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

[SteveSr]

Hello,

Here is a little challenge for your diagnostic noggin on a Saturday afternoon!

The Saw: Husky 41 with new OEM impulse tube, manifold, OEM fuel line, new OEM fuel pickup. Muffler screen is installed and clear. Piston looks good through exhaust port. Engine passes BOTH pressure and Vacuum. Fuel is fresh from the pump 50:1 Non-E with Red Armor. The serial number is 6415588.

The Carb: The saw was found with a Zama C1Q-29E. The carb was disassembled and cleaned with aerosol carb cleaner (Berryman's B-12) and a new OEM kit was installed. I couldn't find this carb model in any of the IPLs but may be similar to a Walbro WT-239 which was listed in later versions of this saw.

Carb Settings - As Found:
"L" = 2 1/2
"H" = 2 1/8
These are the basic settings listed for the WT-239 in the Rev 2 - 12/1995 Operators manual.

Symptoms (i.e. Clues!):
1. Saw will start and run on part throttle but immediately bog when trying to accelerate
2. Saw will not idle and just dies if you release the part throttle lock.
3. If I adjust the "L" screw richer (3 1/8 turn) the saw will idle.
4. The idle is responsive to tuning about the 3 1/8 setting.
5. At this new "L" setting the saw will still bog off idle but you can get it to transition to WOT.
6. WOT tachs out at 13,400 (Max is 13,000) and it sounds like it is 4-stroking at this RPM.
7. Adjusting the "H" screw has little/no effect on the WOT speed.
8. After attempting to tune the saw I removed the spark plug and it was black.

Let me know if I can answer any questions or clarify anything.

Your thoughts on the carb defect(s) that might be causing this behavior. Please justify your answer.

Thanks,
Steve

 

[J D]

Is it possible to transpose the L & H screws on those?
Maybe still some crap in the circuit somewhere... I've had carbs I'd swear I'd cleaned perfectly twice over that then magically fixed themselves after a trip through the ultrasonic

 

[ballisticdoughnut]

Kinda sounds like a bad or missing check valve. Easy to pop them out with carb spray or compressed air. Possibly a missing Welch plug?

 

[Old2stroke]

Did you verify that the transfer ports were open?

 

[singinwoodwackr]

Try starting with 1 turn out, each

 

[Old2stroke]

As mentioned, more likely the nozzle check valve stuck open

 

[SteveSr]

Is it possible to transpose the L & H screws on those?

Great question. Yes! I put them back in the same holes that they came out of. The "L" screw was a little longer and had a finer/thinner needle at the tip

Maybe still some crap in the circuit somewhere... I've had carbs I'd swear I'd cleaned perfectly twice over that then magically fixed themselves after a trip through the ultrasonic

It has not been thorough the sonic tank.

 

[SteveSr]

Kinda sounds like a bad or missing check valve. Easy to pop them out with carb spray or compressed air.

Most of the later carbs used a fiberglass reinforced disc. I have never lost one due to aerosol carb cleaner. Not enough pressure.

Compressed air has not been used on the carb.

Possibly a missing Welch plug?

Welch plug is installed.

 

[SteveSr]

Did you verify that the transfer ports were open?

Yes.

 

[SteveSr]

As mentioned, more likely the nozzle check valve stuck open

If this is true then why is the saw apparently lean at WOT (too high max speed) and the "H" screw has no effect.

 

[SteveSr]

Try starting with 1 turn out, each

Well, it wouldn't idle at L=2.5 too lean and it already exceeds max speed at WOT again too lean. Although I haven't tried 1T I don't think that it would be productive.

