'HH365 Gasoline Chainsaw' can do to their facial expressions, - what a hoot!

I've heard through the grapevine that the chainsaw racers in Oz have created a separate class for the Poleman Carb, just like when they designated rotary valve kart engines to a separate class. Instead of 77cc Unlimited, the Poleman Carb has to run in the 77AA class.


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## Poleman (Aug 1, 2014)

Well its looking like a second carb option will be needed......the sleeper!!!

I'm thinking distressed body, looking like its been cycled to long in an ultrasonic with to much heat and poor choice of solutions!!?? Then and metal parts will receive a rusting plate and chrome flecking. Lastly the carb will receive the leaky sawdust top coat making it look like its never been off the saw or a kit or maintaince EVER done to it!!!

Do you think this will fill the sleeper void??? Did I miss anything??


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## Terry Syd (Aug 1, 2014)

Sounds like the carb on my saw, and I didn't pay extra for the 'look'. You may have a hard sell on that line of carbs.


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## Poleman (Aug 1, 2014)

Lol.....but custom distressed!!!!!! For that barn find look. Look at the price women pay for a worn out pair of jeans...GOLDMINE!!!!! I understand guys are wired differently than women tho...

Just talking this through....most guys could do this themselves....ya, probably wouldn't have much demand...

Dreams dashed.......


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## Terry Syd (Aug 1, 2014)

More ideas. nmurph and I got in a discussion about a C3M on a 346. As I recall the 346 has a 15mm carb. It should also have the same .25/.25 jet combination. I understand that the 346 is considered a good limber, but down on torque for the big stuff compared to some other 50cc saws.

OK, what if a .30/.25 would bring the torque back up. Especially if someone used a 17mm body for the carb where the velocity though the carb would be reduced. If we established a 50cc jetting and we already have a 80cc jetting, then we could roughly compute where a 65cc and 71cc engine would need to be jetted.

I'll try to put a rough guide together using the area of the jets. It is only a starting point, but at least it is a place to start.


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## CR888 (Aug 1, 2014)

Terry this is a great thread! l wonder what you could do to a Solo 681 carb (walbro) for improvments....this thread has got me thinking. l like how you approach a carb modd. There is certainly lots that can be tweaked before boring out venturis. Some *** IMO never reaches its potential due to a compromised carb for epa or whatever reasons.


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## Terry Syd (Aug 1, 2014)

I've got the old Walbro HD-5 that came on my 029 sleeper. It is an emissions carb with two jets, the low speed circuit is limited to ONE .25mm jet. Even a stock Zama is double that flow in the low speed circuit.

I'm still tweaking the Zama that I put on the 029 and I need to go back into the engine to increase the timing on the transfers, but when I've got it where I want it, I'm going to mod the Walbro to see if I can get it as good or better than the Zama.

I'll do a thread for the Walbro folks when I do.


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## Poleman (Aug 2, 2014)

Well....worked on the Zama that will go to Randy today...little biwillfferent than the others. This had a double D Hi screw, than I noticed these carb had a hole drilled in the throttle plate. I measured it at .80mm. 

Drilled the jets to 30/40, double checked needle height and tested this way. Started easily but was running rich and needed more air. It did have idle adjustment the way it was. I then opened up the throttle plate hole to .90mm and retested.....better. One more try this time to 1.00mm. This seemed to do the trick for adjustability and engine performance. The throttle was a little slow now so I tryed a shim and a spring change....nope not it. Left the shim and put original spring back in....that's more like it!!!!

The saw was still a little rich but I figure after Randy gets finished doing his port work on the 7900 it should be real close...or that's my intentions. This carb has a lot of top end to work with!!

Looking to make these as painless as possible to put on a saw and go!! Hopefully I will be getting some feedback soon as to how accurate settings are and power results.


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## fordf150 (Aug 2, 2014)

Sounds like your getting the carbs figured out. Now it's time to move on to learning how to anodize them. Never tried it but supposedly easy to do and no special stuff required for something like a carb


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## RyanTowry_81 (Aug 3, 2014)

Rich, have you looked for anywhere else to get that part number idle screw? I did some searching on my lunch break and found one site that had a list of carb models that needle came on. Seemed to be a lot of the c1a series if I remember correctly. I have it book marked at work, I will have to share the link when I get back to work Monday.


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## Terry Syd (Aug 3, 2014)

Here's what I have for calculations on the area of the hole in the various size jets. I can't measure actual flow, so the area is the default measurement. Each drilled jet will have variations and imperfections from the hand drilled process that will affect flow, but these area figures at least create a starting place to try some variations.

.25mm = .049 (millimetre squared)
.30mm = .070
.35mm = .096
.40mm = .126
.45mm = .159 

The stock Zama has TWO .25mm jets. That equates to an area of .098.

The 30/40 combination is equivalent to .196

What you need to find is the optimum size for a 50cc saw. Then you can make a linear graph for various size engines. For example, let's say a 50cc saw needed a 25/30 combination. That combination equates to .049+.070=.119

The difference between the 50cc (.119) and the 80cc (.196) is .077

There is 30cc difference between the 50 and 80cc engine.

.077 divided by 30 gives - *.00257* for each cubic centimetre increase in displacement over the 50cc engine.

In other words, for a 71cc engine you would multiply 21 (the difference between the 50cc engine and the 71cc engine) times .00257, which gives .05397 (say .054)

Add .119 (the 50cc starting point) and the extra .054 (for the 71cc engine) and the result is .173

A .30mm idle jet gives .070 of area, so you need and extra .103 of area for the 71cc saw (.173-.070=.103)

The .35mm jet is the closest at .096

Thus, a 71cc engine would likely need a jetting of 30/35

It's a starting point...


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## RyanTowry_81 (Aug 3, 2014)

Unless you could find a .375mm bit and be at .110 in area and just a tad rich. Or use the .35 and not be dead steady and probably end up with more near a .36 hole. Lol

I do like the idea of the graph of what sizes work for what CC saws. You could even come up with a algorithm for the Fred line and just have an equation to plug in cc's and get your jet sizes out.


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## Terry Syd (Aug 3, 2014)

It's a starting point. We don't know what a 50cc saw needs (let alone a ported one) and the line may not be linear.

Rich needs to get a 50cc saw and see what it needs, then work out a tentative table and see if it works.

Thankfully, there are other various size drills available which will give him some more options for when the size falls between the metric sizes.


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## Poleman (Aug 3, 2014)

Ryan, I haven't checked on any other suppliers for the slow taper idle screw. Guess I never thought about another source.

I'd like to find a drill source that would be in increments to make smaller steps in drilling the jets to maybe bet a better tune. The slow taper screw is more of a bonus as altering the throttle plate archives the same results. End result I'm hoping to achieve is a nodded carb that can be tuned like a stock carb with the benefits of the larger jet size.

It would also be good to have a graph/chart that one could refer to to get jet sizes that work best for what particular saw and cc size. That would be a tuner/modders dream. 

Terry I like the idea of doing something with a 50 cc saw!!! That is a very popular sized saw with a lot of interest out there to increase power and performance. I do have some 50cc saws (still 026, Makita 520, still 028 and a Dolmar 5105). The 026, 028 and 5105 have all been nodded and run quite well and would be fun to play with.


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## fordf150 (Aug 3, 2014)

Poleman said:


> Ryan, I haven't checked on any other suppliers for the slow taper idle screw. Guess I never thought about another source.
> 
> I'd like to find a drill source that would be in increments to make smaller steps in drilling the jets to maybe bet a better tune. The slow taper screw is more of a bonus as altering the throttle plate archives the same results. End result I'm hoping to achieve is a nodded carb that can be tuned like a stock carb with the benefits of the larger jet size.
> 
> ...



Soon as screws show up here I will let you know and send a couple out if your still in need of them.


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## Terry Syd (Aug 3, 2014)

Rich, you won't get the huge gains you got on the BB, but you may be able to add a bit of torque to make the saw easier to use. I don't suppose any of those saws use a Zama do they?

It looks like a 346 would be a good choice if one became available. Perhaps someone has a spare that they might loan you for some free tweaking. I'd suggest putting a 17mm carb on it (I understand the 357 carb is a modder's option), since you are trying to work out a graph/chart for the 17mm Zama.


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## Poleman (Aug 3, 2014)

I think the Dolmar has a Zama but not sure of model. I do have a 350 Husky that has a BB top on it but had the EPA dished piston in it so it was no where near its potential. Been waiting for a 353 piston for threes week on this but it looks like a C3M. I'll get a look at this to make sure. I like these saw as I built one that was a 346 eater. I was never impressed with the 346 and sold mine.

If its set up the same it would be easy to work with.


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## cgraham1 (Aug 3, 2014)

Poleman said:


> Well....worked on the Zama that will go to Randy today...little biwillfferent than the others. This had a double D Hi screw, than I noticed these carb had a hole drilled in the throttle plate. I measured it at .80mm.
> 
> Drilled the jets to 30/40, double checked needle height and tested this way. Started easily but was running rich and needed more air. It did have idle adjustment the way it was. I then opened up the throttle plate hole to .90mm and retested.....better. One more try this time to 1.00mm. This seemed to do the trick for adjustability and engine performance. The throttle was a little slow now so I tryed a shim and a spring change....nope not it. Left the shim and put original spring back in....that's more like it!!!!
> 
> ...


opcorn:


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## fordf150 (Aug 3, 2014)

5105 has a C1Q-DM13B


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## Poleman (Aug 3, 2014)

Clint,

You want me to send you the carb so you can play with it before you send you saw?? You could get some ideas and include it when you send your saw.

Rich


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## Terry Syd (Aug 3, 2014)

Poleman said:


> I think the Dolmar has a Zama but not sure of model. I do have a 350 Husky that has a BB top on it but had the EPA dished piston in it so it was no where near its potential. Been waiting for a 353 piston for threes week on this but it looks like a C3M. I'll get a look at this to make sure. I like these saw as I built one that was a 346 eater. I was never impressed with the 346 and sold mine.
> 
> If its set up the same it would be easy to work with.



Now that would be a great 50cc saw to work with, put in a flat top piston, intake timing to 160 and one of your 17mm carbs. I'd expect that to be a really nice little work saw with a very broad powerband.


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## Poleman (Aug 3, 2014)

Thats kinda what I was thinking, Terry. I think it has everything needed to be able to play with and see what can be made out of it!!! I know what it can do without the carb change....so this could get very interesting with a 17mm carb.


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## Terry Syd (Aug 3, 2014)

I'll bet once you got the 50cc saw worked out, then it would be even easier to dial in a carb for the 372. Mate, you're gonna have the whole range worked out in no time!

Like Ford150 said - "Now it's time to move on to learning how to anodize them."


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## Poleman (Aug 3, 2014)

Lol.....kinda seemed everyone liked the sneaky look better!!??


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## cgraham1 (Aug 10, 2014)

Two more weeks... and it's off to Tennessee... 

How's that Makita doin', Rich? I just missed another one on eBay.


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## cgraham1 (Aug 10, 2014)

Poleman said:


> Lol.....kinda seemed everyone liked the sneaky look better!!??


Hard to see the carb under all that plastic... What you need is a custom sticker! That's all anyone seems to care about around here... stickers. Just remember, it's gotta look good next to the Mastermind Worksaws sticker.


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## Poleman (Aug 10, 2014)

Clint, I think you right....no one would see the cool anodized carb with the lightning bolt....lol!! The sticker is a way better idea!!!!!
7900 is good!!!! I'm like you will looking for a stock 7900!! 

Been waiting for more carbs.....I ordered some of the cheap Chinese ones to play with....don't have the funds for OEM carb as there about $100 each.

I've been playing with different ideas as to modifications and to keep the stock adjustability....or near stock. Still waiting on the slow taper idle screws also.

Since I've had time I have looked at other carbs and their function. I'm thinking any carb can be modified to supply more fuel increasing performance. I've played with a WT, Q1C and C3 so far. I will need to do a follow up and test to see results. Yet to come will be the HD's and possible older Tillotson. Just kinda playing with what I have.

Just need some nonpartisan feedback and evaluations to see where and what weeknesses are and make corrections and improvements.

Rich


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## psuiewalsh (Aug 10, 2014)

Poleman said:


> Clint, I think you right....no one would see the cool anodized carb with the lightning bolt....lol!! The sticker is a way better idea!!!!!
> 7900 is good!!!! I'm like you will looking for a stock 7900!!
> 
> Been waiting for more carbs.....I ordered some of the cheap Chinese ones to play with....don't have the funds for OEM carb as there about $100 each.
> ...




Can you turn the idle screw on a lathe to get a slower taper?
Would powder coat be an alternative to anodizing? You can get small kits of it reasonably? Or Glock 37 can be a good resource.


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## Poleman (Aug 10, 2014)

I don't think a lathe would work as the taper would need to start earlier and have more area of adjustment. You could probably make one in a lathe...if I had one.....or a possible screw on overlay on original screw.

Carb finish....not sure there. Like Clint said no one really sees just knows its there because of performance. Anodizing would be cool, I think powder coating would be best to but tedious to do to make sure none of the wrong things would get coated or covered up. I've thought of parkerizing like an assault weapon since I can do that already. I guess the main thing to me is make it distinctive and have jet sized embossed on it for recognizeability.

Before all that though I want easy tunability, so if I send a carb out that's been modded that anyone can pretty much get it tuned for their particular saw. I don't think most guys here on AS would have a problem because if your ordering/getting a performance carb you already have an understanding of how a saw works and of modifying them. I think the term is "GEAR HEAD"!!

Rich


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## Terry Syd (Aug 10, 2014)

Rich, I'm going to pull the idle jet on one of my carbs and drill one of the air bleed holes larger. I'll have a play with the size of the holes and see if all the tuning issues get resolved. You wouldn't need a slow taper idle screw and it may give a more stable idle mixture.

Now if Zama had made these carbs with removable jets, drilling out the air bleed holes would be easy...


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## Terry Syd (Aug 10, 2014)

OK, getting the idle jet out isn't very hard if you have the right tool and procedure. I just drilled the first (closest to engine) air bleed hole out from .5mm to .55mm.

I have a diagonal cutters that I had modified for trimming the sprues off my cast lead .62 ball rounds. I had ground the bottom FLAT so that the sprues were cut off flush with the ball.

Initially, I tried to simply pull the jet out by clamping the cutters on the jet - bad idea. The diagonal cutters slipped and clipped the top edges of the jet off.

I then took a small Dremel bit (for engraving) and trimmed around the jet to give the cutters more room and then cut a notch for the cutters to sit it. Worked great, I just twisted the jet a bit as I pulled on it and out it came.

The underside of the jet is unusual. It has a recess area to create a chamber for mixing. I then measured the existing hole in the carb body (.5mm) and went one size larger (.55mm).

I put the carb down on a block of wood and tapped the jet back in. Later, after some trial and error and when I find the size hole I like, I'll use a bit of GB Weld around the jet to make a permanent fix.

I started the engine and found I had about another 1/4 turn of adjustment. The idle appears to be a bit more stable, but I'll have to wait until I go out wood cutting to see how it works with the engine good and hot.


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## Poleman (Aug 11, 2014)

Terry,

What carb is this that your doing this to?? I mean what are the jet sizes drilled too? Just trying to wrap my mind around what the changes did to what carb?

This would be a great fix by making the air bleed hole larger increasing air as needed. I will be interested in results after/during saw use. It would also make the carb more like stock and with less alternate mods to offset increasing the jet size and increased fuel. This would also allow the mixing to be more natural in the carb process as normal, probably making it more stable as you have described.

Rich


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## Terry Syd (Aug 11, 2014)

This is another approach/experiment to modding the Zama I decided to try on my 029 'Sleeper' (390 jug, NK bar). I blocked off the auxiliary port and I'm only using the idle jet. I went too far with the twin jet configuration, so I blocked off the auxiliary and started over, LOL.

There may be an element of idle stability with the consistency of the idle emulsion. I would expect with the size of the idle passages there is 'sweet spot' in the sizes of the air bleeds and the idle jet that gives the most consistent emulsion for the idle circuit. It's just a theory at this point, but I think it is worth looking into.

Did I mention I'm kinda finicky about carburetion?

EDIT: Rich, you could pull the idle jet on one carb and try it. Just go up one jet size on that first transition hole and you will see a difference. I have a feeling that this carb will go up another size for the next trial.


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## mdavlee (Aug 11, 2014)

Lots of good info in here. I'm getting ready to work on a 7900 and trying to decide if a carb swap is in order.


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## Terry Syd (Aug 11, 2014)

See if you can get Rich to build you a carb. At this point most people haven't caught on, he'd probably do you one on the cheap just to build his reputation. He's already got the feel for the low end torque and throttle response mods. If he gets the idle cleaned up (just a matter of time) then there won't be anything out there that will touch it.


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## Poleman (Aug 11, 2014)

Terry, I sure would if I had carbs to work with. Tryed dealers in area and no one had any used ones. I did order some of the knock off ones as the OEM are running around $100 bucks...ouch!!

So my question is...do you find the knock offs to be pretty good and adjust like the Name Brand ones? Maybe just a little rougher around the edges?

I sure wouldn't say no to doing carbs that were sent to me that I would send back when finished. Just waiting to hear back from the ones I've done to see how they worked after I sent them back. It's easy for me to change something I have in hand but harder to tweek it if its not in my possession. I dont want frustrated and angry saw owners because they can't get it adjusted. Thats why I run and test each one so it runs GOOD on MY saw....the only variable is each saw is different so I try to build in adjustability for the slight tweeking it may need.

Bottom line is I just want a BIG ol Smile when the trigger is pulled and a yeehaw for a ported saw!!!! LOL 

Rich


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## Terry Syd (Aug 11, 2014)

Well, if you check with Randy he said he used to throw them in a bin. Sounds like there should be a heap of carbs just sitting waiting to be modded.....

The Chinese Zama copies that I had access to were fine. The main discharge nozzle sat too far into the venturi, but I fixed that by repositioning the nozzle and trimming the back of it so the metering lever didn't hit it. All the Zama parts were a direct fit on the copy. If you could get those at a decent price, even if you had to fix the main discharge nozzle position, they would be good value.

EDIT: The Chinese copies I have aren't named Zama. The name on the carb is AN(A)BA. The A in brackets is actually their logo. Maybe you can find out a contact for them and order them direct. Maybe order with a shorter discharge nozzle, maybe drilled different, Heck, you might even get them to anodize them for you.


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## Terry Syd (Aug 12, 2014)

Rich, looks like Huztl is selling the Zama copies. They have several on offer (including 5 carbs at a time) on Ebay. Just put 'Husquvarna 372 carburettor' in the search function on Ebay. I've looked at the pictures and viewing down the throat of the carb it looks like the Zama twin jet.

You might buy one from Huztl and check it out. If you can work with the carb, then you can contact them for a wholesale price, they mention they do wholesale on the webpage. You might be able to pick them up for $10 a piece. Hmm, a replacement Zama is going for $100 - you could undercut that price with a modded carb, say $90.

Here's a link to give you an idea about what the carbs look like. There is no name on the carb so it is obviously a copy.

http://www.ebay.com.au/itm/Husqvarn...269?pt=LH_DefaultDomain_0&hash=item43c3cd6b2d


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## Poleman (Aug 12, 2014)

Yep, thats where there coming from....had to find an alternative to the OEM. OUCH!!! Lotta wiggle room...LOL.

I will be interested in how they are. I've used a couple aftermarket carbs before and was pleasantly suprised....both wallet and performance!!!

Rich


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## Terry Syd (Aug 12, 2014)

When I was looking at the components in the Chinese carbs, there was little difference to the factory Zamas. If you had a copy and an original opened up side by side, you wouldn't know which one was a copy. However, there may be some slight difference in the air bleed hole configuration under the idle jet, but not enough to affect performance.

On the Walbro copies I found a big difference in the machining of the air bleed holes. The Walbro HD used a different drill angle on the forward hole where it would come out on the edge of the venturi. The copy used the same 90 degree angle that was used on the first air bleed hole and wouldn't have the progressiveness as the original Walbro.

If the Huztl carbs are a good copy, then using them should be fine. A person could get a modded carb with all new components (needle, diaphragms, etc.)


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## fordf150 (Aug 12, 2014)

I checked today about those slow taper screws. no ETA on them yet


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## Molecule (Aug 12, 2014)

the carb has to breathe into the crankcase volume, and the crankcase volume has to exhale through the inlet ports -- IMHO it might be that the cfm of the carb actually needs to match the crankcase volume -- the crankcase pulls a vacuum which then draws the carb -- it only has so much volume available for each cycle -- if the carb throat (venturni) is too large for the crankcase volume, then you will lose power. people put ovesize carbs on motorcycles and they turn into dogs, switch back to smaller carbs with high draw velocity, and they haul.

the low note burble sound ("4-cycling" as it's called) is rich at WOT with no load -- to set the carbs stick it in a good piece of wood to set fuel mixture at full load. As you put it in the wood you immediately hear a phase change in sound, as it goes from burbling (missing) to singing wild and lovely high notes, like some invisible high gear.

great saws


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## Terry Syd (Aug 13, 2014)

Love that 'ignore' function, they just disappear...


