Time To Play... Diagnose This! - Internal Carb Failure

[ballisticdoughnut]

C1Q-W29E IPL incase anyone is curious.

 

[Old2stroke]

Sounds like an overall lean condition as though there is a mysterious air leak somewhere. How about a wrong carb that is just too small for that saw?

 

[Mike Ray]

when I worked for the power co. they replaced my bush axe with a husky 41, to my knowledge myself or anyone else ever got it cranked went back to the bush axe.

 

[mojo jojo]

First thing to do, is find another carb that you know works fine and fits the saw. Install it and run it. That way you will know either your problem is related to carb or to other saw components.
If you confirm that your carb is an issue, and it passes every standard carb test, then just throw it away. If you dont know its past, it could be a broken or wrong H/L needles installed, damaged needle seats, it could leak fresh air thru the throttle plate rod being very worn out, could be wrong carb kit installed (some carb kits include different membranes that fit different types of carbs and you dont use all), maybe you installed membranes and gaskets the wrong way, maybe the carb is faulty and leaking around fuel inlet barb, maybe you forgot to install mesh screen in it...there is a million of little things that could go wrong.
You said your spark plug is black? That is not a sign of a lean condition. Is it black and dry, or black and moist?

I have a more interesting problem with one of my old Tillotson carb, but I will wait until you resolve your problem in this thread.

 

[Chevboy0167]

 

[GMB74]

W29 Zama carb should be correct for this saw. They used a ton of different carbs on the 36/136/137/41/141/142 engines and their Poulan/Jonsered cousins. It will make you dizzy trying to follow the superceded numbers, but they all fit and should work.
Do you have a leaking accelerator pump o-ring? That will drive you nuts.

 

[SteveSr]

Once you have, come back here with a full back story on it mate. Where you got it, why you got it, when the troubles started, what the previous owner said etc.

Unfortunately, the P.O. is deceased and dead men tell no tales! The saw was donated to our trail club by his widow.

From there - start pressure testing it, both metering side and pump side.

On deck next round before I remove it from the saw.

Test the main nozzle check valve holds or drops no quicker than 7 psi over 5 seconds.

How do you check this and come up with numbers? The only test that I have ever seen is with a piece of hose and blow/suck on the "H" circuit and listen for the check valve to click each way. Since these are FRP I wouldn't expect them to seal completely when closed whereas some of the old rubber discs might.

Confirm lever height is set and the seals for the needles if present are in good order.

Done and verified.

 

[SteveSr]

I got a chance to run it again this afternoon and run the "Brake Cleaner" manifold test. The idle did slow down when I accidentally hit the throttle shaft but t hen it is not really sealed and will leak a little bit.

I am letting the bar oil drip off before I remove the carb for more diagnosis of it directly.

Here is one current theory... The main nozzle check valve is stuck mostly closed. This is creating an air leak at idle which requires an increased "L" setting to compensate for. Because it is stuck it is also not opening enough to run correctly at WOT. This may be why it is lean at WOT and not sensitive to the "H" screw.

Your thoughts? Please be gentle!

 

[dboyd351]

It has not been thorough the sonic tank.

Any reason why not?

I have had carbs that looked spotless, but would not work. Put them in the USC, grey clouds immediately poured from the carb as soon as it was turned on. Carb worked fine after USC and reassembled with a carb kit.

 

[Old2stroke]

I got a chance to run it again this afternoon and run the "Brake Cleaner" manifold test. The idle did slow down when I accidentally hit the throttle shaft but t hen it is not really sealed and will leak a little bit.

I am letting the bar oil drip off before I remove the carb for more diagnosis of it directly.

Here is one current theory... The main nozzle check valve is stuck mostly closed. This is creating an air leak at idle which requires an increased "L" setting to compensate for. Because it is stuck it is also not opening enough to run correctly at WOT. This may be why it is lean at WOT and not sensitive to the "H" screw.

Your thoughts? Please be gentle!

Best theory yet

 

[SteveSr]

Well, I took the carb off and did some more testing.

1. The carb passed the fuel inlet pressure test.
2. The carb passed the impulse port pressure test.

I removed the metering cover and the "H" screw. I connected a hose adapter from the Mityvac and a piece of hose to the "H" screw hole. With my finger over the feed hole in the metering chamber I could blow and suck on the hose and hear the check valve move.

It seemed to flow less by vacuum but it was not a tight seal. I couldn't pull a vacuum with the Mityvac. Since this is just a disc I am not sure if it supposed to seal tightly. So this might be the failure (air leak) on the "L" circuit but doesn't explain the "H" circuit being lean.

Any suggestions on how to test/verify the accelerator pump?

 

[J D]

A good start would be to check out this from Tom (@Vintage Engine Repairs)