Saw dies after 8-10 seconds of idle time

This is a new one for me. I have a saw that will start easily and rev just fine. However, if I leave it to just idling after 8 or 10 seconds it will slow and die. It'll fire right back up, but will repeat the slow & die routine again and again. It doesn't have low compression and acts differently than a saw with poor compression as it's been my experience they won't idle for even a few seconds. Here's what I can tell you about it:

• Piston & cylinder are in decent condition (has new Caber rings)
• Passes vacuum and pressure tests on the engine
• Passes vacuum and pressure tests on the fuel line, tank, and tank vent
• Makes about 155 psi compression
• L screw is set to about 1 turn out
• Running the LA screw in to speed it up does not fix the problem
• I have not cleaned or rebuilt the carb
• I have not pressure tested the carb

That's all I can think of for now. I welcome suggestions and questions. Thanks.

 

What kind of saw? Impulse line? Plugged exhaust port/spark screen. I'd be thinking carb rebuild - weak/stiff diaphragms not keeping up with crankcase pulse perhaps. You'd think these issues wouldn't lead to good revving though. Will it rev over and over for a long time and only quit at idle? Cheers!

 

Maybe a dirty fuel filter. Just a guess snd its a cheap place to start

 

Stihl 064. Very recently replaced impulse line (which is also tested during the p/v test). No screen in the exhaust, and a clogged screen has always presented with lack of high end revs for me, not idle issues. It will rev and run at anything above idle just fine. Thanks for the questions/suggestions.

 

Hmmm......I guess I'd lean toward the carb - partially plugged jet? Cheers!

 

Good suggestion, but it's a recently replaced fuel filter, and that has always been a rev/high throttle presentation for me as opposed to idle.

 

That's a valid thought. I've adjusted the L a bit to both sides with no changes. It doesn't sound/feel lean or rich on the L side though; the idle is 'perfect' for a handful of seconds and then it just slows down and dies. The best way I can describe it is if you just pulled the fuel line from the carb; doesn't sputter like it's running out of gas, just slows and stops like the fuel supply stopped all-together...

 

Leaking welch plug.

Leaking check valve--- I have seen this a good bit in some of the C3 carbs.

 

The air filter has been upgraded to an HD2 and it's clean as can be; no prefilter.

 

Could be. Due to limited time out in the shop the carb is the only thing I haven't exhausted as a possibility based on my knowledge.

What is a "C3" carb?

 

You mentioned tank vent, a clogged vent will cause this type of problem.

 

How about some details on how one would recognize or diagnose these conditions?

 

When you first start the saw with choke on, then do your revs warming the saw, then it dies at idle, will it start warm again at normal idle position or do you always need to choke it? My thought is once you are opening the throttle plate and vacuum starts to draw from the main jet the saw is getting plenty of gas, but as the throttle plate closes and transitions to idle circuit, that circuit is clogged and not allowing enough fuel to keep idle. I'm not sure how you would test for leaking welch plug - pressurize carb while submerged with no diaphragms and look for bubbles from the plug before the needle unseats? Never seen a carb pressurized this way. From everything you've said and done, if it were me, I'd do full carb cleaning and if diaphragms are suspect, put in a carb kit. Seems like you've ruled out most everything else. Good luck! Cheers!

 

I haven't messed with welch plugs yet so they're kind of a black box for me. What is under them? What are they sealing away from what?

 

I think the are at the base of the main jet and and seal the carb body from where the main jet was milled - but not positive. Looking forward to seeing what the problem ends up being on this one. Cheers!

 

You're correct, but as I mentioned, the tank vent passed testing; i.e. held pressure, but not vacuum.

 

Very good thoughts & a carb cleaning with possible rebuild is on the list! To answer your question, it'll fire right back up warm with the swith in the 'run' position - no choke or throttle lock.