Dolmar 7900 Carb upgrade?

[Terry Syd]

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.

 

[Andyshine77]

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

 

[Terry Syd]

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.

 

[Andyshine77]

Okay got it.

 

[Chris-PA]

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.

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.

 

[Terry Syd]

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

 

[Chris-PA]

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

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.

 

[makguy]

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?

 

[Terry Syd]

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

 

[makguy]

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?

 

[Gravedigger]

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?

Here's a link to put an unlimited coil on

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

 

[makguy]

Thanks!

 

[Poleman]

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!!

 

[Terry Syd]

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.

 

[Poleman]

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

 

[Terry Syd]

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

 

[CR888]

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

 

[Poleman]

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

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

 

[Gravedigger]

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


Sent from my iPad using Tapatalk

 

[Terry Syd]

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.