Dolmar 7900 Carb upgrade?

[Fly By Night]

What is?

 

[one.man.band]

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.

 

[Fly By Night]

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.

 

[Poleman]

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.

 

[Terry Syd]

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.

 

[Poleman]

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.

 

[Terry Syd]

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.

 

[Fly By Night]

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

 

[one.man.band]

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.

 

[one.man.band]

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?

 

[Terry Syd]

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.

 

[Paragon Builder]

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.

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?

 

[Terry Syd]

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.

 

[Poleman]

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?

 

[Terry Syd]

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.

 

[one.man.band]

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.

 

[one.man.band]

poleman.....experimenting w/electrode gap may be easier than the timing thing?

 

[Poleman]

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.

 

[Fly By Night]

What is this about electrode gap and timing one man?

 

[one.man.band]

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.