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## Poleman (Aug 13, 2014)

Not changing CFM's...just adding more usable fuel to same flow. Speed should stay the same on intake just a richer mixture thus adding more power/torque threw out the RPM range. Hi RPM dosen"t
change just usable increase of power at same RPM. A ported saw would use this and an increased RPM range if it had an unlimited coil...not so on the 7900 with its limited coil. If set at 13,500 the speed in the cut will increase with added power....

Did I say that right??!!


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## ncfarmboy (Aug 13, 2014)

Both of the AM carbs I've used dialed right up w/no problems. Don't remember brand on Tillotson HS copy. Just did the MS290 Zama HD knockoff ($14.00)last week runs great adj. right up. Did notice had to lean low from 1 turn just a little to get it to run right. 
Shep


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## Poleman (Aug 13, 2014)

Thanks....thats good to know and will be very helpful!! Glad they work good. May have to get a baseline before any mods.


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## Poleman (Aug 15, 2014)

Got the A M carbs yesterday....look really GOOD. They are in a box with "Farmer Tec" on them. Didn't do a super inspection but first appearance they look to be of equal quality to an original Zama. Put the Zama embossed cover on and it would be hard for anyone to tell the difference.

I will get pic's and take a look inside to see if any different. Casting and finish on outside is really good....interested to see how they work now. Looks like a 365 BB project this weekend to see if I can get that to perform as it should with a modded carb.


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## Terry Syd (Aug 15, 2014)

Rich, I'm going to start on modding the Walbro HD. It looks like they may have some potential. Here's a thread that opened up that idea - http://www.arboristsite.com/community/threads/need-help-with-ms361-carb-mod-some-other-s.261493/

I'll see what the Walbro can do and write it up. It may be that you will be providing modded carbs to the Walbro folks as well.


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## Poleman (Aug 15, 2014)

Awsome Terry, 

That will be a very good carb to mod!!!!

I WILL be following!!!


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## Poleman (Aug 15, 2014)

Well...heres some pic's of the Farmer Tec carb. For the most part it looks good. It apears a little less finished on the inside but still good.

I guess the test will be performance and not looks.


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## ncfarmboy (Aug 16, 2014)

Thanks guy's this is the stuff I look for and really appreciate the time & effort to do the work and post it.
Shep


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## mdavlee (Aug 16, 2014)

Poleman said:


> Well...heres some pic's of the Farmer Tec carb. For the most part it looks good. It apears a little less finished on the inside but still good.
> 
> I guess the test will be performance and not looks.



Is this going on the 7900?


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## Poleman (Aug 16, 2014)

It could. I have all my saws equipped with modded carbs and trying to get a stock 7900 to test as I have A Mastermind ported 6401 and a 84cc b
Big bore I,'ve been using. I'd like to see if there is any difference. 

I have sent a couple carbs off but no feedback because of more important family health issues and I have one going to Randy to put on a 7900 he will be porting. I think that will give a ton of information and feedback....I expect Randy to put it through its paces and tell me how it is!!

Actually, I was going to start setting this carb up for a 365-372 BB. Mine has a Walbro and I want to see if I can get the same gains out of it as I did the Domars!! The second carb may go on a Husky BB 350 to see what happens there.

The only difference this carb has from the Dolmar carbs is the size of the choke lever attachment....its a little larger for a 
Husky.( see last photo) but that's no big issue.

So until I get a 7900 I will be using these on whatever I can to get results, but yes I would sure use one on a 7900!!!!!

Sorry, long drawn out answer..,..just letting all know what my thoughts are and where I'm going.


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## mdavlee (Aug 16, 2014)

Poleman said:


> It could. I have all my saws equipped with modded carbs and trying to get a stock 7900 to test as I have A Mastermind ported 6401 and a 84cc b
> Big bore I,'ve been using. I'd like to see if there is any difference.
> 
> I have sent a couple carbs off but no feedback because of more important family health issues and I have one going to Randy to put on a 7900 he will be porting. I think that will give a ton of information and feedback....I expect Randy to put it through its paces and tell me how it is!!
> ...



If you want one tested on a 7900 I'm game. I've got one that's going to be worked over this next week or two.


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## cgraham1 (Aug 16, 2014)

Poleman said:


> I have one going to Randy to put on a 7900 he will be porting. I think that will give a ton of information and feedback....I expect Randy to put it through its paces and tell me how it is!!!


My saw will be shipped out next week...


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## fordf150 (Aug 16, 2014)

cgraham1 said:


> My saw will be shipped out next week...



ring show up yet?


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## cgraham1 (Aug 16, 2014)

fordf150 said:


> ring show up yet?


Maybe today? I haven't been to the PO. I'm sure it will be there Monday. Thanks!


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## fordf150 (Aug 16, 2014)

cgraham1 said:


> Maybe today? I haven't been to the PO. I'm sure it will be there Monday. Thanks!


just checked. ready for pickup at the PO. I guess paying the extra for 3 day priority was worth it.


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## cgraham1 (Aug 20, 2014)

cgraham1 said:


> ... my 7900 is headed your way!





Mastermind said:


> I have a carb for it already here waiting.





Mastermind said:


> I wanna see if we get the gains from the carb I'm thinking we will......




I got the ring! It's shipped to Tn. With the saw... This should be interesting!


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## psuiewalsh (Aug 20, 2014)

need a new master monkey videoopcorn:


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## mdavlee (Aug 20, 2014)

Is this a modded stock carb?


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## cgraham1 (Aug 20, 2014)

mdavlee said:


> Is this a modded stock carb?


Yep. 


Poleman said:


> Well....worked on the Zama that will go to Randy today...little bit different than the others. This had a double D Hi screw, than I noticed these carb had a hole drilled in the throttle plate. I measured it at .80mm.
> 
> Drilled the jets to 30/40, double checked needle height and tested this way. Started easily but was running rich and needed more air. It did have idle adjustment the way it was. I then opened up the throttle plate hole to .90mm and retested.....better. One more try this time to 1.00mm. This seemed to do the trick for adjustability and engine performance. The throttle was a little slow now so I tryed a shim and a spring change....nope not it. Left the shim and put original spring back in....that's more like it!!!!
> 
> ...


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## Poleman (Aug 20, 2014)

Drum roll please.........
.............in your Tallented hands Randy!!!!!!


Rich


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## Poleman (Aug 20, 2014)

I think this will have a lot of interest.......hope it's not a disappointment.......


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## cgraham1 (Aug 28, 2014)

Poleman said:


> I think this will have a lot of interest.......hope it's not a disappointment.......


My saw was delivered to Mastermind today! 
Now... We wait. opcorn:


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## Fly By Night (Aug 31, 2014)

I'd like to try these mods on my 7900 do you guys have a feeling for what affects altitude has? I live at 6,500 feet and usually am higher when I'm running my saw.


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## fastLeo151 (Aug 31, 2014)

You can just put a 395 carb on it.... I have a ported 6401 and the 395 feeds it fine.


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## Poleman (Aug 31, 2014)

I think I would start at .30/.35 and see how the saw runs at your altitude. You will be giving the saw a lot more fuel and after modding the carb the air flow is an issue. The throttle plate will need a notch/hole to give it air at idle.....just don't go to big. Also this changes the H setting as the saw is getting more fuel through the idle circuit. I m not sure if at 6000 ft you can get enough air through so saw doesn't run way to rich. I'm at 4000 ft and can use 35/40 drill size.

I think this will work for you, just go slow to see how the first mod works after that its a 10 minute change to increase to larger size.

I have still to pull the idle jet and drill larger air bleed hole in carb as Terry has suggested to alleviate the need for a slow taper idle screw and/or throttle plate alteration.

You will definatly see/feel a lower power increase at this stage!! This mod is all about torque.....saw wont run faster just have a LOT more usable power in the same power curve and hold RPM in the cut!! I also feel this would help feed a ported saw in its added fuel requirement's giving it what it needs from its alterations also. Only limit is the limited coil on the 6400-7900 Dolmars. It may not run past 13,500 but may be able to run close to that in the cut......who needs it to run more RPM's if it can do that!!!!!


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## Poleman (Aug 31, 2014)

Leo,is that a direct bolt on??


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## fastLeo151 (Aug 31, 2014)

You have to do a little fab work. But nothing serious


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## Poleman (Aug 31, 2014)

Interesting.....another carb that will fit!!!!

Does this carb help the upper or lower RPM range? I would think it would loose a little crispness on throttle response but bet great after that!!??


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## fastLeo151 (Aug 31, 2014)

Its wearing a mastermind figerported cylinder so I don't know....


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## Poleman (Aug 31, 2014)

Oh WOW!!!!! A REAL SCREAMER!!!!!!! Finger Porting.......in a league of its own!!!!!


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## Terry Syd (Aug 31, 2014)

Fly By Night said:


> I'd like to try these mods on my 7900 do you guys have a feeling for what affects altitude has? I live at 6,500 feet and usually am higher when I'm running my saw.



For that altitude I'd expect that pulling the idle jet and drilling the first air bleed hole to .55 would be a better option for getting a nice clean reliable idle. The modded carb is already rich at idle and requires some fiddling with the throttle plate to get it cleaned up. With the extra altitude, the richness becomes worse. With the drilled air bleed hole the carb adjusts easily with the stock idle screw.


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## fastLeo151 (Aug 31, 2014)

Terry Syd said:


> For that altitude I'd expect that pulling the idle jet and drilling the first air bleed hole to .55 would be a better option for getting a nice clean reliable idle. The modded carb is already rich at idle and requires some fiddling with the throttle plate to get it cleaned up. With the extra altitude, the richness becomes worse. With the drilled air bleed hole the carb adjusts easily with the stock idle screw.


Do you have any pics of this?


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## Terry Syd (Aug 31, 2014)

I'm going to do a carb for my mate's 372. It will have a 30/35 combination and the drilled air bleed/transition hole. I'll get some pictures of what you need to 'pull' the idle jet. It is really more of a 'twist' than a 'pull', you have to get the idle jet twisting back and forth and then it just pops out.


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## Poleman (Aug 31, 2014)

Terrys right....not hard to get out once you get a hold of it, and it is a twisting motion.

Heres some pic's....in the garage now....lol. Sat on my back side all day now ready to do something.....you guys REALLY get me MOTIVATED!!!!

I did have to releave the area to get a grip...little Dremel work is all. The two orfises under the jet....drill out the one closeset to the edge reinstall jet.

Hope these help..


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## fordf150 (Aug 31, 2014)

fastLeo151 said:


> You have to do a little fab work. But nothing serious


Your hilarious! a little fab work to get the carb on....then a lot of fab work to get an air cleaner on it. but please don't post pics of our custom adapter. not until we get around to making one from aluminum and tig welding it


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## fordf150 (Aug 31, 2014)

Have you guys gotten the slow taper screws yet? I checked Thursday and they still haven't came in at my distributor.


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## fastLeo151 (Aug 31, 2014)

It wasn't that bad...


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## Poleman (Aug 31, 2014)

Nope have gotten any notices or received them yet.....thinking, with Terry's help we've (he) found a way around it and at the same time keeping the carb more stock adjustable. Terry said he like the way this works out better and no need for the slow taper screw!

Just did a carb I will try on a BB365 tomorrow. Went 30/35 with the idle hole opened to .55 under the jet. This will be a starting point for this saw.

I have a 7900 coming in Tuesday I traded my new 2166 Jred for (ouch....this trade hurt) but I needed a stock 7900 to work with as my saws I've been using are a MM 6401 and a BB 6401. I want to see if there are any differences. Besides I'm REALLY LIKING these saws!!!!!!! Slowly trading my Huskys and Stihls off for them....LOL


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## Fly By Night (Aug 31, 2014)

Terry Syd said:


> For that altitude I'd expect that pulling the idle jet and drilling the first air bleed hole to .55 would be a better option for getting a nice clean reliable idle. The modded carb is already rich at idle and requires some fiddling with the throttle plate to get it cleaned up. With the extra altitude, the richness becomes worse. With the drilled air bleed hole the carb adjusts easily with the stock idle screw.


. I'll give that a shot, I take it the tapered screw that opens the butterfly doesn't go far enough in to provide the air needed? 


Poleman said:


> I think I would start at .30/.35 and see how the saw runs at your altitude. You will be giving the saw a lot more fuel and after modding the carb the air flow is an issue. The throttle plate will need a notch/hole to give it air at idle.....just don't go to big. Also this changes the H setting as the saw is getting more fuel through the idle circuit. I m not sure if at 6000 ft you can get enough air through so saw doesn't run way to rich. I'm at 4000 ft and can use 35/40 drill size.
> 
> I think this will work for you, just go slow to see how the first mod works after that its a 10 minute change to increase to larger size.
> 
> ...



Thanks for the suggestion, I'm ordering my bits tonight! If I go to big I can epoxy the holes and try again correct? 

Great info guys, once this all gets figured out I think a new thread should be started with all of the details in the first post so one doesn't need piece together info from hundreds of posts. 


Sent from my iPhone using Tapatalk


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## Poleman (Aug 31, 2014)

Correct. And if you make the idle circuit mod under the Idle jet you will have adjustability without modding the throttle plate to offset the rich idle condition. 30/35 will give you a VERY good start. .30/40 will be even richer and a little trickier to dial in. The H will be almost closed.

Just some things to consider.

Ya, I understand it gets hard to follow...lol


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## fordf150 (Aug 31, 2014)

Poleman said:


> I have a 7900 coming in Tuesday I traded my new 2166 Jred for (ouch....this trade hurt) but I needed a stock 7900 to work with as my saws I've been using are a MM 6401 and a BB 6401. I want to see if there are any differences. Besides I'm REALLY LIKING these saws!!!!!!! Slowly trading my Huskys and Stihls off for them....LOL


 
Red elm is glad you didn't get the one in the trading post. He said it is clean. 
Only problem switching to dolmar is your parts supplier is 800 miles away and we all know how bad dolmar dealers are at stocking parts.


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## cgraham1 (Sep 1, 2014)

Damn Dolmar dealers!


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## Fly By Night (Sep 1, 2014)

Dolmar dealers don't have any stocking requirements, Stihl dealers have to keep something like 20k worth of parts on hand 


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## Terry Syd (Sep 1, 2014)

" I'll give that a shot, I take it the tapered screw that opens the butterfly doesn't go far enough in to provide the air needed?"

The tapered screw that opens the butterfly isn't the screw we have been discussing. We've been discussing the LOW SPEED MIXTURE screw. The idle adjust screw for the butterfly remains the same.

NOTE: Rich has noticed a membrane under the idle jet that I hadn't noticed. It may be the two Chinese carbs I worked on did not have the membrane, or I failed to spot it. That membrane may work like the membrane in the discharge nozzle to prevent backflow of air at idle.

Until we get it figured out as to whether or not you need the membrane, leave it alone - and when you drill the idle jet BE CAREFUL when you finally get through the brass jet not to ram the drill through the membrane.


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## mdavlee (Sep 1, 2014)

Fly By Night said:


> Dolmar dealers don't have any stocking requirements, Stihl dealers have to keep something like 20k worth of parts on hand
> 
> 
> Sent from my iPhone using Tapatalk


 
95% don't keep that stock in saw parts. If I get a 7900 again to mess with I'll mod the carb first on it.


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## fordf150 (Sep 1, 2014)

Fly By Night said:


> Dolmar dealers don't have any stocking requirements, Stihl dealers have to keep something like 20k worth of parts on hand
> 
> 
> Sent from my iPhone using Tapatalk



If that were true I wouldn't have any stihl dealers near me. One stihl dealer only keeps 2-4 saws on the shelf.


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## Poleman (Sep 1, 2014)

Ya....made me almost cry when the party wasn't interested in a trade.....looked like an AWSOME saw!!!!! Red Elm did have me in mind when he got it.....I didn't know that.....and made another deal.Important thing is I think I got a GOOD one coming.

Ya guys I have to be serious about my Dolmar parts man....he may live 800 miles away but the service is FANTASTIC!!!! I have Clint to thank for hooking me up!!!!!

And yes.....I'm slowly in the process of getting rid of the Stilhs and Huskys I have. I have been totally captivated by the Dolmar!!!!


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## Fly By Night (Sep 1, 2014)

Terry, what is the idea behind cutting the butterfly instead of opening it up with the screw? 


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## Poleman (Sep 1, 2014)

After you drill the jets the saw/carb is in a rich condition and needs more air. You've increased the fuel to the saw for added performance and power and you also need more air.

Where the problem is is at Idle because of more fuel...now you needmore air for the saw to idle. The idle screw doesn't allow that much adjustment thus the notch in the throttle plate or better yet pull the jet and drill out the first idle hole to .55. This gives more air through the carb and idle adjustment returns.


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## Poleman (Sep 1, 2014)

Well....just got the BB365 in some wood. I'm impressed with the 30/35 combo and like the idle hole drilled to .55!!!! Very easy to adjust and REALLY RUNS well. I was cutting Pine with 24" bar and could dog in hard and couldn't slow it down...didn't get a tack on it in the cut but it was good and held speed well.

NOTE:I had found a membrain under the jet last night and wasn't expecting to see that. I drilled jet as usuall and it didn't seem to cause any I'll effects, saw idles has excellent throttle response and is FAST in the cut. The carb I modded was a Farmer Tec from a Chinese supplyer.

Rich


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## RyanTowry_81 (Sep 1, 2014)

You are making me excited to get to mine Rich! 

Glad to hear it worked out well. 
What elevation are you at again?


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## Poleman (Sep 1, 2014)

I'm at about 4000 ft where I live just out of town. Work areas are can be 5000+!!

I will have to play a little more with this on the Husky but I'm REALLY happy with the way it runs at preasant!! Hate to go backwards and mess up a good thing!!

It IS a FUN thing!!!


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## Fly By Night (Sep 1, 2014)

Has anyone ordered drill bits for this yet? The ones Terry posted a link to on Ebay ship from the UK and they will not ship them to me here in the US.


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## Poleman (Sep 1, 2014)

Just look on eBay under micro drill bits.....just make sure you get a pin vice.....those drills art tiny. I also drill by hand so I can feel the action....it doesn't take long. I think using a dremel or something you may risk breaking a bit. Also be sure you measure the bits as they may be mixed up on delivery. You want to make sure of the drill size you are using to drill the jets.


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## RyanTowry_81 (Sep 1, 2014)

Poleman said:


> I'm at about 4000 ft where I live just out of town. Work areas are can be 5000+!!
> 
> I will have to play a little more with this on the Husky but I'm REALLY happy with the way it runs at preasant!! Hate to go backwards and mess up a good thing!!
> 
> It IS a FUN thing!!!


Sweet. That is close to where I am, at about 3000, but cut up to 6000. 

Can't wait to play with the zama I have, just have to do the BB kit first.


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## Poleman (Sep 1, 2014)

Well here's what you'll need to make these carb mods....




Next you'll need to identify the idle jet and the auxiliary jet.



This is what I used to pull the jet. Just filed down the edge to make it thinner to be able to grab the jet. Grab firmly and twist...it comes right out. Using this I was able to remove the jet with out doing any grinding to the carb body.

That's about it....45 minutes later (taking pictures also) I had a modded carb.

Hope this is helpful!!!

Rich


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## Terry Syd (Sep 1, 2014)

I like your choice of tool for pulling the idle jet better than the diagonal cutters I have been using. If I was just starting out, that would be the tool I'd get for pulling the idle jet. Just trim the bottom of the cutters flat to get low on the jet and you're good to go.

The only tool I'd add to the list is a set of callipers for measuring the drills. You NEED to measure the drills each time you pick one up to use. It is way too easy to get the wrong size.

There are some micro drill offers on Ebay that have various pin holders. Some of them are much better quality than the one that Rich and I are using.

If you take a look at the picture of the inside of the metering chamber, you will notice a hole (jet) on the back of the main discharge nozzle. That hole provides fuel to the main circuit in parallel with the High speed screw. When you bump up the flow from the low speed circuit the amount of adjustment on the High speed needle gets less and less. On one carb I did, I ended up right on the edge of closing the High speed screw completely closed. I then put a dab of GB Weld on the back of the nozzle to close off the jet, that gave me plenty of adjustment on the High speed screw.


