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

[Vintage Engine Repairs]

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.

View attachment 1216330

View attachment 1216331

The accelerator pump is fed fuel and injects it, not by the L circuit, but by the high. As the trigger is released the spring returns the plunger and creates vacuum on the h circuit at the main nozzle. The check valve seals and can then draw fuel through the high speed drilling (bypass jet if present too) . As the trigger is depressed the piston pushes the fuel into the venturi via the main nozzle.

A little brake cleaner into the piston bore will loosen up the oil that is causing it to stick to the seat.

Skip to 10.30 for an understanding how it works:

 

[GMB74]

Re: accelerator pump.
In this video, Tom explains how the accelerator pump circuit works. He is showing it on a primer bulb carb, but the concept is the same on a non-primer bulb carb like yours.
The problem is air is drawn in to the circuit past a faulty o-ring. In the video he blocks off the pump to cure the problem. Another solution, maybe better is to replace the o-ring to cure the leak.
The second link discusses this. Note that he is stating the ID of the o-ring x the Cross Section.
O-rings can be found cheap on ebay once you know the size you need.


https://www.arboristsite.com/threads/accelerator-pump-o-rings-carburetor-lookup.374153/

Edit: my video link doesn't work and Tom beat me to it anyhow.

 

[Vintage Engine Repairs]

Re: accelerator pump.
In this video, Tom explains how the accelerator pump circuit works. He is showing it on a primer bulb carb, but the concept is the same on a non-primer bulb carb like yours.
The problem is air is drawn in to the circuit past a faulty o-ring. In the video he blocks off the pump to cure the problem. Another solution, maybe better is to replace the o-ring to cure the leak.
The second link discusses this. Note that he is stating the ID of the o-ring x the Cross Section.
O-rings can be found cheap on ebay once you know the size you need.


https://www.arboristsite.com/threads/accelerator-pump-o-rings-carburetor-lookup.374153/

Edit: my video link doesn't work and Tom beat me to it anyhow.

Thanks for sharing none the less

 

[SteveSr]

Well, if the acc pump feeds the main nozzle then if air got into the circuit it would cause the lean at WOT symptom an no "H" adjustability.

However, this doesn't explain why the "L" circuit is also apparently lean at the factory "L" setting but can be "fixed" by opening the screw.

The o-ring broke into 2 pieces when I tried to remove it from the piston. I measured the pump piston diameter at 4.93mm and the o-ring groove at 3.20mm X 1.27mm. So is this the "standard" M3x1 o-ring or something else?

I also tried again to get the screen out using brake cleaner and air pressure but it is stubbornly stuck. I was hoping to be able to see the passages and drillings that Tom mentioned. I can't see any drillings on this side of the screen.

BTW, what is the purpose of the screen? Some kind of capillary check valve I assume?

 

[Vintage Engine Repairs]

Well, if the acc pump feeds the main nozzle then if air got into the circuit it would cause the lean at WOT symptom an no "H" adjustability.

However, this doesn't explain why the "L" circuit is also apparently lean at the factory "L" setting but can be "fixed" by opening the screw.

The o-ring broke into 2 pieces when I tried to remove it from the piston. I measured the pump piston diameter at 4.93mm and the o-ring groove at 3.20mm X 1.27mm. So is this the "standard" M3x1 o-ring or something else?

I also tried again to get the screen out using brake cleaner and air pressure but it is stubbornly stuck. I was hoping to be able to see the passages and drillings that Tom mentioned. I can't see any drillings on this side of the screen.

BTW, what is the purpose of the screen? Some kind of capillary check valve I assume?

Have you even confirmed with certainty that the carburetor is the right one for the machine?

 

[SteveSr]

Have you even confirmed with certainty that the carburetor is the right one for the machine?

I have no way to confirm it. Husky plays "musical carburetors" just like Stihl. Some make it into the IPLs some don't

When you get a non-running piece of equipment do you assume that the it doesn't have the correct carb on it?

