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 It will make you dizzy trying to follow the superceded numbers, but they all fit and should work.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)

 

[Vintage Engine Repairs]

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

Thanks JD, I appreciate you sharing the video!

 

[SteveSr]

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

Tom,

You really need to change the title of that carb "tools" video to what it really shows... testing internal carb check valves.

Also do you have any thoughts on testing for accelerator pump leaks on non-purge bulb carbs? I am having a hard time visualizing the fuel path for the pump. Where does the input come from and where does the output go?

 

[J D]

Tom,

You really need to change the title of that carb "tools" video to what it really shows... testing internal carb check valves.

Also do you have any thoughts on testing for accelerator pump leaks on non-purge bulb carbs? I am having a hard time visualizing the fuel path for the pump. Where does the input come from and where does the output go?

Some people "delete" the accelerator pump by pluging the internal side with rtv. For testing purposes I'd say just packing it up with grease from the outside should give you a reasonable indication

 

[Chevboy0167]

https://zamacorp.com/support-as243.html

https://zamacorp.com/disassemblyservicing-as250.html

https://www.arboristsite.com/threads/accelerator-pumps-on-zama-carbs.147764/

 

[SteveSr]

https://zamacorp.com/support-as243.html

https://zamacorp.com/disassemblyservicing-as250.html

https://www.arboristsite.com/threads/accelerator-pumps-on-zama-carbs.147764/

Thanks for the links.

 

[TheTone]

I think Tom's (@vintage Engine Repair) suggestion to check if the carb is correct for the saw is spot on. The initial carb needle settings may be right for a WT-239 but are they right for a Zama C1Q-29E and is that carb right for the saw? Are you sure you mean 4-stroking at 13,400 rpm? That would go against being too lean as would the black spark plug.

 

[SteveSr]

Sorry for the delay. I am juggling a lot of projects at the moment.

I actually had a venturi pressurization clamp as shown in Tom's video. I use it mostly for checking for welch plug leaks.

However, I used it to run the main nozzle check valve test and surprisingly it passed when wet with fuel. So that would tend to rule that out as the primary cause of the observed symptoms.

This leaves the accelerator pump. So I removed the throttle shaft and the pump plunge and spring. The screen is still in there and doesn't want to come out even with air in the "L" needle hole. I was hoping to remove it so that I could look for drillings to understand how the pump is supposed to work.

So what I am trying to figure out is how air could get in to the "L" circuit through a bad ACC pump. Somewhere there should be a one-way check valve to prevent this from happening.

So I am wondering where the check valve is hiding. I did notice what appears to be a plug (possible check valve?) in the air filter face of the carb. Is the check valve hiding under here?

Here are a couple of photos of the metering chamber and the AF face welch plug. Note that the only feed holes holes in the metering chamber go directly to the "H" and "L" circuits respectively. This may account for the larger openings on the screws since ALL fuel has to go through these feed holes.

 

[SteveSr]

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.

Well, I normally reserve the US tank fo carbs with obvious dirt/crud issues upon disassembly.

I have had the US tank ruin carbs by vibrating welch plugs loose and it may not be kind to the fiberglass main nozzle check valves.