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## Poleman (Sep 1, 2014)

Looking at my post I see about half of it was missing.....oops.

Terry is right...you need a caliper to make sure the drill your using is the right size. Plus if your a clutz like me you'll tip them over a time or two or three. I did right after the first drill under the jet and had to sort them all....again.

I think knowing you can alter the H setting by blocking the second orfis will help in over drilling and still getting good results.


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## fordf150 (Sep 1, 2014)

You guys are way above my pay grade on this stuff but if I keep reading I may just learn something


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## Terry Syd (Sep 2, 2014)

Rich, I pulled the idle jet on one of my carbs, and yep, there was the membrane. It also had a small hole in it from when I drilled the jet. It doesn't seem to affect the idle, as I was saying, I let it idle for 40-50 minutes while I lapped in the clutch and it never missed a beat.

The hole in the membrane was very small, in fact I couldn't see a distinct hole. Perhaps when I pushed through the membrane it wasn't cut but simply pushed through and then closed back up without removing any material.

The Zama engineers obviously know a lot more about that circuit and decided to install the membrane. Perhaps when we drill the idle jet we just need to be careful about pushing through when that last bit of the brass jet gets drilled out.


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## CR888 (Sep 2, 2014)

lts really good what everyone is doing in this thread. l like the idea of making a carb more usable and adding more power throughout the rpm range. Terry what is your thoughts of the walbro carb fitted to a Solo 681? Do you think l could basically do the same thing you guys have been doing to the Zammas? Cheers Colin.


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## Terry Syd (Sep 2, 2014)

I should receive a Walbro for a 460, probably this week. I think it has some potential and will be modding it. I'll explain the concepts as I go and the concepts can be applied to any of the HD series. I can start a separate thread or just do it on this thread and keep all the carb tuning ideas on one thread for future reference.


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## Poleman (Sep 2, 2014)

Your right Terry.... I have seen no ill effects from drilling throught the membrane on the carb I just did. It idled and ran good!!!

If your like me you'd still like to know its purpose, but I think your right in thinking it's to stop bleed back.


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## Mastermind (Sep 2, 2014)

Just now catching up guys. Yesterday was a holiday, so I used it for some other stuff. I'll be getting into Clint's saw this morning.


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## cgraham1 (Sep 2, 2014)

Mastermind said:


> Just now catching up guys. Yesterday was a holiday, so I used it for some other stuff. I'll be getting into Clint's saw this morning.


opcorn:


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## Fly By Night (Sep 2, 2014)

Your buddy there just broke through my screen, you owe me a new one Clint! 


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## cgraham1 (Sep 2, 2014)

Sorry. All my money is tied up in saws!


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## RyanTowry_81 (Sep 2, 2014)

Terry Syd said:


> Rich, I pulled the idle jet on one of my carbs, and yep, there was the membrane. It also had a small hole in it from when I drilled the jet. It doesn't seem to affect the idle, as I was saying, I let it idle for 40-50 minutes while I lapped in the clutch and it never missed a beat.
> 
> The hole in the membrane was very small, in fact I couldn't see a distinct hole. Perhaps when I pushed through the membrane it wasn't cut but simply pushed through and then closed back up without removing any material.
> 
> The Zama engineers obviously know a lot more about that circuit and decided to install the membrane. Perhaps when we drill the idle jet we just need to be careful about pushing through when that last bit of the brass jet gets drilled out.


 
Terry do you think that membrane could be a gasket to ensure the brass jet seals when pressed in place?


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## Poleman (Sep 2, 2014)

Dont think so......this is what the underside looks like. Membrane is in middle surrounded by brass.


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## CR888 (Sep 2, 2014)

Terry Syd said:


> I should receive a Walbro for a 460, probably this week. I think it has some potential and will be modding it. I'll explain the concepts as I go and the concepts can be applied to any of the HD series. I can start a separate thread or just do it on this thread and keep all the carb tuning ideas on one thread for future reference.


That will give me a little time to order some tools and be ready, l am sure whether l get good results or not my understanding of the inner workings of a carburettor will greatly improve. The OEM carbs in oz can get pretty pricey which is kinda a shame for practice/research purposes. lts threads like this that make AS.


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## CR888 (Sep 3, 2014)

So been looking for my carb modd tools and this is what l think l need; micro drill set 10pce 0.1mm-1.0mm, micro hand drill & pin vice. There seems to be many options on ebay, is there any brands/kits to look out for or ones to stay away from? On a side note Randy just did a build thread on a 3120xp and turned out a H needle on his lathe.....pretty kool!


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## fastLeo151 (Sep 3, 2014)

Good question


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## Poleman (Sep 4, 2014)

Have only purchased one set of Micro drills and thats the one pictured. They work really good and I havent had any trouble and have used them on several carbs. Be careful using a drill...I just use the pin vice and drill by hand for the feel. You'll see what I mean when you get the drills....there really small. I didn't want to break one off in the carb.

Another word of caution... I was modding a knock of carb and couldn't get it right. Upon pulling the jet I discovered the membrane had come loose and was blocking a couple of the openings in the carb. Be careful drilling these carbs...I drilled the jet by feel and quit when I just got through the brass....apperantly I dislodged the membrane at that time. The first one I did I drilled through it not knowing it was there.

Just a word of caution,

Rich


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## weedkilla (Sep 4, 2014)

Without going through this thread with a fine tooth comb again I can't seem to find the relevant venturi sizes of the carbs in this discussion. Anyone care to enlighten me? Is there anything else that should be considered when guessing air flow capacity? (I don't have access to a flow bench atm)


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## Fly By Night (Sep 4, 2014)

Does anyone have a link to a suitable kit in eBay? I've searched eBay and amazon and can't quite find anything that's just what's needed for this. 


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## CR888 (Sep 5, 2014)

Just put 'micro drill' into ebays search engine. These drills are typically used in jewelery.....there should be heaps.


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## ncfarmboy (Sep 5, 2014)

weedkilla said:


> Without going through this thread with a fine tooth comb again I can't seem to find the relevant venturi sizes of the carbs in this discussion. Anyone care to enlighten me? Is there anything else that should be considered when guessing air flow capacity? (I don't have access to a flow bench atm)


Dolmar 6400-7900 Zama C3M-DM12=17mm
Husky 372 Walbro HD-12=17.76mm
Shep


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## SawTroll (Sep 5, 2014)

weedkilla said:


> Without going through this thread with a fine tooth comb again I can't seem to find the relevant venturi sizes of the carbs in this discussion. Anyone care to enlighten me? Is there anything else that should be considered when guessing air flow capacity? (I don't have access to a flow bench atm)



What carbs is the question about?


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## SawTroll (Sep 5, 2014)

ncfarmboy said:


> Dolmar 6400-7900 Zama C3M-DM12=17mm
> Husky 372 Walbro HD-12=17.*4*6mm
> Shep



Fixed the misprint (bold).


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## weedkilla (Sep 5, 2014)

Thanks - puts Terry's early posts in perspective for me.
I've got the answers I was looking for now!


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## fordf150 (Sep 5, 2014)

drills and pin vise looks like they have a pretty good selection but I don't know what all sizes you need. http://ehobbytools.com/contents/en-us/d8.html


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## cgraham1 (Sep 5, 2014)

cgraham1 said:


> Can't wait to run my 7900!





Mastermind said:


> It's all done......I wanna run it with the stock carb, then the modded carb.





cgraham1 said:


> Sweet! Rich (Poleman) and I can't wait to hear the verdict on the modified carb. I hope you're making videos...





Mastermind said:


> We'll try to get out a decent log over the next couple of days.


opcorn:


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## Poleman (Sep 5, 2014)

Me either....all the other feedback has been good so far.....now for the real test!!!


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## Mastermind (Sep 5, 2014)

Jon is taking off early in the morning with his family for the weekend. It might have to wait till he gets back.


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## cgraham1 (Sep 5, 2014)

It's just a saw... We can wait. I'm sure his family is more important! Besides, I got a new 288 to keep me occupied! 

Thanks Randy!


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## ncfarmboy (Sep 5, 2014)

SawTroll said:


> Fixed the misprint (bold).


 Thanks!
Shep


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## Poleman (Sep 6, 2014)

Well did the carb mod with the knock off and that turned out great!!!! One time shot....so I figured I'd start with the other knock off for the 7900 Dolmar.

Made the same mods 30/35 jets but drilled out their bleed hole to .60 instead of .55. I figured this saw could handle more air.

Couldn't get the saw right at these setting and couldn't figure out why.....I've done bigger jets for other saws and they worked fine.

Took the drilled idle jet back out and found the membrane had come out and was blocking a couple of the holes in the back. Took that out and replaced the jet and tried it again....worse. It was so rich both H & L were closed. Tryed a 1mm hole in the throttle plat better but still rich. Removed the jet again...JB Welded the .60 hole I had drilled and closed off the jet on the H. (Jet in the back on other side of needle). Re drilled bleed hole to .55, crossed my fingers and retryed........ Not sure what went on but the saw liked it!!!! Dialed right in, lot of adjustment all the way around. It even wanted more fuel on the H side!!!

I will run this like this and see if it holds and if I think more power can be obtained. I like the performance the way it is....pull good and excellent throttle response. Very smooth to operate and fun!!!!

Just another chapter....not sure what was causing the super rich condition but was able to reverse it and get a good mod that the saw liked!!!

Rich


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## Terry Syd (Sep 6, 2014)

Rich, good work on exploring what works and what doesn't. I've used that .55mm air bleed hole on both a 64 and 77cc engine and it seems to work well with a nice reliable idle. I'm going to continue using it for the time being.

As far as the High side tuning, once you block off the jet on the back of the discharge nozzle, the needle then has to provide all the fuel for the High side. The High speed needle may go from being closed to two turns out.

The various Zama carbs (and their clones) have different size jets on the back of the discharge nozzle. You may be able to get by on a carb from one saw that has a smaller jet, but on another carb with a larger jet you will need to block it off. You won't know until you fire it up that you have to go back in and block the jet on the discharge nozzle.

Now that you have the High speed circuit turned down to where you can tune it, you may find that you can go up to 30/40 on the Low speed circuit and pick up some more low end torque.


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## Poleman (Sep 6, 2014)

Kinda what I'm hoping for Terry. I think there is room for improvement. I think I'll try .35 on the idle circuit and leave the auxiliary alone and see what that does. The one thing I like now is the crispness of the throttle now.
Still trying to decide if the membrane was the issue or the .60 hole instead of .55. I have a habit of doing to many things at once then can't figure out what, what change did...


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## Terry Syd (Sep 6, 2014)

The 35/35 works on my 365BB. It may be that the 35/35 works better than the 30/40, but I don't know. You would have to have two carbs to try back to back to see what combination works the best.

The .60 hole should have leaned out the idle even more than the .55 hole, so the extra richness you were experiencing was likely from the rich High speed circuit (which you have now adjusted by blocking off the nozzle jet). If you go to the 35/35 combination, you might drill out your GB Weld and try the .60 hole again. I'd like to find out if it does work better, I'd go back in and do mine.

Oh, and if you do try the 35/35 - wait until you see how the saw runs before you go back in and try the .60 hole - like you said, one thing at a time. As far as the 35/35 combination, when you open up the idle jet you may find you have to add a bit of pre-load (washer/s) to the metering spring to lean the throttle response back to where it was.

EDIT: The larger you go on the idle jet, the more fuel you will get in that first 1/4 of throttle. You will probably be OK on your 77cc saw, but on a smaller engine you may have to use the stronger spring I referred you to earlier in the thread. The stronger spring will allow a slower ramp in of the fuel to allow for the smaller displacement.


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## Poleman (Sep 6, 2014)

I will try that tomorrow.....I promise...1 Change at a time. I will do the idle jet to .35 and see then do the idle hole to .60.

I really thought I was at a point of no return when both Hi & Lo were all the in.....sure glad it came around. I really like it now!!!! Just want to see if I can get the killer torque and still stay the same in other aspects!!


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## Poleman (Sep 7, 2014)

Well tryed the .35 for the idle jet and that worked pretty good. Not really any changes too the saw. Throttle response seemeda little slower tho.

Next I did the air bleed hole to .60 from .55. Now needed some adjusting and again throttle response slowed. Best setting was with L about 1/4 turn open. I could adjust for more power in wood by giving it more H....seemed to really like the H opened up!! I was also running 1 washer under the spring.

This is a good setup but I think I prefer the 30/35/55+1 for its throttle response and quickness with still good pulling power in wood.

The .35/35/60+1 was good just a little slower response and a torque power curve.

Now it seem one can taylor for speed or power and any combo inbetween. Don't get me wrong when I say slower....its still seems faster than stock response.

I do feel the Big Bore 84cc Dolmar kit makes more torque than the stock 7900, I guess which it should. But neither are peaky, they both have a good power band. I could still have the bar buryed in a cut, stop it let the saw idle then throttle with no bog or hesitation...it would just start throwing chips again. Thats the kinda power/torque I like!!

This was all conducted with a 24" bar and Oregon 72lg chain.

Rich


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## cgraham1 (Sep 7, 2014)

So I must've missed it, Rich... Are you testing this on the 7900 or the Big Bore Makita, or both at the same time? 

I really appreciate your enthusiasm, but I think you may be addicted to modifying chainsaw carburetors. You should seek mental help before it's too late.


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## Poleman (Sep 7, 2014)

Clint....it is to late for me....I'm past help!!!

I'm working with a stock 7900...sorry if I didn't mention that. I've worked with the BB and have a baseline for it now I've got a stock 7900 thats in like new shape. I've also been working with the knock off carbs with this. From what I can tell there just as good as the Zama's. 
Some of the mods have somewhat changed and been made to make carbs more adjustable and more like a stock carb but only better!!!

Hope this clarity's things a little. I was kinda thinking out loud with my thoughts on the 7900 compared to the BB.

Rich


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## Terry Syd (Sep 9, 2014)

I just got the 460 carb and it is an 'emissions' carb. Crikey, the idle/low speed jet is only .25mm! Even on my 029 with the 64cc jug on it I will have to open it up to something like .40 or more.

I noticed in another thread there was discussion about the 461 having a butt-load of torque compared to the 460. The same thread mentioned that the 461 carb had a *bigger low speed jet* and a smaller high speed jet.

I'll have to play around with this carb a bit and see what the air bleed/transition holes need to be to get a good idle when I open up the low speed jet. Unfortunately, each time I go in to change the transition hole I will have to pull the welch plug.

The Walbro low speed circuit is much different than the Zama. On the Walbro emission carbs the low speed jet is a LIMITING jet to the low speed circuit (ie: emissions reduction). As far as modding this circuit, I could just drill out the jet to 1mm and use the low speed needle to set the flow in the low speed circuit (which I will probably do).

I also note that Walbro put a bit of material behind the main nozzle in order to reduce turbulence. With the larger venturi, the offset nozzle and the material behind it, this carb is likely to flow better than the Zama.

Although this is a Chinese knock off, the second air bleed hole is drilled up towards the back of the venturi so I won't have to be redrilling that bleed hole.

All in all, this looks like it is going to be a good carb, I just wish I had a selection of Walbro metering springs to play with.

I've been gone a few days, did Randy ever try Rich's carb on the ported 7900? If so, can someone provide me a link?


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## cgraham1 (Sep 9, 2014)

Terry Syd said:


> I've been gone a few days, did Randy ever try Rich's carb on the ported 7900? If so, can someone provide me a link?


I got an email from Randy a couple days ago saying the port work was done, but he hadn't had time to get to the carb testing quite yet. That's the last I heard from him.


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## Terry Syd (Sep 10, 2014)

I just drilled out the Walbro low speed jet to .35mm as an experiment to see how much this 64cc engine wants. I'll take it up in increments and maybe do some mods to the transition holes as I go. Maybe I'll learn something by taking it up in increments.

Oh, I made another observation about this Chinese knock-off of a Walbro - it has a diaphragm cover on it that has the word ZAMA cast on it, LOL!


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## SawTroll (Sep 10, 2014)

Have anyone looked into the possibility of putting the RWJ-4 of the 372xpXT (less divider) on the 7900?

It is possible on the 372 (non XT), 385 and 390 at least, and should be a nice upgrade with a 19mm+ venturi. You need some 385/390 parts to do it on the 372 though - and I don't know about the Dolmar...


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## mdavlee (Sep 10, 2014)

SawTroll said:


> Have anyone looked into the possibility of putting the RWJ-4 of the 372xpXT (less divider) on the 7900?
> 
> It is possible on the 372 (non XT), 385 and 390 at least, and should be a nice upgrade with a 19mm+ venturi. You need some 385/390 parts to do it on the 372 though - and I don't know about the Dolmar...


Yes I did it a few weeks ago.


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## SawTroll (Sep 10, 2014)

mdavlee said:


> Yes I did it a few weeks ago.
> View attachment 367738



Nice! 

Did you need any additional parts (or mods) to do it?


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## mdavlee (Sep 10, 2014)

There is a few tweaks to make to use the 372 boot. I'll try a 385/390 boot on the next one.


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## SawTroll (Sep 10, 2014)

mdavlee said:


> There is a few tweaks to make to use the 372 boot. I'll try a 385/390 boot on the next one.



I more or less expected something like that.


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## mdavlee (Sep 10, 2014)

SawTroll said:


> I more or less expected something like that.


Not a bad swap and it seemed to help. I'm beginning to think a lot of saws could use more carb.


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## SawTroll (Sep 10, 2014)

mdavlee said:


> Not a bad swap and it seemed to help. I'm beginning to think a lot of saws could use more carb.



I am sure you are right, I understand the carb size often are on the small side to keep the noise level down.


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## mdavlee (Sep 10, 2014)

SawTroll said:


> I am sure you are right, I understand the carb size often are on the small side to keep the noise level down.


That may be part of it. It would be nice if there were more carb choices in the 17 - 20 mm venturi size.


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## SawTroll (Sep 10, 2014)

mdavlee said:


> That may be part of it. It would be nice if there were more carb choices in the 17 - 20 mm venturi size.



Yes, it is of course not the only reason - and that would have been nice. One option is if course to open the 17.5mm carbs more, but you have to know what you can and cannot do.


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## ncfarmboy (Sep 10, 2014)

Builders have been doing that for a looong time. Esp 385 Tillotson HS296 for 372 carb upgrade before the RWJ4 came into being.
Shep


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## SawTroll (Sep 10, 2014)

ncfarmboy said:


> Builders have been doing that for a looong time. Esp 385 Tillotson HS296 for 372 carb upgrade before the RWJ4 came into being.
> Shep



Yep, but that Tilly has just a 17.5mm venturi - same as the HD-12.


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## Terry Syd (Sep 11, 2014)

I got a chance to fire up the 029 with the modded 460 carb and things are looking good. First off, the idle was fine and the throttle response was acceptable. The throttle valve was almost all the way closed, so I may have to put a bit of solder around the inside of the cutaway on the valve to reduce the airflow. I can then drill a hole (if needed) on the other side of the valve to lean out the idle.

I ended up with the High speed needle all the way in, so I will have to go back in and block off the auxiliary high speed jet. Thankfully this carb has a removable jet, so unscrewing it and blocking it won't be a big deal. I may use solder to block it off, that will give me the capability to drill the solder to a smaller jet size if the High speed needle ends up too far out.

I should note that the idle/low speed 'jet' appears to be thin copper (easy to drill, its not brass) . - OK, my gut impression is that I've seen this before in lots of EPA approved cars, trucks and motorcycles - an easily modified EPA restriction. It may be that the carb body is drilled proper for the 76cc 460, but they put in the restricting 'jet' to pass the tests. Being a thin copper piece, it allows the operator to pop out the restriction and run the carb the way the engineers designed it. 

I've got a few other things on my plate at the moment, but I thought you guys might want to know these first impressions. I won't be able to do some cutting with the saw for a week or more.


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## Terry Syd (Sep 15, 2014)

I just took the idle jet out to .4mm (then .7mm) and tried running it again. It required the same low speed needle setting. In other words, the drilling in the carb body is controlling the idle mixture. So, you can't just pull the idle jet, you also have to remove the welch plug and drill the transition hole.

I also drilled out the idle jet on a HD-5 I had and found that there was a 'membrane' behind it. OK, what is going on here?? Both the Zama and the Walbro 'emissions' carbs have a .25mm idle jet with a membrane valve behind it. I got a feeling that this .25 jet with valve is an EPA mandate. As far as it being functional, on the Walbro with the wet side of the carb on top, the valve is always off the seat. Anyway, with it all drilled out it works just like the earlier carbs that didn't have such a jet and valve. I don't know what it is for, but the carb works fine without the jet and valve.