 

[cookies]

Might be worth trying lifting the needle arm and applying a tiny bit of aviation sealant on the needle threads, there could be a ridge of crap under the check valves disk preventing a good seal at higher rpm. Also possible crank bearings are flexing causing vac leakage, rings could be sticking or the ring lands worn. Some of the carb kits have had different buttons on the diaphragms, thickness variances in gaskets, bad needles, bad levers, incorrect springs all causing issues. Did you pressure test the carb on the fuel inlet side? I have found leaking covers more than once doing this, replacing them solved the issue
I have been fighting a carb that fails to properly fuel the H side off and on for 3 weeks...carb was discontinued years ago with no aftermarket support. Honestly I am quickly moving away from chinese carbs after all of the frustration from wasted time waiting on shipping to receive and swap out bad carbs, returning them only to buy a oem or direct manufacturer replacement to fix it right.

hope this helps

https://jackssmallengines.com/jacks-parts-lookup/part/husqvarna/545013503

 

[ballisticdoughnut]

Most of the later carbs used a fiberglass reinforced disc. I have never lost one due to aerosol carb cleaner. Not enough pressure.

Compressed air has not been used on the carb.


Welch plug is installed.

True. They’re made of teflon with fiberglass reinforcement. Having said that they are still quite thin and flexible. Carb cleaner sprayed directly into the nozzle via high speed circuit can absolutely pop them out, I’ve done it myself. If you’ve verified it’s still in place then obviously you can rule that out. Have you pressure tested the carb?

 

[PV Hiker]

Float level and needle not working properly?

Carb not sealing to manifold properly?

 

[SteveSr]

True. They’re made of teflon with fiberglass reinforcement. Having said that they are still quite thin and flexible. Carb cleaner sprayed directly into the nozzle via high speed circuit can absolutely pop them out, I’ve done it myself. If you’ve verified it’s still in place then obviously you can rule that out.

I have not verified that the disc is still in there. However, if it was completely AWOL I wouldn't think that I could get it to idle at all and it shouldn't be lean at WOT.

Have you pressure tested the carb?

Good call, not yet. Usually leaking needle and seat will cause flooding or over rich conditions what I am seeing appears to be lean.

I am surprised that no one has asked but by the Zama parts list the W29E has an accelerator pump. I don't know if this could factor into the symptoms or not.

 

[SteveSr]

Float level and needle not working properly?

It is not flooding and responsive to the "L" screw so I think so but will check it next chance I get.

Carb not sealing to manifold properly?

Pretty simple setup but I'll get out the brake cleaner next time I run it. Tomorrow or Monday.

 

[PV Hiker]

I am surprised that no one has asked but by the Zama parts list the W29E has an accelerator pump. I don't know if this could factor into the symptoms or not.

Looking at Google search it could be a issue.

 

[J D]

Any issues with accelerator pumps that I've struck affected the the priming & L side of things. That said it may allow air to bleed into the circuit causing a lean condition... as would a gasket leak, or an adjustment screw that wasn't sealing properly. Check for a partially obstructed impulse too. I had an impulse barb come loose & leak on a carb once... that was fun to track down.
Tom @Vintage Engine Repairs may have some worthwhile input

 

[Vintage Engine Repairs]

The Carb: The saw was found with a Zama C1Q-29E. The carb was disassembled and cleaned with aerosol carb cleaner (Berryman's B-12) and a new OEM kit was installed. I couldn't find this carb model in any of the IPLs but may be similar to a Walbro WT-239 was listed in later versions of this saw.

Carb Settings - As Found:
"L" = 2 1/2
"H" = 2 1/8
These are the basic settings listed for the WT-239 in the Rev 2 - 12/1995 Operators manual.
.

Thanks,
Steve

Steve, re read what you wrote that I have highlighted in red… don’t spend any more time trying to get that carb working before you have confirmed and are certain it’s the right one for the machine…

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.

From there - start pressure testing it, both metering side and pump side. Test the main nozzle check valve holds or drops no quicker than 7 psi over 5 seconds. Confirm lever height is set and the seals for the needles if present are in good order.

 

[singinwoodwackr]

Well, it wouldn't idle at L=2.5 too lean and it already exceeds max speed at WOT again too lean. Although I haven't tried 1T I don't think that it would be productive.

Reason I suggested it is I’ve never had a saw that needed 2+ turns out other than some Zama carbs that were problematic to begin with.