 

[J D]

I believe the series (36, 41, 136, 141, etc) were released with half a dozen different Walbro WT carbs & also the Zama C1Q-W29E

 

[GMB74]

I had a 141 with the Zama C1Q-W29E carburetor and it ran fine. In fact it started and ran great.
I also owned a 142 with a W29E.
I found the W29E listed in an IPL for Husky 142. The only difference between a 141 and 142 is the latter has a catalytic muffler and an easy-pull starter. They are identical otherwise.
As stated above, Husky used several different carbs over the years, subbing whatever they had available currently to replace the older carb number. They all work fine, assuming there are no defects with the specific carb in question.
So yes, this Zama carb is correct for this saw.

 

[stihltech]

Nope, no check valve on the accelerator pump. Just an Oring and plunger. The air enters when the oring leaks. To check, hold the plunger hole op, spray something in the hole to fill it. Work the throttle lever. It should NOT suck and of the fluid in the hole.

 

[SteveSr]

Nope, no check valve on the accelerator pump. Just an Oring and plunger. The air enters when the oring leaks. To check, hold the plunger hole op, spray something in the hole to fill it. Work the throttle lever. It should NOT suck and of the fluid in the hole.

So whats the purpose of the mesh screen if not a capillary check valve?

 

[dboyd351]

So whats the purpose of the mesh screen if not a capillary check valve?

To filter out anything that got past the fuel tank filter before it gums up the carb.

 

[SteveSr]

To filter out anything that got past the fuel tank filter before it gums up the carb.

Wrong mesh screen... The accelerator pump is already on the "clean" side of the main filter screen.

 

[Vintage Engine Repairs]

It’s just a strainer. Stops wear on the o-ring if debris does make it past into the metering chamber.

 

[SteveSr]

I decided to go ahead and order a new carb just in case I destroyed (or couldn't repair) the original. Here is what showed up...

It appears to be identical to the original with the exception of the markings on the side. It would appear that Zama has changed t heir part numbering system. Has anyone figured it out?

Interesting that it is also labeled "W29E" just like the old one.

I am still working on getting some o-rings to see if the acc pump was the root cause of the issues with the old carb.

 

[trains]

my money is on the accelerator pump o ring being the culprit.
Fixed many issues with odd running like you describe with the Accelerator pump o ring, often its intermittent too just to make it more fun.

 

[Old2stroke]

Hard to believe a worn o-ring would leak enough air to cause this wide spectrum fault. Missing ring maybe.

 

[SteveSr]

my money is on the accelerator pump o ring being the culprit.

Will find out as soon as I get some replacement rings.

 

[SteveSr]

Hard to believe a worn o-ring would leak enough air to cause this wide spectrum fault. Missing ring maybe.

Ring was there.

 

[trains]

Hard to believe a worn o-ring would leak enough air to cause this wide spectrum fault. Missing ring maybe.

Dosent take much to mess up the internal flow of fuel / air when your relying on pressure differences for it all to work correctly.

 

[J D]

I had a carb I put to the side a while ago I suspected had an issue with the accelerator pump... decided to pull it apart yesterday. The primer would suck air & it was impossible to tune. First indication the O-ring wasn’t sealing was the fact the little piston would fall into/out of the hole with no resistance. To confirm it was leaking I removed the spring & placed the piston in the hole upside down (so the O-ring sat where it should to seal). I then spraywd soapy water into the hole & pumped the primer. The soapy water was sucked past the seal & into the primer bulb so it clearly wasn't working. On closer inspection the O-ring was cracking & hard as a rock.
I rummaged through a few other carbs & found another C1Q carb that appeared to be identical with the exception of the accelerator pump piston being smaller. Got me wondering if there are common sizes for these (& therefore common O-ring sizes to keep on hand). Original carb was 5mm piston, 5.5mm bore. Second carb was 4mm piston, 4.5mm bore.
Curious to know what size O-rings you've come across Tom