Whereas the Zama has a separate idle circuit from the low speed circuit that doesn't affect the low speed circuit fuel flow, the Walbro is a *combined circuit*. With the Walbro, if you adjust the idle with the low speed screw, you are also adjusting the WHOLE low speed circuit fuel flow.

OK, to tune the carb what I'm going to do is find a log and keep opening the low speed needle and turning in the high speed needle until I find a combination that works good. I'll have to turn the idle screw up to keep the engine running and the chain will be turning. A bit dangerous, but I'll just have to be careful.

Once I find the combination I like, then I don't touch the mixture screws - that's the setting I want for cutting. I then have to go into the carb and open up the transition hole and maybe the throttle plate to get the saw to idle. Then, I may have to look at the pre-load on the metering spring to clean up the throttle response. - Wish I had a dyno to set the carb up, it would be a lot quicker and safer.

Now about those transition holes - these sizes are as close as I can get it using my drill set to measure the holes. They should be considered approximations, close, but not exact.

On the HD-5 which is used on the 54-64cc 029-039 series the holes are .6mm for the first hole and .5 for the second.
On a 044 carb (Zama, non twin jet) for a 70cc saw the first hole is .75mm and .6mm for the second.
On the 460 carb for a 76cc saw the first hole is .6mm and the second is .65mm.

Of the three carbs the 044 is the only non-emission carb. Admittedly the configuration of the carb is different (especially the venturi), but I'm only looking at the idle circuit bleed holes so it likely has some relevance to this discussion.

Using the 044 carb as a baseline for non-emission carbs we have a .75/.6 combination - for a *70cc engine*. Those are larger bleed holes than for the 460 carb at .6/.65 - on a 76cc engine. I would love to find out the size of the bleed holes on the 461 as the guys running a 460 might want to drill their carb to those specs.

Here's what's cool about putting the 460 carb on my 64cc saw, it looks like it is a perfect match .

HD-5 at .6/.5
460 at .6/.65 
044 at .75/.6

The 460 bleed holes are a bit smaller than the non-emissions 044 carb for a 70cc engine, which makes the 460 carb about right for my 64cc engine.

I'll get around to testing it and dialling it in, but I expect it to be close. Here's another cool thing, I can drill out the second bleed hole on the HD-5 to .65 and it should work like the 460 carb - which means that all the guys with 039/390s could drill out their carbs and not have to buy another carb.


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## billyj (Sep 18, 2014)

intresting reading, id love to try a modded carb on my 7901


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## Terry Syd (Sep 18, 2014)

I had a chance to check the transition/bleed holes on a WJ for a 385. There were three holes -

First - .83
Second - .77
Third - .63

That is a lot of air to add to the idle mixture, I expect that the low speed circuit flows a lot of fuel.

Those figures should give an indication that to mod a Walbro HD for a 77-84cc saw you will need some serious drilling for those transition holes. Perhaps even put in a third hole like the WJ.


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## mdavlee (Sep 18, 2014)

That's a lot more fuel needed for those. The last 385 and 390 I had come with tillotson carbs.


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## cgraham1 (Sep 18, 2014)

Just an update for everyone interested...

Mastermind has finished the porting on my 7900 and it is currently on it's way to Rich for some fine tuning.


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## Mastermind (Sep 18, 2014)

I'm interested. 

But does anyone care.......no.


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## mdavlee (Sep 18, 2014)

cgraham1 said:


> Just an update for everyone interested...
> 
> Mastermind has finished the porting on my 7900 and it is currently on it's way to Rich for some fine tuning.


Nice. I hope he can figure out a good fix on the stock carb for more power.


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## Poleman (Sep 18, 2014)

I DO TOO!!!

Have made some changes for the better since that carb was modded to make them more user friendly!!! With the way that one was modded ( Like the BB carbs) I know there is a happy medium. Have a good set for a stock 7900 that works well now to see what that does on Clints saw and go from there. I'm thinking it may be as easy as drilling the air bleed hole out under the idle jet to bring it to usability.

Rich


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## mdavlee (Sep 18, 2014)

Poleman said:


> I DO TOO!!!
> 
> Have made some changes for the better since that carb was modded to make them more user friendly!!! With the way that one was modded ( Like the BB carbs) I know there is a happy medium. Have a good set for a stock 7900 that works well now to see what that does on Clints saw and go from there. I'm thinking it may be as easy as drilling the air bleed hole out under the idle jet to bring it to usability.
> 
> Rich


That would be nice. I need to get a set of those micro drill bits.


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## Poleman (Sep 18, 2014)

Mike...Get a pair of magnifying glasses too when wou get the drills...it helps cuz there small.....


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## mdavlee (Sep 18, 2014)

Poleman said:


> Mike...Get a pair of magnifying glasses too when wou get the drills...it helps cuz there small.....


I got 20/15 vision. I need a set that will go up past .062" also. I got something I want to try on another carb.


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## Poleman (Sep 18, 2014)

Good!!! Then you'll have them after your done!!! I get the cross-eyed syndrom!! LOL

I think my sets run fro .o3omm to 1mm by half steps...another set is decimal which fills in sizes between some of the others of the metric set. Harbor Freight has some that start at .5 and go up...dirt cheap and work good also. If you happen to have one close by to stop and pick up. Just make sure you get a pin vice or if you have a Dremel to get the small collet set to hold it. There kinda like putting a hair in a drill chuck.


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## mdavlee (Sep 18, 2014)

Poleman said:


> Good!!! Then you'll have them after your done!!! I get the cross-eyed syndrom!! LOL
> 
> I think my sets run fro .o3omm to 1mm by half steps...another set is decimal which fills in sizes between some of the others of the metric set. Harbor Freight has some that start at .5 and go up...dirt cheap and work good also. If you happen to have one close by to stop and pick up. Just make sure you get a pin vice or if you have a Dremel to get the small collet set to hold it. There kinda like putting a hair in a drill chuck.


I found a 49 piece set on eBay. Should work fine. Up to 3mm.


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## Poleman (Sep 18, 2014)

AWSOME.....lots of them out there to do the job!!!


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## Terry Syd (Sep 18, 2014)

After thinking about the transition holes on the WJ and what it would take to mod a HD carb, I'd try using a 'torque port' (like the one Zama added to their carb) on the HD. I mentioned the concept in the Chinese Carburettor thread where I'd put in an extra jet to increase the low speed circuit fuel flow on a Walbro.

Adding air requires big holes to flow, but adding fuel is easy. When you have mixture ratios of 12.5 to 1 BY WEIGHT of air to fuel, it is easy to see that a little extra fuel flow goes a long way.

OK, say you take a 460 carb, drill out the idle jet and set up the idle. THEN, you drill a separate hole through the wet side into the carb body to a point at about 1/4-1/3 of the throttle movement. Take a dab of GB Weld (GBW) and smear it into the hole from the inside of the carb, put a piece of tape over the hole, turn the carb upside down and let the GBW set. Now you can easily drill a jet hole through the GBW to make a torque port.

Keep drilling out the port until you go too far, now you know the size of jet you want. Clean out all the GBW from the hole and reapply a fresh dab. Now drill the hole to your desired jet size.

Or, you could just get a Zama with the jets nicely laid out for you to work on.

EDIT: If the GBW tended to get eroded by the passage of the fuel, you could make a nylon jet for the port. I used some nylon jets for Mikunis years ago and they worked fine, you just couldn't apply much torque when tightening them.

Just take a bit of nylon fishing line and GBW it in the hole, then drill through the nylon.


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## fordf150 (Sep 22, 2014)

I was just getting ready to ship a couple parts out and ran across this. dolmar concrete saws...PC-7414 come with a walbro wj126 carb. 17.46mm Venturi. while searching for specs on this carb I ran across a metering spring assortment pack in an organizer. part # 400-606 for anyone that is interested.


Additional for anyone that doesn't know. The concrete saws share a lot of parts/ design with the chainsaw. PC 7414 is a 7300.


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## Terry Syd (Sep 22, 2014)

Using the WJ would likely increase the low speed circuit flow, at least it would be easy to increase. The problem is the WJ mounting holes sit at a 45 degree angle to the Zama/HD carbs. The WJ is also a bit longer than the HD. Somebody would have to take all the parts, manifold, aircleaner, choke and see how it fitted up.

Using a modified Zama on the 7900 looks like the way to go. It is easy to work on and capable of flowing more fuel than the 7900 can swallow. The low end torque increase has to be felt to be appreciated, it turns a good work saw into a great work saw.


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## fordf150 (Sep 22, 2014)

I didn't look or think about the mounting screws. I did think about the size difference between the carbs. When I get another carb back in stock I will take a few minutes to see how it will mount up. Likely similar to that 395 carb we fitted to the Makita BB. Custom adapter for the air filter. I know what needs made for that but don't have the tools needed.


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## Terry Syd (Sep 22, 2014)

If I was going to use a Walbro, I'd use the HD from the 460. I expect that it would be a lot easier to drill a torque port in the carb and incrementally step of the flow of the low speed circuit until you had the jetting you like. You would have the same size venturi as the WJ, but you would be able to fine tune the carb to where you wanted it. - And, you wouldn't have to go through all the mounting issues that using the WJ would create.

I suggest you try modding the Zama first (or get Rich to make you one). After you try the modded Zama, I think you'll find it does everything you want. I could get a Walbro and mod it for my 77cc BB, but the Zama on the saw already has enough power to run a 24" bar with 8-pin in Aussie hardwood. I can bog it if I take a really big bite using the dawgs, but I don't have a problem with bogging during normal cutting - and I tend to use the dawgs to keep the revs down and spit bigger chips.


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## Poleman (Sep 24, 2014)

Well just recieved Clint's saw straight from Randy..... Randy said it was a "BEAST" .....I dissagree.........It's a "MONSTER"!!!!! OMG!!!!! 

Looking at getting a carb set up for the port work on this saw and hopefully better guestemate on future ported saws. The carb I sent didn't perform like I had intended. Clint was kind enough to let me play with his saw on its trip back home...just a short vaca before going to work!!!

Have intentions of putting on a HD 10 carb to see where it stacks up....just because I'm interested in what it does. Then I will get into modifying a Zama to get the best performance I can for this saw. I have ideas of where to go and how to proceed from Randy's feedback. Also I want to correct the difficult tuning that Randy experianced from the carb I sent. First Gen..... Have some refinements that I believe will take care of this problem. Second will be as I said the performance, in given RPM range since it is limited. So we're looking at increasing torque!! I think and forgot saw was limited to a specific rpm range so I overdrilled the jets on the carb I sent. I will back up a step and see if that is closer...I know the tunability problem will be corrected.

I will give you results of what turns up!!! Clint, Thanks for the use of your saw, Randy for porting and the MAGIC work you did!!! All in the name of "RESEARCH" right!!?? I feel anyone with a Dolmar 7900 stock or otherwise will benifit. Because there there to be had!!

Rich


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## Mastermind (Sep 24, 2014)

Be sure to keep me in the loop. 

As you can see, I get big gains with the stock carb, so reworking the stock jetting has never been a priority in my mind. Make a better air pump and you get more airflow.....then you get the additional fuel that is needed by design. 

I'm still very interested in what gains you may find though.


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## Poleman (Sep 24, 2014)

I will most deffinatly!!! Your gains are amazing with the stock carb!!


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## psuiewalsh (Sep 24, 2014)

Rich, can you provide info on the carb sent to mdavlee for the other 7900build?


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## Poleman (Sep 24, 2014)

I will. The one I sent to Mike was jetted smaller than What I sent to Randy, it also had the air bleed drilled to clear up the tuning issues. I did run this carb and tune it on a stock 7900 and it ran good!! I will have to go back and check notes to make sure what I sent, I dont want to give wrong info.

Rich


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## Terry Syd (Sep 24, 2014)

Does Mike have a thread of the 7900 he is building?


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## mdavlee (Sep 24, 2014)

No thread yet. I haven't got started on it? I still need a 385 carb boot for the xtorq carb for the other one.


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## Terry Syd (Sep 24, 2014)

So, what are you doing? Building two 7900s, one with the modded Zama and the other with the Xtorq carb?


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## mdavlee (Sep 25, 2014)

Terry Syd said:


> So, what are you doing? Building two 7900s, one with the modded Zama and the other with the Xtorq carb?


Actually 3 but yes on the carbs on those 2. Mine will probably be the stock to compare with for now. It's going behind those 2 and one other saw. I just need a 390 carb boot to get here for the one swap so I don't have to rotate the carb and all will fit just like oem.


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## Terry Syd (Sep 25, 2014)

Cool, this will be a real world comparison of performance. I'll be putting my money on the Zama. Maybe the Xtorq carb would have bit more on top (it's 19mm venturi as I recall), but the Zama will likely have better torque down low. The Xtorq carb is after all an EPA approved carb for a 71cc engine so it may be lean on the low speed circuit.

If that proves to be the case, then you might try taking an air bleed hole on the Xtorq carb out a bit and making the low speed circuit a bit richer.

EDIT: Does the Xtorq carb have that damn .25mm low speed jet with the valve? You might be able to just ream it out.


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## mdavlee (Sep 25, 2014)

I don't know about the small low speed jet. Both saws will be the same with carbs being the only difference.


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## Poleman (Sep 25, 2014)

Well...I ran Clints saw straight from Randy......IMPRESSIVE TO SAY THE LEAST!!!!! Clint your gunna LOVE this BAD BOY!!!! I DO!!!!!

I did do the mod's I wanted to do on the carb I had sent that didn't work out. I thinkmit was 30/40 and was unadjustable and rich. I drilled out the air bleed hole and tryed that.....OMG the power this saw produced was scarey. It was now adjustable, had plenty of idle adjustment. Only issue was hot restart....it wanted throttle to be open!!??? Tweeked a little after that.
I put on a 35/35/55 AM carb that was to rich for a stocker and it seemed to like that!! I will get that in some wood tonight and see how close that is to the bigger carb??? It should be close and may go with it if I can't bet the rich/hot start resolved.

This is getting VERY INTERESTING...stock saws you can tell a differance but a ported saw....all I can say is you need to run it to know what I mean as I have NO words to describe how it is!!!

GOOD WORK, Randy on another AWSOME saw!!!! I wish you could run it with the BIG carb....I think it would make you blink!!!!


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## Poleman (Sep 25, 2014)

Also, I have some "TEST MULES" out there to get feedback on from these carbs. This will give me an idea of there tunability/adjustability and ease to use. I will get back treir results, finding, likes, dislikes ets. It will be a no BS tell it like it is report. Thanks for that guys!! What I find out only makes these better...and will/can be used by any one using this carb be it Dolmar or Husky. Some BIG results can be had!!

Rich


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## fordf150 (Sep 25, 2014)

I got one.....now to find time to install and run this thing...I substituted a sharpie for the blue anodizing


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## Poleman (Sep 25, 2014)

lol I thought about it!!!


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## fordf150 (Sep 25, 2014)

Almost sent the thing back to rich. Wasn't anodized and I didn't even get a sticker!


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## Poleman (Sep 25, 2014)

Proto type.......unmarked before production!!! Lol

Man, I gotta sharpen up to stay ahead of you.....

Hows this???


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## Poleman (Sep 25, 2014)

Hey Nate...this better?????

Clint......you have carb #00001!!!! Top of the line jetting for a top of the line saw!!!!

I like the little smaller carb, it was good but had a funky idle. The others I've done with that jet combo didn't have the idle issue. Bad carb I'm thinking. Was AM. carb.

Put the bigger carb on that had the hot start issue after making a couple changes.......WHALA!!!!! Problems solved!!! I was running and cutting and it was just under 95 degrees here today and restarted great!!! Hard to believe less than two weeks ago we had 6-12 inches of snow on the ground. 

All came togather good!! Think I will duplicate carb and see if they are consistentwith what I've done.

Clint...your really going to like the sound of your saw too!!!!! Randy ported and installed a huge muffler bearing!!!!!

Rich


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## Terry Syd (Sep 25, 2014)

Rich, I take it after your testing you prefer a .30mm idle jet rather than the .35mm - is that correct?


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## Poleman (Sep 25, 2014)

On most carb yes. The 35 seems to be to rich. I've done two carbs with 35/35 combo and it worked good but the second has been a headache. It runs good just has a really bad idle that I don't like. Have yet to check into it and try to find the problem. Leaky Welch plug??

That 35/35 deffinatly makes more power/torque but a little harder to get just right on a stock saw. It worked excellent on the ported saw I have the opportunity to test on and that's where I was going but the changes to the 30/40 I did changed that carb around and it deffinatly makes more power!!!

Still want to try the .6 air bleed again hoping I could use bigger jet sizes but after the first try with it I'm reluctant.
Have you tryed it with any good results??


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## cgraham1 (Sep 26, 2014)

Poleman said:


> Hey Nate...this better?????
> 
> Clint......you have carb #00001!!!! Top of the line jetting for a top of the line saw!!!!
> 
> ...


 Can't wait to run this against the 064...


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## Terry Syd (Sep 26, 2014)

Poleman said:


> That *35/35 deffinatly makes more power/torque* but a little harder to get just right on a stock saw. It worked excellent on the ported saw I have the opportunity to test on and that's where I was going but the *changes to the 30/40* I did changed that carb around and *it deffinatly makes more power!*!!



OK, that is a bit ambiguous, which carb makes the most power on the 7900?

No, I haven't tried the .6mm bleed hole. Heck, I'm just a firewood hack that doesn't get to cut that much, with all the other stuff on my plate I haven't cut anything in a month. Good thing we've got you to keep the ball rolling.

I still haven't tried the 460 carb on my 029 'Sleeper' saw. Hopefully, I will get a chance on Sunday afternoon.

I've thought about modding a 460 carb for the 77cc saw and put in a torque port, but there hasn't been much interest in modding carbs, probably only a handful of people have shown any interest.

I think the torque port would be the way to go on the Walbro. It is a straight forward mod that can be easily reversed with a dab of GBW (unlike drilling the bleed holes) and it would be easier and safer to tune than drilling the bleed holes. There would also be no issues with trying to get the idle right.


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## Poleman (Sep 26, 2014)

Terry, 

So far the 30/40 carb on Clints saw is tops. You have to consider the saw is ported also. Its a good combination!!

On a stock 7900 the 35/35 produces more power and its a good alternative to the larger carb. Little more finiky to tune.
The 30/30 is a good step up from stock of you hadnt ran any of the others and very easy to tune. It probably has the quickes throttle responce other than Clints saw.....it goes if you just think about it!!!

It does seem the bigger the jetting the harder to tune but I think that just a part of the beast!!!

Little things I try seem to work so far....just continue to think about the two 35/35 carbs that act so different. I will make up one more and see where it falls.


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## ncfarmboy (Sep 26, 2014)

Guess I need to post to let you know of my interest in these carb mods. This is the very best thread I've read in a loooong time here. Check in everyday to read results. I have 2-7900's and a 681 to do the carb mods to. Already bought micro drills just need the time to do one. I have HD12's on all three. Thank you Terry Syd, Poleman cgraham1, mdavalee and others for posting. Please don't quit. Learning a great deal about carburetion right here @ the school of CARB.
Thanks, Shep


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## mdavlee (Sep 26, 2014)

I'll be starting the tear down of the 7900s after a video or two tomorrow or Sunday. I have one 6400 to test against the 665 and then it will be torn down. I'll pull the carb off mine and pop the one he sent me and try it out.


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## Terry Syd (Sep 26, 2014)

Rich, remember what I said about running the larger idle jet and using one of the stronger metering springs. If you have a metering spring with a stiffer rate (thicker spring, more gap between the coils), it will give you a slower ramp for the fuel. You may be able to get better throttle response as it won't be feeding the fuel in as fast.

I'm using the stiffer springs in the two carbs I've drilled to .35.

I have to laugh at us getting so anal about the throttle response. In the real world it doesn't mean anything, but once you feel how quick you can get it, it seems you can't resist trying to get it as fast as you can.

EDIT: Rich, about those two different .35 carbs. There's two things to check that may be different - the metering spring and the size of the metering orifice. A smaller metering orifice will ramp in the fuel slower than the larger orifice. If one has a shorter or weaker metering spring, it can affect the idle and make it a bit richer. The pre-load on the spring is analogous to raising or lowering the float on an old style float bowl carburettor. If you drop the float, it takes just a little bit more 'suction' to pull up the fuel from the bowl (or in our case, move the needle off the seat).


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## Poleman (Sep 26, 2014)

Yep Terry....one gets hooked on Quick throttle response once you experience it!!!!! Especially when there's power behind it!!!!

Usually when first testing a new carb I try to set it up for quick response but with larger jets I know it slows it down but try to compensate on the first go. If its not there I know that's something I can improve. My main objective is to get carb to adjust and tune it to the saw and get an idea of what's left and where you can go with it, rich_lean so it's adjustable. Then more fine tuning starts if there's something I don't like.

