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.
Dolmar 7900 Carb upgrade?
- Thread starter plowin-fire
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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.
plowin-fire
ArboristSite Operative
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.
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...
importjunk
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Terry Syd
Addicted to ArboristSite
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.
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!
Terry Syd
Addicted to ArboristSite
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.
Terry Syd
Addicted to ArboristSite
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.
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.
Terry Syd
Addicted to ArboristSite
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.
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.
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
Chris-PA
Where the Wild Things Are
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.
Terry Syd
Addicted to ArboristSite
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.
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.
Terry Syd
Addicted to ArboristSite
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.
Chris-PA
Where the Wild Things Are
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.
Terry Syd
Addicted to ArboristSite
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.
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.
Chris-PA
Where the Wild Things Are
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.
