MS039 - runs on after throttle release, fuel boils out vent, and more

[TreedFella]

I recently inherited this saw from my father. Before he passed, I remember he replaced the carb but he got sick before he got it adjusted. I've tried the standard stihl procedure and multiple other methods I saw on YouTube but I cant get it to idle without running away, and revving dangerously high (didn't have a tach but it sounded over 16k). The trigger response is as bad as could be, where it almost dies, goes nuts, and then keeps going for a bit after the throttle is released.

Im suspecting either an air leak behind the carb, allowing it to suck more fuel without the trigger, or some issue with the low circuit "jet". It's almost like it's not atomizing the fuel and responding like liquid fuel. It's like a slowed down echo to the trigger pull plus initial hesitation, if that makes sense.

I also have the low speed valve at a only 1/10th of a turn, out, from the safety stop. This is the only setting it doesn't start moving the chain and getting up into 8k, plus. I can get it to idle, but it varies between 2800-3700, when I'm aiming for 3500, and this is an unstable state.

I also have an MS192TC that I love because it can eat trees all day and leaves my back out of it.

In honour of dad's love for the woods and our family's plan to log a small area of it, I'm going to try to do the firewood part of it as a small business, and possibly get out the sawmill if I can find a buyer for green wood (mostly maple and black cherry).

The offer for the firewood from the logger was $15/bush cord and my dad will haunt and curse me if I gave away our woods. The work is really helping me work through his loss so im hoping I can make money doing it...

Got distracted, sorry, what is going on with this saw? It even ran away on me after I turned it to off before I reached over and pushed the circuit closed (luckily had the cover off), but there's something weird about this saw. Should the fuel appear atomized coming out of the Low circuit? It looks like it's pouring out which might explain the general problem. When the revs go above 10k, fuel starts coming out of the vent, too!

Thanks for any help. Will record video if needed.

 

[stihltech]

Look carefully at the fuel line for cracks. I stock at least five at all times. Occasionally there is a cracked pulse hose. Check fuel for dirt aand water. REPLACE fuel filter. Disassemble carburetor. Look for hard diaphragms, inlet lever height, dirty inlet screen and corrosion. This saw also needs a vacuum and pressure test before going further. I know I sound like a broken record, but no carburetor will function properly with a leak in the motor. This is a start.

 

[TreedFella]

Followed your advice to the letter! General consensus was a leak in the saw because it wouldnt seal on the intake side, but the compression was fine. I replaced the lines (fuel and impulse), but both looked brand new. I was very hopeful about a bad impulse tube, but nothing.

I was just about to drop 1k on a new saw when i decided to replace the spark plug (the one in there wasnt old, was properly gapped, and i'd checked the spark, so dismissed it).... but then the saw fired up just fine!

Im thinking this means the saw is on its way out, unless theres a way for air to leak in THROUGH a plug, rather than around it through thread wear, but at least ill get one more season out of the old girl and i learned a lot about chainsaw maintenance and 2-cycle engines and carbs in the process. Brilliant design, really!

Thanks again for the help! Hope youre enjoying the weather, too

 

[J D]

General consensus was a leak in the saw because it wouldnt seal on the intake side

If it has a leak best address that before running it further or you'll likely be up for a piston &/or cylinder (& still have a leak to address)