The two carbs that are so different are different....ones a true Zama and the other is AM. I'm thinking maybe I just got a bad one. I did put togather another carb so I can check it out Sunday. Tomorrow were going to Custer State Park with the grand kids for the Buffalo Round Up then to a cabin with relatives near Custer!! Should be a good time...little hot for this time of year tho!!! Lol

Terry, one thing I've been doing on the AM carbs is to remove the membrane under the idle jet. The AM jet is open more compared to the Zama's. Once drilled I've had to go back in twice to remove it after it comes out and plugs the back two holes. So I just take them out. 

I still have the one carb I used JB weld on to block off the Hi jet on the wet side. I want to play with it more and get a better understanding of affects of blocking different bleed holes.


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## Terry Syd (Sep 26, 2014)

ncfarmboy said:


> I have 2-7900's and a 681 to do the carb mods to. Already bought micro drills just need the time to do one. I have HD12's on all three.



I've got an HD-5 off my 029 that I could do the mods on so that you would see how to do it on the HD-12 for the 7900. The Walbro HDs are basically the same so you can mod any of them for different applications.

Actually, I might use a cheap Chinese copy of a Walbro that I have. It at least has the right hardware for mounting on the Husky 365.


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## LegDeLimber (Sep 27, 2014)

A friend of mine ordered these for me right before my knee port & polish job. I'm just now getting around to taking a glance at them.
The blue boxes are duplicate ordered sets and I think the 2 smaller black pouches (left &right ) are also duplicates.
The larger black pouch (center) looks to cover the larger diameter drill range.
They were all from ebay sellers and I think the entire cost was under $35~ish USD.

Looked at a couple of the packaged as "Indian made" pin vices. The threads on them seem decent, actually look quite noticeably better than typical stuff in this price range
when marked as Chinese origin.
The collets will definitely have to be deburred though.
That one diagonal shot is a pin vice lain out in the order of disassembly exactly as received.

Haven't grabbed a magnifier and looked at the bits yet, rear of pouch says China.
Usual problem I find on low price drill bits is relief angle behind cutting edge is bad.
have even seen a few negative i.e. reversed ones, almost like it the grinder set up for a left hand twist but yet right hand helix to the fluting.
The next thing to check is for consistant diameter of fluted area from tip to shank region mainly and the rest of drill too.
Then how is the grinding quality are the flute edges sharp and the overall geometry clean etc. I guess You guys get the ideas that I'm blabbering over.

sorry that the pics aren't among my usual, better (I hope) looking efforts.
The knee job has had some distracting after issues , So I've slipped up and let all of my batteries get low and the camera gets sorta iffy to work with when the cells are on the edge of quitting. 

Keep up the postings folks!
hopefully I'll get back to tinkering here and maybe can add something to the pursuit.

Edit: of course it wont be the same saw by any means. 
My budgetary options are a very small sampling of the homeowner sort poulan and husky plus whatever redneckery I can inflict upon the weedbeaters.
...funny how much lower the cost runs for the carbs that are in the range for such lawn mania experiments.


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## Terry Syd (Sep 27, 2014)

That's a lot nicer set of drills and pin vices than I have. The cheap 'made in India' pin vice works, but you just have to be careful getting everything lined up.

I just drilled out a 'torque port' on my HD-5 carb. The Chinese clone of the Walbro wasn't exactly a 'clone'. So I drilled the real Walbro to give you guys an idea of what I'm on about. The pictures will be in the next post.


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## Terry Syd (Sep 27, 2014)

Here's a picture of where I drilled the torque port in the wet side of the carb. I used the edge of the relief for the metering pin as my vertical axis. The horizontal axis was measured from inside the carb where I wanted the hole and then that distance was transferred over to the wet side to coincide with the vertical axis. The distance from the edge of the carb was 12mm.


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## Terry Syd (Sep 27, 2014)

Here's a shot of the inside of the carb with the 1mm drill sticking through the hole. You'll see that it is located on this side of the venturi and will open after the last air bleed hole for the idle circuit is opened. I used a 1mm drill, but it could be too small for the GB Weld, you just have to try and see what works. The hole didn't go exactly vertical through the carb, but it still ended up in a good position. A small drill press would have been great for precision work instead of the big-ass hand drill that I used.


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## Terry Syd (Sep 27, 2014)

I should mention that I used a couple of small Dremel ball cutters to trim back the edge of the slot for the metering pin. After I had trimmed back the edge and got the ball down far enough in the carb, it provided a nice depression to start the drill in.


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## LegDeLimber (Sep 27, 2014)

Yeah I had wondered how well the collets will center in the tool body let alone repeat.
took a closer look at the black pouches and realized they are all one set- looks to range from .3mm up to 3.0mm From what I can remember*, this combination of sets Seemed to give the best spread of diameters for that amount of money. 
The blue cases are duplicates, ranging number 61 through 80 diam.
Seems like they were about $4~5 including shipping each.

*I'm sorta learning, the hard way, that each round of anesthesia appears to extract its toll from my gray matter faculties.
I have slowly come to hate hearing the words "unique" along with "that so rarely ever happens", when you're talking to a DR.
I cant seem write an entire paragraph or read and process a long one at the moment. lots of re-editing for now.


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## Terry Syd (Sep 27, 2014)

I don't go under anaesthesia if I can help it. I don't like someone else messing with my consciousness. The last time I had a big sebaceous cyst taken out of my back the doctor realised it should be done under general anaesthesia it was so deep. I just told him to go ahead and cut it out. I learned how to 'look at the pain' when I got shot in 'Nam and how to get the endorphins flowing. 

They can do a nerve block to your legs. If I eventually have a knee replacement, I'll have the nerve block and stay conscious.

Back to the carbs - I've also got a set of the 61-80 drills, it gives me more options for the size of holes I can drill.


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## Poleman (Sep 27, 2014)

Me Too! I also found a pretty good set at Harbor Freight...They have the collet installed and a colored rubber bishing on the shaft. They go down to .24mm, I've been using them to measure everything!!!

I have the same pin vice with the swival end whis is great but the smallesr collet is hollow and I've found it imposible for me to bet a bit centered...but thats me. It does work on 1mm and larger very well.I do all my drilling by hand'm afraid of breaking a bit off in a jet. Slow as you go....and it dosent take long.


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## ncfarmboy (Sep 27, 2014)

One porter I follow generally mods all the carbs on the ported saws he does. He pulls some massive hp & torque out of his saws. Working on a Walbro C1Q-DM14 from a 5100 Dolmar this AM. Thought I could swap throttle shafts etc. from a 55 Husky C1Q-EL7 both 13.5mm venturi everything matched up til I got to the choke shaft. Back to the drawing board.
Thanks Terry for showing Hd12. They are on for later today & tomorrow afternoon. Want to try and get one done for Mon as I have help on Mon.
Shep


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## Poleman (Sep 27, 2014)

Shep,

Your modding a C1Q??? I've looked at those as I have 2 5105's & a 510. Played with one of these but haven't put it on yet to see what happens. 
You'll have to keep us posted on this!!! A lot of saws use this carb. 

Wow...Zama C3M, Walbro HD and now C1Q mod's!!!! AWSOME!!!!!!!


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## Terry Syd (Sep 27, 2014)

ncfarmboy said:


> Thanks Terry for showing Hd12. They are on for later today & tomorrow afternoon. Want to try and get one done for Mon as I have help on Mon.
> Shep



Great stuff! Looking forward to what you come up with for jetting on the torque port. You might want to do two carbs and increment them up together. Say, one with .30mm and the other with .35mm - find out which works best, then mod the 'loser'. That way you always have an objective reference point to refer to. If you just mod one carb you end up with subjective evaluations that may or may not be correct based upon your memory of the previous mod.

If you don't like the torque port, just fill it in with GB Weld. I modded the HD-5 just to demonstrate the concept, however, I'm going to fill in the torque port and mod the bleed holes to run on a 64cc saw (duplicating the 460 carb). I figure a lot of guys with the 039-390 series would like a cheap and easy mod of their existing carb.


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## VINIFIREWOOD (Sep 29, 2014)

Not to derail this thread away from the Zama and the 7900
But
With all the discussion about the 660 and how much power can be had by a simple switch to a dual port muffler. A guy has to wonder about the carb on those also.


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## Terry Syd (Sep 29, 2014)

If the stock carb is an 'emissions' carb, then there should be a fair bit of power waiting to be released. Take a look in the carb and see if it has a low speed jet installed.


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## Derf (Nov 30, 2014)

I've read this whole thread and I'm interested in learning some more about carb porting.

For me specifically, I'm interested in seeing gains made with the HD12 since it has the horizontal (non-offset) bolt pattern that fits my 372xp (among other saws). My saw is a ported 75cc jug, and I'm going to be doing a muffler mod to help with the breathing. But after that, I'd really like to dial in the breathing through the carb.

As SawTroll eloquently put it in a post somewhere, 'the manufacturers are all playing games with us somewhat, on where to detune a saw to meet EPA requirements: either on the carb, the exhaust, or the porting.' I know that the 372XP x-torq came with a different carb than the 372XP pre-xtorq.

For the 372xp
The air filter boot to carb has a 23mm ID.
The rubber boot between the carb and the jug has a 22mm ID.

-------- Pre-Xtorq -------------- Xtorq
Carb : Walbro HD12 ---------- Tillotson RWJ-4
Carb intake : 22mm ------------ 22.5mm
Carb venturi : 17.46mm --------19.1mm
Carb exhaust : 20.5mm--------- 22.5mm

Quite a few people have explored putting the Xtorq carb onto the pre Xtorq saw for better flow.
It seems clear that these saws, especially ported, can benefit from some additional flow through the carb. I'm going to do a little experiment to see if I can bore out the HD12 to close to RWJ-4 dimensions, and would love some feedback from the crew here when it comes to tweaking the jets, and perhaps adding a torq-port.

I'd rather put the effort in to modify the HD12 than adopt the RWJ-4, because 1) even though it is possible with alternative mounting pieces from other saws to accommodate the offset bolt pattern, you run into some fitment issues with the HD filter and the choke lever sticking out a little far, and 2) I have access to a machine shop.


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## Terry Syd (Nov 30, 2014)

You may want to look at the 460 carb as it is drilled for a 76cc engine. All you have to do is drill out the low speed jet to get it in the ballpark for jetting. You will have to swap the shafts - and the impulse line goes to the lid, so you may have to run a longer impulse line or drill and GB Weld a pipe in the side of the carb for the impulse line.

I bought a cheap Chinese replica for around $20 on Ebay. I'm running it on a 64cc engine (Stihl 029/390). That's an easy swap for guys that want to upgrade their 64cc Stihls. Excellent idle and throttle response and good torque down low.


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## Derf (Nov 30, 2014)

Terry Syd said:


> You may want to look at the 460 carb as it is drilled for a 76cc engine. All you have to do is drill out the low speed jet to get it in the ballpark for jetting. You will have to swap the shafts - and the impulse line goes to the lid, so you may have to run a longer impulse line or drill and GB Weld a pipe in the side of the carb for the impulse line.
> 
> I bought a cheap Chinese replica for around $20 on Ebay. I'm running it on a 64cc engine (Stihl 029/390). That's an easy swap for guys that want to upgrade their 64cc Stihls. Excellent idle and throttle response and good torque down low.



Thanks for the advice, Terry.
I'm looking at this page from Walbro and they show four different carbs that went on the MS460. 
HD-8-318 (15.08mm venturi)
HD-9A (17.46mm venturi)
HD-16-318 (17.46mm venturi)
HD-24A (NLA)

I'm not that familiar with the history of Stihl saws very much, but I assume these changes had to do with EPA regulations changing. Which carb were you suggesting, and why is that better than modifying the HD12 if I have to make changes including JB welding an impulse pipe to the carb?


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## Terry Syd (Nov 30, 2014)

My carb is the 17.46mm venturi. I liked the 460 carb as it has a cleaner venturi than the earlier Walbros. The venturi is offset to the side and will likely flow better than the earlier ones. You are looking for more flow, so that seems like a better carb to start with.

The idle and throttle response should be spot on for your engine. The engine might like a bit more fuel flow on the low side for better low end torque. You can try a torque port and see if you pick up some more low end.

Although it wasn't necessary to block off the extra high speed jet, since I only had about 1/8 adjustment on the high speed needle, I blocked it off to make sure I could get more adjustment if I needed it. I ended up with about 2 turns out. I cut at 2,000-2,500 feet of altitude and I won't be cutting any higher, so I should be fine. You may have to do the same mod if you put in a torque port. The torque port will increase the fuel flow and you might not be able to lean out the high speed needle enough.


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## Derf (Dec 1, 2014)

Terry Syd said:


> My carb is the 17.46mm venturi. I liked the 460 carb as it has a cleaner venturi than the earlier Walbros. The venturi is offset to the side and will likely flow better than the earlier ones. You are looking for more flow, so that seems like a better carb to start with.
> 
> The idle and throttle response should be spot on for your engine. The engine might like a bit more fuel flow on the low side for better low end torque. You can try a torque port and see if you pick up some more low end.
> 
> Although it wasn't necessary to block off the extra high speed jet, since I only had about 1/8 adjustment on the high speed needle, I blocked it off to make sure I could get more adjustment if I needed it. I ended up with about 2 turns out. I cut at 2,000-2,500 feet of altitude and I won't be cutting any higher, so I should be fine. You may have to do the same mod if you put in a torque port. The torque port will increase the fuel flow and you might not be able to lean out the high speed needle enough.



If I can just clarify, when you say the "venturi is offset to the side" I assume you know the venturi is the smaller diameter of the barrel designed to reduce volume and increase flow rate. I assume you meant the main jet is offset in the venturi, and you are implying that it therefore allows better air flow through the venturi because there is less turbulence or because it blocks less air?

As for offering more flow for my saw, do you mean it offers more flow compared to the HD12? I'm not sure I would conclude that the offset main jet creates a significant difference in flow since they both have the same venturi diameter. I wish I had a flow bench to measure though.


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## Terry Syd (Dec 1, 2014)

Yeah, I meant that the main discharge nozzle was offset to the side. I think I discussed it in the thread why there would be less turbulence past the nozzle.

It would be great to have not only a flow bench handy, but a dyno to see what difference each carb and jetting made. I expect that some of differences would be easier to pick and then where the next mod should made. Maybe one day Chadiman will have some time to run a few different carbs on the same saw to see what the different powerbands look like.


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## hwrdpromac7900 (Dec 1, 2014)

Has anyone actually figured out how much more power you're going to get from the modded carb? Any videos?


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## Derf (Dec 1, 2014)

I don't know if anyone is really making calculations. If I was to hazard a (wild) guess:
+20% from porting the cylinder (better flow, higher compression = more HP)
+5% porting the muffler, and +8% if porting the muffler and the jug is already ported
+5% from porting the carb, and +7% if the muffler and jug are already ported


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## Terry Syd (Dec 1, 2014)

hwrdpromac800 said:


> Has anyone actually figured out how much more power you're going to get from the modded carb? Any videos?



If we're discussing just changing the jetting in the carb, then you won't see any increase in the peak horsepower. However, there is increased torque when the torque peak was formerly running lean. The resistance to bogging is also increased.

Rich discussed the effect in his posts where he could now stop the saw in the cut, pull the trigger and the saw would take right off.


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## Derf (Dec 2, 2014)

I agree with Terry that there is more torque, but since HP is the relation of torque to RPM, if the RPM is the same, and there is more torque, it stands to reason that there is more HP, no?

OK, I just bought into this carb-modding-voodoo-magic-hocus-pocus. Got a bunch of (old) toys coming from the 'bay.
Going to be making a bunch of measurements, modifications, and tests.

Also, Terry. I have an idea that a centered or offset main-jet may not be a make-or-break feature. If I lower the main jet in the bore so its opening is closer to the venturi wall, I think I will put the jet opening in a higher air velocity stream and have i) better laminar flow, ii) better vaporization/emusion of fuel.

Are these jets just press fit into the body? Have you ever removed one? It looks like there is a slot for a screwdriver in the back side, but I see no threads. [Speaking about a Walbro I'm looking at at the moment]


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## Terry Syd (Dec 2, 2014)

Derf said:


> I agree with Terry that there is more torque, but since HP is the relation of torque to RPM, if the RPM is the same, and there is more torque, it stands to reason that there is more HP, no?
> 
> Also, Terry. I have an idea that a centered or offset main-jet may not be a make-or-break feature. If I lower the main jet in the bore so its opening is closer to the venturi wall, I think I will put the jet opening in a higher air velocity stream and have i) better laminar flow, ii) better vaporization/emusion of fuel.
> 
> Are these jets just press fit into the body? Have you ever removed one? It looks like there is a slot for a screwdriver in the back side, but I see no threads. [Speaking about a Walbro I'm looking at at the moment]



There won't be anymore torque at peak horsepower (because you are already tuning the high speed needle to get the most power at that RPM), however as you move down the powerband towards peak torque, many of the carbs are going unto the lean side of the mixture curve, that's where you can fatten it up a bit and pick up more torque. Look back on the various posts in the thread to see some of the fuel mixture charts I referred to.

The idea of lowering the main jet may be a good one, I did that on a Zama to clean up the venturi. [There is also a diagram of laminar flow (I think it was in the Chinese carburettor thread) as it goes through the venturi.] Yes, the main discharge nozzle is a press fit and you can reposition it. On one of the Zamas I had to modify the discharge nozzle a bit to make sure it would still pick up fuel from the carb passages. Just measure where the entry hole is on the nozzle and make sure it can get fuel.

Good luck, sounds like you're keen - we need more blokes like you and Rich that want to give the carb mods a go. If you get that HD-12 cranking out more power, you're gonna get the attention of a few folks.


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## Derf (Dec 2, 2014)

Terry Syd said:


> Good luck, sounds like you're keen - we need more blokes like you and Rich that want to give the carb mods a go. If you get that HD-12 cranking out more power, you're gonna get the attention of a few folks.



Thanks for the encouragement, Terry. I downloaded a Walbro service manual and have read through it twice so far. I'm sure I know what I need to do to tear down the carb. If I can get the HD12 making more HP, I'll be happy if people sit up and take notice.


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## bandres34 (Jan 28, 2015)

First I want to say thanks to everyone for all of the great information on this site. I have to say I have caught the CAD bug. About a year ago I did a MM on my MS390 thanks to info from this site and now I have purchased a HD Makita 6421. I am in the process of putting a 79CC top end on it so I am very interested in this thread. I was wondering if anyone that has modded their jets for the 79cc top end has any feedback on the 30/35 or 30/30 orifice sizes before I go boring holes in what I used to think was a perfectly adequate carb!.


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## fordf150 (Jan 28, 2015)

bandres34 said:


> First I want to say thanks to everyone for all of the great information on this site. I have to say I have caught the CAD bug. About a year ago I did a MM on my MS390 thanks to info from this site and now I have purchased a HD Makita 6421. I am in the process of putting a 79CC top end on it so I am very interested in this thread. I was wondering if anyone that has modded their jets for the 79cc top end has any feedback on the 30/35 or 30/30 orifice sizes before I go boring holes in what I used to think was a perfectly adequate carb!.


Contact @Poleman for some guidance. He has taken these carbs allot further than what is in this thread. I have one of his old trial carbs but have yet to really test it.


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## 460cixy (Feb 1, 2015)

Gidday all I have a zama terry modded for me and was ment to get out this weekend and give it a good work out but just never got there. So watch this space and ile let you know how it goes on a stock 372xp


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## Terry Syd (Feb 1, 2015)

I hope you have a chance to run it with a stock carb, back to back, so you can make a comparison.

Here's a carb tuning story - A couple a years ago I rebuild a mate's 365 to 77cc with a modded carb. He was impressed with my saw, so I reworked his saw. Well, his brother came by and was saying the saw was running like crap.

I tried firing it up and it wouldn't idle and couldn't rev out. The saw was filthy, so before I started tuning I thought I'd check the air filter - freaking absolutely covered, like two years worth of dust. I blew down the saw and cleaned the air filter, then tried running it again. It still wouldn't idle, so I checked the idle screw - it was more than 1 1/2 turns out. The carb was tuned for the idle screw to be 1/2 turn out. Then I realised the throttle plate was all the way closed, so I dialled in the idle.

Once the saw was idling, I tried revving it out - still wouldn't go above 11,000 rpm. Screwed in the High speed needle to 13,400 and the saw was alive again.

When he came by to pick up the saw I asked him about why he was running the High speed needle so rich. He said that was the way he always tuned a saw - he liked the saw to four-stroke when he lightened the load on the bar. I said "don't you like cutting smaller logs at 12,000?" His reply was, "that sounds like it is revving too much"

So, after doing all the work to blend in the two fuel curves, he decides to tune the saw his way - whether it works better or not.

Sheesh, I'll bet he's back in another two years....


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## 460cixy (Feb 2, 2015)

I sort of destroyed the old carb trying to work out what they had done on the low speed side.


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## LegDeLimber (Feb 2, 2015)

Terminally damaged carb. Perhaps a candidate for a viewable cutaway?


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## 460cixy (Feb 2, 2015)

It could be fixable maybe I just wanted to see what was under the welch plug where the brass jet was ment to be. Now I don't have any plugs to fit it everything is too big or small. Anyhow road tested the big chingvarna tonight off idle response has improved I haven't changed the metering spring yet so that may improve it more. The top end power is what I'm amazed at much better and can actually be tuned to 13500 now where it should be but I tuned it in the cut for the most power that turns out to be around the 12400/12500 mark this is a chinga copy of a 372xp every thing looks to be identical except when pulling the cyl it has open transfers instead of closed so I don't think it will ever make great power with out some work but it sure does not embarrass its self that's for sure.


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## Terry Syd (Feb 4, 2015)

I drilled that carb as a 30/35 and thought it would be about right for a 71cc saw. I thought it would be tuned a bit higher, so maybe a 30/40 would have worked better, or maybe I should have dropped the main discharge nozzle down further (it could have gone another 2mm).

OR, maybe the powerband on the open port jug is determining the tune. The closed port Husky 365/372 jug has nicely angled ports with the back ports slightly angled upward so the flow characteristics are completely different than that open port jug.

Bugger if I know, that's why a dyno would be so good to dial in the carbs. You could graph the powerband from one change to another until you had a combination that was as good as it was going to get.

As far as that jug being open ported, I remember the first BB jug I got from Matt. It was 54mm bore size, but the Chinese had copied the smaller ports from a 365. I wonder on your saw if they copied the open port 365 jug to make the 372 copy. If so, you may want to try one of the new BB jugs (77cc) that Matt is selling instead of messing around with porting that open port jug. The new jugs (with a little bit of work) flow quite well and have better compression.


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## 460cixy (Feb 4, 2015)

no idea terry but it is better i will chainge the spring after another couple of tanks


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## Fly By Night (Mar 17, 2015)

Id like to get some bits coming to do these mods on a ported 7900 that will cut from 6,000' to 11,500' in elevation,. Rich do you have an idea which sizes will work well in that range and not be too rich up high?


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## Poleman (Mar 17, 2015)

If you've got the drills etc. to do the jets, I would probably start small....30 main, 35 aux and do your hole under the main jet to 55. It will be easy to change once that is done because you would only be redrilling the top jets.

My highest working height where I'm at is only around 6000 ft so I wouldn't know how a saw would act up high. That's kinda why to start small. A ported saw can really benifit from larger jets, just not sure what would happen at such elevations.

If saw runs or seems to run normal at those elevations I don't see why bigger wouldn't work. If saw is running rich it's probably not getting enough air for the fuel it's getting.

In your situation tho it may be tough to get a carb dialed in with such elevation changes. Back to the thinking of smaller because of it. A little increase in torque is better than stock. I just think you may be limited with such elevation differences.

I'll be following to see how things shape up for you.


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## Fly By Night (Mar 17, 2015)

Thanks Rich,

I am going to order the drills, probably tonight, I want to make sure I get kits with the sizes I do/may need. Home base is at 6,500' between there 11,000 will probably be my primarily operating elevations, I just want to make sure I don't create something that is unusable up high. 

Do you have links for the drill bit kits you use readily available? I just looked some and nothing seems to cover 30-55+


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## Poleman (Mar 17, 2015)

*Here's the one I purchased. It has all the sizes you will need. If you need a larger hole in the throttle plate for idling you can just drill two. Than a pliers, side cutter or such to pull the main jet. Might want to get if you don't have a caliper to measure the drill bits to make sure they are truly the size the say...mine came kinda messed up as far as sizes went.

Hope this helps.




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## Terry Syd (Mar 17, 2015)

DEFINITELY have a calliper or micrometre with you when you start messing with these micro drills. It is way to easy to get the wrong size drill if you rely upon their position in the box.


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## Fly By Night (Mar 17, 2015)

Thanks Rich, they are on their way to come meed Dolly. 

Ebay says that seller has sold 9 of those kits, im guessing most have gone to other AS members. 

P.S. Does anyone else think Rich is an ironic name for a guy who has spent a lot of time putting more fuel through carburetors?


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## Paragon Builder (Mar 18, 2015)

Fly By Night said:


> P.S. Does anyone else think Rich is an ironic name for a guy who has spent a lot of time putting more fuel through carburetors?


That's pretty clever![emoji106]


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## Fly By Night (Mar 19, 2015)

Rich, 

Do you have a part number for the accelerator springs? I just re read most of this thread and cannot find a part number. 

If you'd help formatting this, adding arrows and reference numbers to pics ect to post the final findings to the library I'd be happy to assist.


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## Poleman (Mar 19, 2015)

Lol....we were flying by the seats of our pants on most of this.......adding pictures as they occurred and comments like wise.

I'm pretty sure Terry posted the number for the spring. I players a lot with that and found I didnt need it in the most part. I only had 1backwards carb that didnt make sense on tune.....it turned out to be a favorite carb.....thats a whole mother story on that one as I had junked it but somehow found its way in the mix. I had totally tore it down and took the dremel to it in boredom.....


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## Terry Syd (Mar 19, 2015)

The metering spring is used for ramping in the fuel, ie: throttle response. For the most part, you can shift the starting point of the ramp by using the pre-load on the spring - and that will be enough adjustment to get the majority of tuners in the ball park.

Rich and I joked about how anal we could get over getting the throttle response as quick as we could, when in the real world of a bucking saw it wasn't something that you'd even care about while working.


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## Poleman (Mar 19, 2015)

Isn't it AMAZING.....theres more out there like us...... LOL

My kinda people!!!!


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## Fly By Night (Mar 19, 2015)

I will pass on the springs for now per riches recommendation but I think they might be helpful at my altitude. Has anybody else been building these? I do not recall seeing anyone else posting their results. 

Looks silly easy for the results you guys are seeing, the hardest part sounds like deciding which pliers to take to the grinder


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## Poleman (Mar 19, 2015)

I'll see if I can find the number but after my testing I don't think you'll need it. Only had one strange setting where I had to really mess with the spring rate. It can help.


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## fordf150 (Mar 19, 2015)

Fly By Night said:


> I will pass on the springs for now per riches recommendation but I think they might be helpful at my altitude. Has anybody else been building these? I do not recall seeing anyone else posting their results.
> 
> Looks silly easy for the results you guys are seeing, the hardest part sounds like deciding which pliers to take to the grinder


May look that way after the fact but I imagine there are to many hours invested in this testing to even count. Rich and Terry invested allot of time and money in figuring this all out and openly share this info.


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## LegDeLimber (Mar 19, 2015)

Hate to say that my drill kits and things are stuffed in my old kennedy top box.
just been a handfull of Dr type #%*# that I've slid on and let ruin my concentration
towards a great many other things around here.


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## Poleman (Mar 20, 2015)

Terry Syd said:


> "The metering springs are as follows 0019014 30gm / 0019002 16gm / 0019009 19 gm / 0019012 25 gm / 0019013 35 gm



Is this what your looking for???? I hope.


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## Derf (Mar 20, 2015)

My progress has been halted over the winter too.

I have a friend who can x-Ray CT scan a carb so I know how much clearance I can bore out before running into adjacent passages, but connecting with him has been difficult. I may just chance it and bore it out without the CT data. 

I plan to increase the bore on an HD-12, then drill the jets holes. I also need to pick up a couple welch plugs- can anyone tell me how to order those? Saw some on eBay but they were too expensive for the plugs alone. 

My elevation is very low, below 6000' for sure, so I likely won't be messing with springs. But it is a consideration that working environments change for these tools, so we should take that into consideration so that they work under a broad range of conditions. 



Sent from my iPhone using Tapatalk


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## Mastermind (Mar 20, 2015)

I see y'all ain't give plum up yet.


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## LegDeLimber (Mar 20, 2015)

This walbro pdf shows the carburetor bodies and the variuos bores and venturis.
If I were trying to open up a carb, this is about the best info I have found
that might tell how far you might risk going with trying to enlarge one.

I think there is a page on the Zama site that lists the diaphragm bodies
and the bore and venturi size ranges, also.
i'll keep looking to see if I have the link saved.

Edit: The Zama link to the carb bodies and bore ranges.
Same problem as Walbro, with No ideas as to internal passage locations.
http://www.zamacarb.com/page/products


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## Derf (Mar 20, 2015)

Hi Randy,
I know you said you think the ported 372XP/375XP runs better with the x-torq 372XP carb, which is bigger than the Walbro HD-12. Other people have said they think it runs better with a 390XT carb, also bigger. However, I'm OCD and don't like the x-torq carb in a non-xtorq saw (fitment issues, mixed and matched parts), so I just want to see if I can mod a HD-12 to flow more air and add more fuel and yield better throttle response/more power.

It would probably help anyone else with BB 372 kits, or the 7900 guys as well. Except Terry and Rich are working on the Zama carbs I believe.

--edit--
thanks LegDeLimber, but that won't show what I need. I want to see how much clearance between the air-flow hole and the fuel passages there are. 
Example 1) the "jets" are tiny holes that are drilled through only a few mm of aluminum before they reach a resevoir. If you open up the bore to much you cut into the reservoir.
Example 2) the fuel on the top side of the carb is brought down to the bottom of the carb by a passage running near the venturi. If you open up the venturi too much you cut into that fuel passage.


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## LegDeLimber (Mar 20, 2015)

Derf, I follow what you're after, 100%.
and yes it's frustrating not to have that passage mapping!
Just saying that this gives the best ideas of what bodies Walbro started with.
And ltes us know not to go too far with a certain body type, at least.

I've got some Walbro WT-1048 sitting here. plain jane carbs that will need some transition passeges to make them match the engines that they end up on.
Also would like to know how far out to take the bore & venturi, but I have the same reservations as you, because of that lack of passage mapping.

But I think we all know how Walbro dosent even want to give out any info about the mods of any of their specific carbs, let alone a good diagram of the genreral body passages.

Seems we're stuck with a bit of blind working, unless someone has that hookup
with the X-Ray dept!


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## LegDeLimber (Mar 20, 2015)

Another thought is to find some very strait wires that are the correct size
(i.e. those drill bits)
and gently place them into the intial openings of the passages.
Now take look at them, like the crime scene crews do at a shooting.
That gives us some starting ideas of the passage projections.

From there, you're down to looking for those pressed in balls that are the blockers from where the manufactuer drilled an intersecting hole, to make a corner or angle change, then pressed that ball in to close the unwanted entry hole, left from that secondary drilling entry.
With that in mind, you can try to make a mental map of the passages.
That's how a lot of our old Mikuni carbs on our dirt bikes, were modded.


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## Derf (Mar 20, 2015)

LegDeLimber said:


> aanther thouight is to find some very strait wires that are the correct size (i.e. those drill bits)
> andgently palce them into the intial openings of the passages.
> Now toake look at them like the crime scene crews do at a shooting.
> From there, you're down to looking for those pressed in balls that are the blockers from where the manufacuer drilled an intersecting hole, to make a corner or angle change, then perssed that ball in to close the unwanted entry hole, left from that secondary drilling entry.
> ...



Great minds think alike. I already did that! I used a piece of sewing thread and marked it with a marker on either side where it entered the hole and then took it out and measured the distance. I'm confident on how much I can open up the bore now, but I'm unsure of the venturi. Hence why I'm on the fence about just modding, or waiting for my x-ray friend.


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## Paragon Builder (Mar 20, 2015)

Mastermind said:


> I see y'all ain't give plum up yet.


Ain't gonna happen!
I'm just trying to wrap my head around all this before I start digging in.


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## Mastermind (Mar 20, 2015)

Derf said:


> Great minds think alike. I already did that! I used a piece of sewing thread and marked it with a marker on either side where it entered the hole and then took it out and measured the distance. I'm confident on how much I can open up the bore now, but I'm unsure of the venturi. Hence why I'm on the fence about just modding, or waiting for my x-ray friend.



On that carb just remember to open it in a D shape........I've cut thru a few.


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## Derf (Mar 20, 2015)

Mastermind said:


> On that carb just remember to open it in a D shape........I've cut thru a few.



THANKS RANDY! You're talking about a D shape in the venturi, right, to avoid the fuel passage on the side... ?


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## Mastermind (Mar 20, 2015)

Derf said:


> THANKS RANDY! You're talking about a D shape in the venturi, right, to avoid the fuel passage on the side... ?



Exactly.


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## Mastermind (Mar 20, 2015)

Do keep this in mind though........a larger venturi will decrease the pressure differential and can make pulling enough fuel tough. I'd rather see the throttle plate enlarged in most cases.


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## Derf (Mar 20, 2015)

Mastermind said:


> Do keep this in mind though........a larger venturi will decrease the pressure differential and can make pulling enough fuel tough. I'd rather see the throttle plate enlarged in most cases.



I plan to do both


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## LegDeLimber (Mar 20, 2015)

speaking of venturi to bore ratios: looking at this I have a C1Q-W29 sitting here. 
The venturi is 14mm and the throttle bore ( the engine side) is roughly 15.5mm
and the choke bore is bout 18mm (cheap plastic calipers readings)
I've seen some mold flashing nearly matching that amount of diameter change.

it's No surprise that the throttle plate only has a rather tiny notch on each side and there are 3 transistion holes in the bore.
And then the choke plate has a much smaller hole than you usually see.

I'm prone to soldering choke plate holes shut, as I prefer the charectoristics 
of that type choke system.
You might guess that I also just loathe all of the uni-lever, spring back open crap that's the trend on things now.


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## Poleman (Mar 20, 2015)

Just modded a carb last night and ran it today.....really bad bottom and transition. I put in a lighter spring (Walbro 98-530) and it straightened right out. Fast throttle and responce!!
I'm finding every once in awhile a weaker spring is needed for some unknown reason. All mods are the same as other carbs that work fine with stock spring.
Just put this out there in case others have problems tuning a carb on a modded saw or if using a modded carb.

Springs can make a difference!


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## fordf150 (Mar 20, 2015)

Poleman said:


> Just modded a carb last night and ran it today.....really bad bottom and transition. I put in a lighter spring (Walbro 98-530) and it straightened right out. Fast throttle and responce!!
> I'm finding every once in awhile a weaker spring is needed for some unknown reason. All mods are the same as other carbs that work fine with stock spring.
> Just put this out there in case others have problems tuning a carb on a modded saw or if using a nodded carb.
> 
> Springs can make a difference!



And your probably the only person with an assortment of springs to choose from for tuning


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## Poleman (Mar 20, 2015)

I might just be......I got a hellava support crew!!!!!


Wonder where I got that kit from???


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## Mastermind (Mar 20, 2015)

I need one of those kits huh?


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## fordf150 (Mar 20, 2015)

Mastermind said:


> I need one of those kits huh?


They are nice kits and have quite a few springs to choose from but I think rich is the only person nutty enough to buy one and actually use it


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## Mastermind (Mar 20, 2015)

Birds of a feather.


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## Poleman (Mar 21, 2015)

Flock of turkeys gotta stick togather!!!! Power in numbers.......


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## Chris-PA (Mar 21, 2015)

This works here because of the way these carbs are made, but won't work the same on other carbs. A shot from the Dolmar tutorial shows what happens:



Notice the fuel outlet in the center - it is fed from a fixed jet, and so its mixture is only a function of the pressure in the metering chamber, while the H and L are fed from the adjustment needles. 

If you use a weaker spring, the fuel pressure in the chamber will get closer to atmospheric (i.e. it will rise), and the mixture will get richer. You can then adjust the needles to get it back on the H and L, but the center jet stays richer. So at part throttle, and as the throttle plate moves through there during acceleration, the mixture will be richer. 

This won't work on something like an HDC, as the transfer outlets are fed from the idle circuit and are adjusted by the L needle:


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## Mastermind (Mar 21, 2015)

Excellent post Chris......

What you are saying makes perfect sense.


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## Mastermind (Mar 21, 2015)

There's a lot going on in a little area in these carbs. lol


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## LegDeLimber (Mar 21, 2015)

Just a quick freehand sanp, not my usual, more fussily lighted crap.

WT-285 body: "L" needle regulates the total flow into the chamber
undeneath the Welch (Not Welsh ) plug.
Red is inlet from "L" needle.
Green are transition ports
Yellow is the idle port
Black is the supply to the L needle.
Throttle bore (not the Venturi) and plate, are seen in the bottom of the frame.

Makes ya wonder how much the metering lever motion influences the idle
by it's pulsing right over the inlet.


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## Derf (Mar 21, 2015)

Can't you just clip a few winds off a spring instead of buying a spring kit?


Sent from my iPhone using Tapatalk


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## LegDeLimber (Mar 21, 2015)

Some people do and seem to get ok results.
But springs work better with a few, unseperated turns at the ends.
I've read some of the reasoning and seen a couple high speed vids of how springs move
and the way that cutting those ends off or using a different winding rate at hte ends affects them.
Using this one from a WYA-155 (from EBZ8500 Redmax) as an example.
I would not cut more than about 2.2 ~2.5 turns off of an end.
A turn meaning starting from the yellow line and counting the loop.


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## Terry Syd (Mar 21, 2015)

Poleman said:


> Just modded a carb last night and ran it today.....really bad bottom and transition. I put in a lighter spring (Walbro 98-530) and it straightened right out. Fast throttle and responce!!
> I'm finding every once in awhile a weaker spring is needed for some unknown reason. All mods are the same as other carbs that work fine with stock spring.
> Just put this out there in case others have problems tuning a carb on a modded saw or if using a modded carb.
> 
> Springs can make a difference!



Rich, it may be that the carb had a smaller metering orifice than you're used to. You might check it and get back to us.

Remember that the C3M carbs are put on 50-80cc saws. It may be that there are optimum ranges of metering springs and metering orifices that correspond with engine displacements. For example, I was given four C3Ms one time and there were three different size metering orifices in that batch of four.


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## fordf150 (Mar 21, 2015)

Derf said:


> Can't you just clip a few winds off a spring instead of buying a spring kit?
> 
> 
> Sent from my iPhone using Tapatalk



i would prefer you buy a kit....http://performanceoutdoorequipment.com/products/walbro-metering-lever-spring-kit-400-606


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## Poleman (Mar 22, 2015)

You can clip the winedings but its harder to duplicate if you want to do so. 

The kit is good and am glad I purchased it. Makes things easier. On wish is that it would have a spring rate listing. Its still easier than cutting winds.


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## Poleman (Mar 23, 2015)

Poleman said:


> Well finally.....
> 
> I know a few of you have been waiting for this and others just want to know/see how it's done.
> 
> ...



Thought there would be more interest in the how too.....


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## fordf150 (Mar 23, 2015)

Don't worry...it will get looked at and used. This is still the experiment/question thread


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## Paragon Builder (Mar 24, 2015)

Well I read it through carefully and I'm about to start looking at some of my saws. I'm sure I'll have questions!


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## Poleman (Mar 24, 2015)

It's pretty easy once you see the layout of the carb and t you'll be doing. Hardest thing to do is probably removing the main jet without destroying it....small steps there, and the right tool! I suggest ordreing a couple am carbs to practice on as there not as expensive as full blown Zama's......and seem to work good for me. You have to watch the membrain on those tho as they come out quite easy when your drill drops threw when you drill. Little more open design than the Zama jet.

Any questions let me know!!!!


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## Fly By Night (Apr 2, 2015)

I was able to get my aux punched out to .35 last night, it went really smooth. I couldn't get the main jet to budge, I have some small old side cutters that get in there well but will not twist the thing! I put some penetrating oil on it overnight. Ive thourght about maybe epoxying a nail or something to that jet to remove it.Also my 3 smallest bits all measure right around .35 on my calipers, I was going to go to .30 on my smaller jet has anyone else found that with their drills?


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## Poleman (Apr 2, 2015)

I have too..... I find if I bear down on the caliper the measurement will change.....a little.
On your side cutters can you gently just pry the jet up so it pops out? I've never used those but just a thought.


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## Terry Syd (Apr 2, 2015)

You may be able to put a small notch in each side of the cutter jaws using a Dremel wheel. That will give the cutters a better grip on the idle jet so that you can turn it. Once you get the jet to turn, it can be worked back and forth as you pull it out.


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## Fly By Night (Apr 2, 2015)

Poleman said:


> I have too..... I find if I bear down on the caliper the measurement will change.....a little.
> On your side cutters can you gently just pry the jet up so it pops out? I've never used those but just a thought.


I tried rocking it side to side and front to back with no luck. I thought about cutting a relief. I really don't have any cutters I want to grind on.


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## Paragon Builder (Apr 2, 2015)

I've been really thinking about all this for a while. It seems we are looking for midrange fueling on our saws. Idle is simple, at WOT if it can 4stroke you are getting enough fuel. I'm thinking out loud now. So my question is this: if we want more fuel in the cut why do we tune to clean up when touching the wood? Why wouldn't running rich achieve the same goal as drilling the carb?


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## Poleman (Apr 2, 2015)

Very good question. I see it like this...
From idle through midrange a saw can use more fuel to give it more torque, which in turn feeds the top somewhat. The saw can be richer up in that range to improve performance but the way it needs to be done is by modified the jet size and mixing carituristics, something you can't do with just the needle adjustment alone. By the time the saw is reaching top rpm's the low and mid are also combining with the Hi jet to feed the saw. The changed mixing carituristics in the low circut enhances performance threw out the power band until hi rpm is encountered in which the saw has reached its full fuel air demand. If it's to lean the saws rpm's will increase but heat is also increased because of more friction, more speed and lack of fuel/oil which cools the combustion chamber and maintains proper lubrication. Without the fuel/oil mixture being correct meat to metal contact occurs and seizing and galling occures.

On the other hand to much fuel after optimum will slow a saw down because of to much fuel being delivered and it's inability to use it, thus excess smoke, unburned fuel, and a basic flooding condition with incomplete combustion.

My take on it as I see it. Hope it makes sence.


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## Chris-PA (Apr 2, 2015)

Paragon Builder said:


> I've been really thinking about all this for a while. It seems we are looking for midrange fueling on our saws. Idle is simple, at WOT if it can 4stroke you are getting enough fuel. I'm thinking out loud now. So my question is this: if we want more fuel in the cut why do we tune to clean up when touching the wood? Why wouldn't running rich achieve the same goal as drilling the carb?


Running rich does not make more power at a steady speed/load. There is a very narrow range of fuel/air mixtures that will run, and you can optimize a bit from the lean to the rich side, but more fuel does not equal more power. There is one correct mixture that gives max power - but these carbs are nowhere near accurate enough to hold that as rpm changes.

The goal here is to improve acceleration, and to accelerate you need to provide more fuel than would be needed at a constant speed. Since these carbs have no accelerator pump system, the only way to do that is to fatten up the mid range fixed jet (since you must go through mid range to get to WOT) - this also makes it too rich at a steady part throttle, but since one rarely runs there it's a decent compromise.


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## Fly By Night (Apr 2, 2015)

I tried it with my little dikes again, it still wouldn't move, I cut a relief to get more bite, still nothing, I cut 2 strait sides on the jet to try to get it to twist to no avail. I wonder if I can drill it out? Someone please tell me you can get just the jet.


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## Terry Syd (Apr 3, 2015)

Don't drill it out, you need the jet for the idle and transition. What I've done on difficult carbs is to take a very small Dremel bit and grind away on the aluminium at the edge of the jet. That will reduce the depth of the hole that the jet sits in and thus reduce the resistance to twisting. When I re-install the jet, I put a small amount of GB Weld around the jet to make sure it is sealed and can't fall out.


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## mdavlee (Apr 3, 2015)

Terry did you do a walbro HD for stihks? I'm wanting to mess with the carb on my 046 soon.


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## Terry Syd (Apr 3, 2015)

No, I just drilled out the low speed jet on a Chinese 460 carb and run that on my 64cc 'Sleeper'.

Here's a link back in this thread where I discussed the HD series and possible drillings. - http://www.arboristsite.com/community/posts/4953791/

If I was going to work up a 046 carb, I'd find some old HDs laying around and start drilling them. Forget modding the HD-12, you might as well mod some other piece of crap that you don't mind 'going too far' with. Near as I can tell, the HD series are basically the same, it's just the drilling of the transition holes that need to be worked out for the displacement. If you can find a crappy emissions carb with the bigger venturi, just drill out the low speed jet and start modding the transition holes.

Whereas the Zama in nicely laid out to work on, the Walbros will require you to remove the Welch plug each time you want to try a different jetting.

For the 046 I'd start with the 460 drilling and go up from there. I'd also try to make the transition hole next to the throttle plate always the smaller hole (like on the 460 carb). What that does is create a more progressive rich mixture as the throttle plate moves across the holes. The result would be similar to what the Zama does with the auxiliary jet coming in after the transition holes.

If you had two carbs to work with you could always keep a 'baseline' to refer to. Mod one carb and see which works the best. Then mod the 'loser' and test again.

The 460 carb has the first hole at .6mm and the second at .65mm. I'd try a .6mm/.7mm next, then a .6/.75, etc.. If the carb started bogging right off of idle from being too lean, then you would have to open up the earlier transition hole. Say, if it started to bog at .6/.75, then you may have to go back and try .65/.7

When you go up to a 385 WJ carb, you can see they have a LOT more flow in the low speed circuit. It may be that the HD will require a fair bit of drilling to get that kind of flow out of the low speed circuit.


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## mdavlee (Apr 3, 2015)

I have a Wj 6 from a 066 but I don't think it will fit on the 046. I do have a velocity stack so I'm sure it could be made to work.


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## Terry Syd (Apr 3, 2015)

Aw, forget the WJ - Lash out and take a HD emissions carb off of a 029 and build a killer carb, you'll be a legend! - Women will swoon at your approach...


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## Poleman (Apr 3, 2015)

I don't believe you can get just the jet...I've looked, maybe not hard enough. I went to a Husky dealer and went through his box of junk Zamas and found a C3 has the jet you would need.....and yes, I have ruined a couple. Thought I'd just take the jet from an A.M. carb....wrong, there slightly bigger than the Zama.
I've had two instances like yours where I DID ruin a jet. They were both older carbs and the jet was set very tight. I did drill mine out just to get it out but I had found replacements in the junk carbs. A slight setback but if you can find old carbs to rob the jet from you'll be good. Seems there's not many junk Zama C3M's out there as the dealer didnt have any so that how I found the jet from the C3 worked.


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## Fly By Night (Apr 3, 2015)

Thanks rich. 

Is the air bleed hole by chance directly Below the hole in the jet?


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## Poleman (Apr 3, 2015)

I think I know what your thinking....to drill through the main and go into the air bleed won't work. One you'll need a bigger grill for it then you would have to JB Weld the main and redrill it. Unfortunately the bleed hole is offset toward the edge of the main jet. Actually there rather sporadic lyrics spreed out. Zama's are the worst for consistency where as the A.M. are uniform.


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## Fly By Night (Apr 26, 2015)

My idle mixture screw doesn't seem to make much of a difference in tuning. I have it to where it will idle nicely for 10 or so seconds after I come off the throttle, it will idle up for a few seconds (leaning out) then pretty quickly idle down and die, any thoughts on that? I may just need to bite the bullet and figure out how to get to that air bleed, or tack down some slow taper idle screws.


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## Poleman (Apr 26, 2015)

You will have to get to the air bleed and enlarge that to .55. That's what is causing the issues.
Did you put a hole in the throttle plate and is your idle slowly slowing down until it stops?

I'm trying to figure where your at with it. I've done carbs at first without drilling the bleed home....they just tune harder.


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## Fly By Night (Apr 26, 2015)

All I have touched on it at this point are the the two jets.


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## Poleman (Apr 26, 2015)

So is you idle to slow with the idle screw all the way in? If that's the case you can put a notch or drill a hole ( which I prefer because you can Solder it shut and go back to the last size that worked). That will bring up your rpm's and give you some adjustment on you idle screw. I usually go to where I get to the middle of the screw. You will know if you go to big if you've got the screw all the way out and it idles to fast yet...thus the drill hole...easy to fix with solder.

I'm guessing that's what your issues is....


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## Poleman (Apr 26, 2015)

What size did you go? 

Can't remember....your saw ported??


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## Fly By Night (Apr 26, 2015)

I am getting plenty of air, my issue seems to be getting the mixture right so it will idle stably


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## Fly By Night (Apr 26, 2015)

I am at 6,500' I ported it my self so its not as carried away as most. I drilled .30/.35


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## Poleman (Apr 26, 2015)

Ok. That's good for your altitude. What's your adjustment like on you idle screw? It must be pretty good? 
Is your idle kinds lopy and irratic? That was an issue without drilling the bleed hole. 

You had a problem getting your main jet out didn't you? That issue occurred to two carbs I did....got a couple replacement jets from junk carbs at dealer.


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## Fly By Night (Apr 26, 2015)

Adjustment on the air screw is fine, the mixture screw is unlike what im use to. The idle is fine for a few seconds then gets weird. You are right, I am the one that couldent get my main to budge. I wonder if I can heat it? How much were the Chinese carbs you were playing with?


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## Poleman (Apr 26, 2015)

Are you meaning the L adjustment and possibly the H? It will make it totally strange. Remember the H sometimes may be totally closed because of the increased fuel from the main jet.

They range from $11 to $20.. I've had good luck with them.


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## Fly By Night (Apr 26, 2015)

In my position that seems like a no brainer to get a new one, mod it and be done. Where were you getting them? I am talking about the low, I am use to knowing where I am when tuning an idle mixture, this thing I have no idea whats going on.


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## Poleman (Apr 26, 2015)

I was getting them from Hutzl on ebay. They are good quality and Farmer Tec brand.

Don't be afraid to really mess with the carb and make some radical settings. The setting dont seem to make sence at all on these carbs. This what you are describing is why I didn't like the first carbs I did before drilling the bleed hole. Drilling the bleedf hole larger straitens out these problems and the carb acts like a stock carb after that.

I do have one extra jet left and I do have 4 more carbs on the way....I can send you the jet and you can drill out your problem one after you drill the bleed hole, just make sure it is the right size. I could also mod a carb for you and send it unless you want to do your own modding I could just send a carb.

I'm still thinking about you not having to put a hole or notch in the throttle plate...I had to open it up for more air to get them to idle right because of the increased fuel at idle and low speeds.


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## Fly By Night (Apr 27, 2015)

Thanks for the offer Rich. I will turn in my idle speed screw today and see where that gets me. What's odd to me is that it idles very stabley for a minute before it gets wonky. I'm not sure what it's deal is. I will report back tonight With my findings.


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## Poleman (Apr 27, 2015)

I've found on some of the modded carbs they have a funny idle. You may try reseal in the Welch plug with clear fingernail polish...if that is leaking it will idle funny.


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## Fly By Night (Apr 27, 2015)

I'll give that a shot when I have a minute. I need to pull that thing apart and put a kit in it at some point.


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## Fly By Night (Apr 27, 2015)

I cranked my idle screw all of the way in today, the L mixture still doesn't seem to do much until it gets really far one way or the other. I think sealing that welch plug is priority #1 for getting that straitened out. Im sure drilling that air bleed is going to need to be done regardless.


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## Poleman (Apr 28, 2015)

Sounds like you may need a rebuild then start over on the settings. Sealing the Welch plug will be insurance.
You say you had your idle screw all the way in as fast idle??? Did you have a high rpm idle or did it still act funny. If it still acres funny you may need to drill a small hole in your throttle plate for more air and to stabilize idle.


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## Fly By Night (Apr 28, 2015)

The diaphragms didn't look too bad but while I'm taking the carb apart and messing with it i don't see why not. 

With my idle speed screw all of the way in it still acted funny, it would idle normal for 5-10 seconds then idle up and cut out.


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## Poleman (Apr 28, 2015)

I'd go a rebuild and needle and seal the Welch plug. I think you will have to drill a small hole in the throttle plate to get your idle adjustment back because it sounds like you have no more adjustability on it other than leaning out you L needle which usually doesn't solve anything.

You have to remember with drilling the jets your introducing more fuel into the idle circut and upwards. To counter react that you need more air also at idle so saw will idle right. To me it sounds like it's over rich and stalling. How's throttle responce when running?


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## Fly By Night (Apr 30, 2015)

When running throttle response can be pretty darn good, after idle its boggy during Normal idle and when idling up before it dies which I suspect is it being lean it will try to die if you don't feather into the throttle lightly. I was playing with it the other night with the sun in my eyes and realized it's smokey at idle when warmed up, I suspect thats a pretty good way to tell she's rich! How large of a hole do you suggest I start with?


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## Poleman (Apr 30, 2015)

Yep...sounds like your rich and needing more air at idle. I would probably start with a .80 hole in the throttle plate across from the jet or on top. That should help your idle and idle screw adjustment. If it idles to fast and the adjustment screw won't bring it down the hole is to big. Simple solder it shut and drill a smaller on in the solder. I have drill 2mm holes in some ported saws plates to get enough air to them at idle. After that the plate opening feeds the need....


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## one.man.band (May 1, 2015)

Fly By Night said:


> My idle mixture screw doesn't seem to make much of a difference in tuning. I have it to where it will idle nicely for 10 or so seconds after I come off the throttle, it will idle up for a few seconds (leaning out) then pretty quickly idle down and die, any thoughts on that? I may just need to bite the bullet and figure out how to get to that air bleed, or tack down some slow taper idle screws.



sounds like it's check valve related. possibly because the whole idle circuit is out of whack now.


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## Terry Syd (May 1, 2015)

If it was the check valve it would idle for about 10-15 seconds before the metering chamber sucked in too much air past the check valve - then the engine would die.

He needs to drill the transition/air bleed hole. Drilling the throttle plate will help, but the transition hole mod cleans up the idle.


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## Fly By Night (May 1, 2015)

Which check valve is this? That's what it will do Terry, it idles rich for 10 or so seconds, then idles up from leaning out from what I can tell and dies


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## Terry Syd (May 1, 2015)

The check valve is in the main discharge nozzle. The symptoms of a malfunctioning check valve are like the saw running out of fuel (because it does).

The valve sometimes get damaged when someone uses compressed air on them. However, it may have something underneath the thin flutter valve that is keeping it from sealing. If you punch it out of the carb, you can try flushing it out with some spray liquid like WD-40, brake cleaner, contact cleaner, etc. Once you have the nozzle out of the carb, check to see that there is still a valve in the nozzle and that it isn't stuck or damaged. If it looks OK, then with the nozzle out you can get access to the little hole that feeds fuel to the check valve. Try spraying the liquid in the hole. - Or, put a replacement nozzle in the carb.


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## Fly By Night (May 1, 2015)

This is at idle only or in all situations? 

Im so tempted to order a farmer tec and donate this carb to Rich for R&D.


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## one.man.band (May 1, 2015)

fly........did it idle and die out before you did any drilling?


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## Fly By Night (May 1, 2015)

I smoked the top end 3 years ago, I ported the jug and the carb at the same time. It idled before but who knows what's changed in that carb.


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## one.man.band (May 1, 2015)

that's my point.


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## Fly By Night (May 1, 2015)

What is?


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## one.man.band (May 1, 2015)

Fly By Night said:


> I smoked the top end 3 years ago, I ported the jug and the carb at the same time. It idled before but who knows what's changed in that carb.



have you ruled out that it may be the motor? comp; crank seals etc?
aluminum dust from drilling in the idle passages could do it along with the check valve?
walbro service manual.

just throwing it out there. good luck fly.


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## Fly By Night (May 1, 2015)

I vacuum tested the engine, I know it holds air. 

Something changes in the idle, I just don't know what it is. Ill try to get you guys a video tonight.


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## Poleman (May 1, 2015)

Never had issued with aluminum or brass dust from drilling of jets or passages in these. I still think the bleed hole under the main jet is causing most of the issue, with throttle plate hole also being needed.

I have encountered two carb that stumped me for awhile....that ran ok at the top but was really boogy on the bottom and wouldn't idle right. I put in way softer needle springs which brought them around. The only thing I could come up with was they (particular carbs) needed way more fuel for some reason. Both were done and drilled the same as all the others with same procedure. I think one was an AM carb and the other was a Zama.

I just changed carbs on a Mikita 7900 that had a 35-35-55 drill and it was soft on the bottom also. I changed the coil to unlimited on it and carb stayed the same as I thought it would. I JB Welded the main shut and redrilled to 30 and I have a "HOTROD".... saw responce is instant and has excellent power buryed in wood even over the previous drill which should have given more pulling power.

Sometimes things don't make sence but don't be afraid to try something different....some thim,e you hit on things and another step is taken.


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## Terry Syd (May 1, 2015)

Rich, I wonder if that Makita had a slightly retarded ignition timing than is normal.

Going leaner gave it a faster rise time in the cylinder, so throttle response improved. Likewise, with the low-end torque where the richer mixture slowed down the rise time even further (eg: acted like a further reduction in timing).

If the 35/35 works well on most 7900s then the only thing I can think of that would affect the power that much would be the ignition timing. 

Sometimes the 'manufacturing tolerances' can be quite large in a saw. I was stunned when I looked at a Stihl manual and it stated that the range of ignition timing on my 029 was between 23.5 and 28.5 degrees - that is a freaking 5 degree spread!

I really wish these saws had a backing plate for the coil where you could easily try different ignition timing.


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## Poleman (May 2, 2015)

Very good point Terry....I never considered manufacturers tolerances being that different....that is a BIG spread especially in timing.
Maybe that explains why I get different results on the different saws I experiment on.


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## Terry Syd (May 2, 2015)

I don't know what component would be the most likely to show the most variance, but it possibly would be the flywheel. It is not just the machining of the 'woodruff key', but the casting of the magnets in the flywheel that could cause a fair amount of 'manufacturing tolerance'.

I wonder if anyone on the forum has done research on this point. Perhaps some manufacturers are more precise than others.


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## Fly By Night (May 2, 2015)

to me it doesn't seem like it would be that hard to put a mark in there somewhere that you can align with say the front of the magnet to compensate for keyway, magnet ect. Variation


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## one.man.band (May 3, 2015)

Fly By Night said:


> I vacuum tested the engine, I know it holds air.
> 
> Something changes in the idle, I just don't know what it is. Ill try to get you guys a video tonight.



if it is not the motor, i would test the check valve to make sure it is working correctly before doing much else.


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## one.man.band (May 3, 2015)

after reading many earlier posts, confused on a thing or two.

say the stock carb, with stock hole sizes.....is set for typical 1 needle turn out for low speed/idle jet.

....you guys are drilling the idle holes out bigger.......then turning the idle needle jet in, so it is only 1/4 turn out.

what is the gain here, seems like a wash?

what am i missing?


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## Terry Syd (May 3, 2015)

A couple of things. First, is this is a Zama C3M carb where the idle circuit is separate from the low speed circuit. When you move the low speed needle to change the mixture strength of the idle, it ONLY changes the idle mixture.

Some carbs, like most Walbros, when you tweak the low speed needle it changes not only the idle mixture, but also the total fuel flow for the entire low speed circuit.

On the C3M, in order to bump up the fuel flow for the low speed circuit, Rich is drilling out both the idle and auxiliary jets to increase the fuel flow.

When the idle jet (the one that needs to be pulled to get to the transition holes) is drilled larger, the idle circuit becomes very rich and difficult to tune. That's why the idle jet is pulled and one of the transition/air bleed holes under the jet is drilled out to .55mm - to increase the air through the idle circuit and lean the idle mixture back out.


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## Paragon Builder (May 3, 2015)

Terry Syd said:


> A couple of things. First, is this is a Zama C3M carb where the idle circuit is separate from the low speed circuit. When you move the low speed needle to change the mixture strength of the idle, it ONLY changes the idle mixture.
> 
> Some carbs, like most Walbros, when you tweak the low speed needle it changes not only the idle mixture, but also the total fuel flow for the entire low speed circuit.
> 
> ...


Terry, so if the idle is separate from the low speed circuit, why aren't we just drilling the low speed circuit out and leaving the idle circuit alone?


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## Terry Syd (May 3, 2015)

There is the issue of how the engine transitions to power. The idle jet is working in the first 1/4 of throttle plate movement, then the auxiliary comes in. On a bigger engine we need more fuel in that first 1/4 movement to get a quick throttle response.

There's two jets in the system. Both jets are .25mm from the factory. We need a lot more fuel in the low speed circuit to bring the torque up. On a smaller saw you may be able to get by with a 25/30 or 25/35, but the bigger engines require a lot more fuel.

I don't know what the maximum size would be on the auxiliary jet before you ran into idling problems. A large auxiliary jet could give you the same problems that a malfunctioning nozzle could give you - allowing air back into the metering chamber at idle. However, using the idle jet for more fuel flow ensures that air can't go back through that jet as fuel is being pulled through it.

I've been running 35/35/.55 on two 77cc saws and the engines idle just fine. I think the longest I let one idle was for 10 minutes while I lapped some new clutch shoes with valve grinding compound.


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## Poleman (May 3, 2015)

Very Good explanation Terry!! The combination of drilling the main and auauxiliary jets gives the increased fuel flow and drilling the bleed hole stabalizes the idle and makes it it all come together adjustment wise.

Terry, do you drill the throttle plate on your carb as I do to get the air to the engine at idle to regain adjustment?


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## Terry Syd (May 3, 2015)

No, but I did nip a bit off the edge to help lean it out. Drilling a hole is probably a cleaner and more easily reversed mod.


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## one.man.band (May 4, 2015)

ok, you are talking zama . w/the walbro, could not see how this could work as described... almost impossible to balance. runs out of check valve real quick.

i don't have any more patience to mess with it any more than i already have. unless drilling another air bleed maybe.

typical good idle/bad acceleration or adjust to good accel/poor idle.

good luck w/those TS. 10-12 hours is enough of that bs for me.


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## one.man.band (May 4, 2015)

poleman.....experimenting w/electrode gap may be easier than the timing thing?


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## Poleman (May 4, 2015)

When using a Husky coil I was retiming and moved it 30 degrees to make it work...way more than I could get that way. Had to grind off locating tab and move flywheel those degrees on the crank.


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## Fly By Night (May 4, 2015)

What is this about electrode gap and timing one man?


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## one.man.band (May 5, 2015)

Fly By Night said:


> What is this about electrode gap and timing one man?



slight timing change. as i understand it from something i read.....if to rich or too lean, more initial may be necessary to get the flame going, along w/this also comes a greater delay. another trade off.


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## LegDeLimber (May 5, 2015)

Smaller gap should fire a Gnat-sneeze earlir , due to breakdown voltgae (of air gap) achived sooner.
Probly get far more result from finding longer reach plig and some spacer/washers
to set the depth/disrtance that plug protudes into chamber.

But gatta be Damn sure to chek cleances!!! dont want to have the piston hit and get a hole/crack in it.

then there's also the indexng of the strap/ground elctrod factor to also to alter and look for any improvmnts.

sorry i'm having al ittle messy typing day, fellows


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## Fly By Night (Jun 14, 2015)

I just looked for a farmer tec carb and cant find them on Ebay, do you happen to have a link Rich?


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## Poleman (Jun 15, 2015)

Hutzl. On eBay carrys the Farmertec carbs and priducts. I also use the 272 coils they have to unlimit the 7900's.


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## Paragon Builder (Jun 15, 2015)

Fly By Night said:


> I just looked for a farmer tec carb and cant find them on Ebay, do you happen to have a link Rich?


I couldn't find any carbs for the dolmar saws on ebay either. Plenty of carb kits but no carbs. Is there a Zama part number we can search for?


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## Poleman (Jun 15, 2015)

Item: Husqvarna Chainsaw 362 365 372 371 372XP Carburetor 503 28 32-03, 5032818-01

URL: http://pages.ebay.com/link/?nav=item.view&id=291047828269

Alt URL: http://www.ebay.com/itm/Husqvarna-C...buretor-503-28-32-03-5032818-01-/291047828269


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## Paragon Builder (Jun 15, 2015)

Poleman said:


> Item: Husqvarna Chainsaw 362 365 372 371 372XP Carburetor 503 28 32-03, 5032818-01
> 
> URL: http://pages.ebay.com/link/?nav=item.view&id=291047828269
> 
> Alt URL: http://www.ebay.com/itm/Husqvarna-C...buretor-503-28-32-03-5032818-01-/291047828269


Great! Thanks. I didn't realize they shared a carb with those Huskys.


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## Poleman (Jun 15, 2015)

I use the A.M carbs from a Husky 362-372....the only difference is the choke hole is larger where the choke lever attaches on the carb. I have never had an issue with it coming out.

OEM, are $75-$105 in comparison to the aftermarket. The casting quality isn't quite the same but the machining is as good or better than the Zamas.

Of all the carbs I've worked with (AM) I've only had two that weren't right. I have very good luck with them.


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## Poleman (Jun 15, 2015)

With the older ones they do. This mod also work on them.
The new extorque uses a different WJ carb and are totally different.


Item: New Replacement Carburetor Carb Fits Husqvarna 362 365 371 372

URL: http://pages.ebay.com/link/?nav=item.view&id=281066370635

Alt URL: http://www.ebay.com/itm/New-Replacement-Carburetor-Carb-Fits-Husqvarna-362-365-371-372-/281066370635

Here's another supplier.... Their carb have Zama on them and the have very prominent main jets and east to pull. The jet is also a slightly different design which I like also.


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## cgraham1 (Jul 2, 2015)

Bump!


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## Paragon Builder (Jul 2, 2015)

cgraham1 said:


> Bump!
> View attachment 433587


Nice! Did you just get her back?


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## cgraham1 (Jul 2, 2015)

Paragon Builder said:


> Nice! Did you just get her back?


I've had it for quite a while. I just saw this in my watched threads, and thought it could use a bump.


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## cgraham1 (Jul 2, 2015)

Paragon Builder said:


> Nice! Did you just get her back?


Its been done for quite a while. I just saw this in my watched threads, and thought it could use a bump.


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## Paragon Builder (Jul 2, 2015)

Guess I'm not the only one getting double posts this morning...
Is that saw randy sent to rich to play with after he was done?


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## mdavlee (Jul 2, 2015)

I got a couple of these carbs to play with soon. With all the rain this weekend I may have more time inside and less outdoors


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## fordf150 (Jul 2, 2015)

Paragon Builder said:


> Guess I'm not the only one getting double posts this morning...
> Is that saw randy sent to rich to play with after he was done?


clint just did that to up his "like" count. i had to like all 3 posts


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## cgraham1 (Jul 2, 2015)

Paragon Builder said:


> Guess I'm not the only one getting double posts this morning...
> Is that saw randy sent to rich to play with after he was done?


Yep. 


fordf150 said:


> clint just did that to up his "like" count. i had to like all 3 posts


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## Paragon Builder (Jul 2, 2015)

Did you get to try it with and without the carb mod?


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## cgraham1 (Jul 2, 2015)

Paragon Builder said:


> Did you get to try it with and without the carb mod?


No, I did not. In fact, I put my stock carb on a different saw and sold the saw.


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## Terry Syd (Jul 2, 2015)

As long as you bumped this thread, I've tried something else with the Walbros.

I have a mate who bought a cheap Chinese saw (71cc) with a 15mm HDA on it. (The HDA doesn't have the better staged transition ports that the HD has). The carb needed a heap more low speed circuit flow and I turned up the low speed needle. That made the idle way too rich and I had to turn in the idle screw a bunch to keep it idling (rough as it was). The throttle response was also too rich - but it did cut wood well.

I was thinking of drilling out the transition holes - and then had an idea after looking at the lightweight spring in the carb. The carb would take a Zama spring and I had a good selection of stronger springs. If I used a spring that was much longer, it would lean out the idle and give a better transition. I looked through the springs and found a packet of three springs that are used on stationary engines.

Not only was the spring longer, but it was a thicker wire and wound with more pitch - it was a lot stronger than the stock spring. The stronger rate of the spring would also slow the ramp in of the fuel from the low speed needle. OK, if it was too strong I could always trim it back. So, I put the spring in the carb and started the saw.

Since the idle screw had been turned in so much, the saw was idling quite high. I turned down the idle screw - and the freaking thing idled perfectly. Then I hit the throttle and the response was crisp and clean. It was like the spring was made for that saw!

So, for you Walbro enthusiasts, you may want to get one of those spring kits with all the different Walbro springs in it. You may be able to use a different spring to clean up the idle/transition rather than pull the welch plug and re-drill the carb.


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## one.man.band (Jul 2, 2015)

very nice TS and tyvm.


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## Gravedigger (Jul 10, 2015)

Finally ran full tank of fuel through my Makita 84cc with Walbro carb . That thing was cuzing through 20"+ honey locust with a 24 inch bar. Dozer tuned the carb at PA gettogether . The saw 4 stokes and runs ggreat.


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## Derf (Jul 15, 2015)

Sorry to post this question, but I need a welch plug for my hd12b. 

The plugs themselves are cheap, around $0.40 each online, but everywhere wants like $6-$8 to ship me one. Does anyone have one they could send me in a regular #10 envelope with a stamp?


Sent from my iPhone using Tapatalk


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## acp104 (Jul 21, 2015)

I'd been following this thread for awhile, and finally decided to give the mods a try on one of these aftermarket carbs URL: http://pages.ebay.com/link/?nav=item.view&id=291047828269 per Poleman's recommendation. I purchased 2 sets of bits for the project because the first set wasn't measuring out very well, and the .30mm bit was more like .34mm. When the 2nd set came in not much better, I ended up drilling the main to 0.33mm (due to lack of a 0.30), auxillary to 0.35mm, and mixing chamber hole to 0.55mm. I set both H & L screws to one turn out, and the saw fired right and stayed running. While letting it warm up, I blipped the throttle a few times and thought throttle response seemed pretty good. After I felt like it was warm, I started to evaluate the idle. It was idling a little fast and wanted to turn the chain very slightly so I first attempted to slow the idle with the idle screw. It didn't matter how far out I turned the idle screw, it would not reduce the RPM. At that point, I started to back out the L screw to richen it up, which did reduce the RPM and stop all chain movement at about 1.5 turns out. Throttle response still seemed about the same (good). Idle seemed a little rough to me but I played with the screw in both directions and ended up leaving it at 1.5 turns out. I haven't played with the H screw much other than to lean it out enough to where I thought it sounded like it was hitting the limiter. Poleman, what are you thoughts on my situation with the idle screw being backed out and still haveing to use the L to reduce engine speed to stop the chain completely? It was 80+ degree last night, and humid here in northern IL, so I assume I'll have to richen her up more come cold weather . Does having the low at 1.5 out sound at all close to what you've had to set yours at on a stock 7900? I would have thought that drilling the main to .33 would have pushed it towards the rich side vs the lean side. Seems I'm not lacking air.


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## Poleman (Jul 21, 2015)

Good report!!! It does happen sometimes where a certain carb/saw combination is a struggle. When you mod these the idle sometimes is a bit funky but not unusable. You did good at richening the L to help adjust idle. That happens sometimes on ported saws sometimes also.
The only thing that doesn't seem right is you are getting enough air without the throttle plate hole and you should be rich with out it.
I'm thinking possibly you may have an air leak. Your carb should be just the opposite.....with your idle screw all the out to fast idle instead of all the way in. In my case if I don't drill the throttle plate I don't have enough screw to make it idle by giving it more air at idle. After the throttle is cracked the hole is not required as air is then supplied through the ventura along with more fuel.
The only time I had a problem like yours was I got in a hurry swapping carbs and forgot to tighten the carb attaching screws......ran great but wouldn't idle down. Another thing to check it the throttle cable....make sure it's seated in its pocket or it will pull the throttle open slightly enough to cause a fast unadjustable idle.


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## acp104 (Jul 21, 2015)

What I call the idle screw (large one above L & H) should increase engine speed when turned in clockwise as the taper of the screw moves the throttle linkage/butterfly opened correct? And in my case, backing it out so that the taper is not engaging the throttle linkage is necessary to keep the idle speed minimized. Even then, I need to have the low backed out 1.5 turns to keep engine speed from engaging the clutch and turning the chain a little. I think I'll loosen up the attachment screws this afternoon and re-tighten them a little looser because if anything, I may have tightened them too much last night. I doubt it, but just wondering if being a little too tight could distort the sealing surface and allow it to suck a little air. I'm not hopeful, but it'd be nice if that made a difference. One thing I should also mention is that when first attempting to drill the mixing chamber hole, I started to drill the one at the very edge/corner of the hole that goes down at an angle. I wasn't using any magnification, and saw that hole as the one closest to the edge so I started to drill it. I hadn't noticed that it went in at an angle so I started using the pin/drill to drill straight down like the others. I drilled a bit before realizing it was actually an angled hole and I was just drilling straight down into the casting. I didn't drill through, so was hoping no harm done. I then moved to the vertical hole closest to the edge and drilled my .55 hole. Any thoughts about negative effects of my error?


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## Poleman (Jul 21, 2015)

No I don't believe there are any Ill effects from the drilling error...be Suprised if there was.​Good thought about possibly over tightening and distorting the plastic flag around the boot.
Is your idle screw clear down to the threads on the screw where there is no taper? Can you see if the throttle plate is cracked open at idle letting in more air? Can you push the throttle arm where the cable attaches down more? Is the plastic cable piece on the carb secured and flat with its tab and squeeze attachment securely into the throttle arm?

Just trying to think of every possible cause.....

Also another tip.....you can JB weld the jets, carb body and redrilling if need be...just let it set over night. I use JB weld to close the Hi jet orifice behind the low .30 jet. I find anything over .40 it needs to be closed or saw will run to rich at wot only getting to about 12,8k or so, and that with the H jet totally screwed in tight. Plug it and it's fully adjustable again. JB Weld is my friend!!!


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## acp104 (Jul 21, 2015)

Yes, I believe I've got the idle screw backed out so that the throttle arm is just sitting on the threads with no taper. I will look at it closer tonight and try to determine if any of the things you mentioned are causing the throttle plate to stay opened a little bit. What do you think about me drilling the main jet to .33 since that was the smallest bit I seemed to have? If anything, I gather that going larger with the main would be causing just the opposite of what I'm experiencing, and the need for drilling a hole in the plate to get more air. I did have to push the main jet membrane back in as you cautioned as I must have pushed through. I'm guessing I went too far with the auxillary as well, but obviously didn't have the luxury of viewing it from the opposite side like the main. Hopefully something didn't happen to the membrane under the auxillary causing it to block the jet. Guessing that probably isn't the case either based on some of your other comments in this thread.


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## Poleman (Jul 21, 2015)

No I don't think the .33 is causing the problems your having, in fact you are spot on thinking it should have the reverse effect like you stated. I always use a caliper on the drills before I use them to make sure of the sizes. They are like you say....a .30 is .33 etc. I've found the same issues and do the closest to what I'm wanting.
I glad you caught the pushed out membrain, that will cause a headache if not caught. I've also only had the main do it and not the auxiliary.
I've also noticed the carbs I've been using has the jet sticking out a little higher making it easier to pull and a redesigned jet with a more closed back helping this situation. It seems to be ok if you drill a hole in the membrain just not push it loose as it comes loose and lodges in the back two holes behind the on that gets drilled....least from my experiance.
Hopefully you'll check things over and find something simple, then be able to proceed from there. Your looking for you idle screw to be topped out and not on the threads. Then you just drill a small hole (.8mm-1.2mm) to straighten the idle out...that the usuall.


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## acp104 (Jul 21, 2015)

The carb I got is the last one you recommended in this thread that you indicated had a more prominent jet and you liked. I had no trouble pulling the jet with a small thin pair of side/wire cutters. They got a good close bite, and I was able to rock it out with just a couple back and forth motions.


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## Derf (Jul 21, 2015)

Is that how I pull out the main jet on the walbro hd12? Just grab it through the Venturi with some pliers and wiggle it out? I thought I'd have to push it from behind. Any risk of damage grabbing it too tightly?


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## Poleman (Jul 21, 2015)

I'm thinking it would be best to tap it out instead of grab it..


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## blsnelling (Aug 26, 2015)

I have not followed these carb threads, but have been reading up the last couple of days. I'm interested as I'll be porting a new 7910.

Help me understand something. I can see how these mods would make a difference at partial throttle poor a very flow transition from idle to WOT. I don't understand how they could make much of a difference of going to WOT instantly. As I understand it, your not altering the total fuel supply at WOT, just altering the fuel curve at partial throttle, as it transition from idle to WOT. Help me understand.


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## Poleman (Aug 26, 2015)

Brad, 
I riches the lower off idle through midrange curve giving more fuel to increase torque.....on a stock saw.
On a ported saw with the larger mod .040 aux jet it does affect the H. I find to get adjustability out of the H I need to JB weld the bleed inside on the H jet. That will give you adjustability back as the saw is usually running rich at WOT and need to be leaned but in most cases the H is already totally closed not allowing saw to be leaned out and unable to get it to ideal rpm's.


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## blsnelling (Aug 26, 2015)

I understand that these mods will affect where the H needle will end up, but total fuel at WOT will not change. So back to my original question, how does this mod help if transition from idle to WOT is instant? Your instantly flowing from all jets.


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## Poleman (Aug 26, 2015)

Not sure on that one but, figuring from idle to wot it still has to transition through circut system and probably flowing through all as you state.

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## blsnelling (Aug 26, 2015)

BTW, I'm not trying to criticize your work, just trying to understand it.


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## Terry Syd (Aug 26, 2015)

Fuel flow is dependant upon air flow, that's what carburettors do, meter the fuel to the amount of air going through it. - Just because the throttle valve may be wide open, it doesn't mean that fuel automatically goes full on.

There are two circuits in the carb, the high speed circuit through the venturi and the low speed circuit through the straight section of the carb - each circuit has it's own flow characteristics depending upon the pressure differential working on the circuit. The result is that the two circuits (one venturi, the other straight section) have two different fuel flow curves based upon the rate of air flow.

The mod increases the low speed circuit flow so that the engine can run richer when the engine is loaded down, that is, when the air speed through the carb is slower.

It appears that the factories were 'persuaded' to decrease the fuel flow (lean it out) in the low speed circuit in order to reduce emissions. At lower RPM there is a greater tendency for mixture to escape out the exhaust port. By leaning out the low speed circuit they could cut emissions at those lower speeds AND keep the operator of the saw using the engine at a higher speed (because the engine lost low speed torque).

The mod brings back that lost low end torque.


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## Poleman (Aug 26, 2015)

I know....not reading it that way. I'm not sure I totally understand what's all going on with the carb through transitions and operating conditions. I'm just telling you what I do know......and sadly what I dont...lol


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## Poleman (Aug 26, 2015)

Terry to the rescue!!!!! THANK YOU!!!!
Good explanation!!


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## blsnelling (Aug 26, 2015)

Thanks Terry. That's exactly the explanation I was looking for. Makes perfect sense now.


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## CR888 (Aug 26, 2015)

Terry explained that simply and well. The high speed starts flowing fuel well after the low as the pressure changes and air velocity increases....doing these mods gives better fuel delivery before the transition to wot running conditions. While Graham Bell has a whole chapter on carburation, he explains this subject well on pages 124 and 125. He basically does this exact same mod to mikunis giving bikes/carts better bottom end power for tight tracks where power out of the corners can be improved.


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## Terry Syd (Aug 27, 2015)

If someone was just 'cookie cutting' and only using the top part of the powerband, then they probably wouldn't think the mod meant that much. However, if you are a wood cutter, then the mod makes a BIG difference.

Brad, if you want to see how much the mod changes the powerband, try this. Have a modded carb on the saw and then do about an hour of serious wood cutting. Then put on a stock carb and try to continue working. My bet is that within 5 minutes you'd stop and put the modded carb back on.

All that extra torque does make a difference in the real world of wood cutting - it's like having a bigger saw to cut with.


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## weedkilla (Aug 27, 2015)

Terry Syd said:


> If someone was just 'cookie cutting' and only using the top part of the powerband, then they probably wouldn't think the mod meant that much. However, if you are a wood cutter, then the mod makes a BIG difference.
> 
> Brad, if you want to see how much the mod changes the powerband, try this. Have a modded carb on the saw and then do about an hour of serious wood cutting. Then put on a stock carb and try to continue working. My bet is that within 5 minutes you'd stop and put the modded carb back on.
> 
> All that extra torque does make a difference in the real world of wood cutting - it's like having a bigger saw to cut with.


I'm certainly not arguing - but I'd like to add a bit to this explanation. 

There are two main times I notice this aspect of a saws performance, when I'm felling and you are feeling the saw up to the hinge. You can be full bar buried and start and stop a cut a few times. 
And when I'm in a tree, feeling my way up to the point a branch will lean over, trying to get it to peel back the bark and drop vertically into a little hole below. 
I blocked a tree down like this today, and if I couldn't start and stop easily in the cut it would have been a painful day. 


Thanks guys, plenty of us reading and learning and mostly keeping our mouth shut and our ears open.


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## Terry Syd (Aug 28, 2015)

It's not just the increase in throttle response and power off the bottom of the powerband, it is the increase in peak torque that makes the saw feel like it is a bigger saw. As the dyno runs I linked to in the thread indicated, instead of a torque PEAK there is a torque DIP on many of these saws.

What is happening is that the low speed circuit flow is petering out before the high speed circuit can pick up the slack. When the low speed circuit is opened up, the low speed fuel curve extends further up the RPM range and the 'dip' is eliminated.


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