Arrgh! Stihl 039 acts like it's leaking air.

Saw would run fine for five minutes, then quit. Replaced spark plug. Saw ran fine a couple days ago for several hours of intermittent use. I fired it up yesterday and tore into some 16" logs, sustained flat-out running. Saw ran for about twenty minutes, then quit. It would pop after about seven pulls with the choke on. Opened the choke and started the saw, but it kills as soon as I open the throttle. I've cleaned the carb, put a diaphragm in. Seems to me that it must be sucking air around the crankshaft seal (seems to be revving a bit high.) Any other ideas of where air can get in? If it's the crank seal, how big a job is it? Can you pop the seal out after you get the clutch and whatever out of the way?
I saw a download for a service manual for the 029/039, but couldn't get it tonight...site down for maintenance.

Check your fuel line and filter first, tank vent, cylinder bolts are tight and muffler free of carbon.

Before diving in to a crank seal job I'd want to be sure that is the problem. This requires a vacuum/pressure test. If you don't have access to a mity vac type tester you may want to tear the saw down (remove muffler and carb) and have the dealer do it.

I should have mentioned, I checked the fuel line and filter, new fuel cap, blew air through the return line from carb. I'll check the bolts and muffler next.

I may pick up a vac tester. How is the test performed?
What's involved in the seal replacement? Can you pluck the seal out, like I just did on my car, or do you have to split the case?

Sounds like a fuel line is collapsing/ leaking air. Same thing happend to me on a weedeater about a 10 min job to repair and runs great.

You do not have to split the case. I have been able to gently pluck seals out, but they also make a special tool. I recommend the mity vac available at Amazon. You want a tester that does both pressure and vacuum. The process then entails blocking the exhaust and intake port (they make special plates, you can make your own or I use Gorilla duct tape). Then you conduct the test through the impules line or if there is not an impulse line, through the spark plug hole. If you go through the spark plug hole you will need to make a fitting. An old spark plug with the porcelin removed works or I found a couple fittings from and air compressor that I was able to use after connecting the pieces together and using soime JB Weld to seal them.

All of this stuff is available at your stihl dealer but it is expensive.

If you need some additional info when it is time to test, send me a PM. I have a few links that will guide you through.

Great info. Thanks, HS!
The impulse passage is one thing I saw mentioned that I haven't checked yet. It seems like a restriction of that passage would give weak performance all the time, not just when the saw gets hot. Of course, as usual, I could be wrong. :lol:
Another thing I considered; The carb is mounted to the handle assembly, and there's a rubber sleeve going from the carb to the case. Years ago, I dropped the saw out of a tree and broke the bottom leg of the handle. Now the handle/carb moves a bit in relation to the case. I wondered if the rubber sleeve could have fatigued and cracked. I tried to pop the sleeve out, but didn't get it yet. I rather doubt that the sleeve is cracked, though; It seems to be pretty thick material.
I ordedered an 029/039 Stihl factory service manual, and I'm getting a Mityvac. I guess I want to test the case using vacuum, not pressure.
I had a lot of room to work pulling the crank seal on the car, so it was easy to avoid scratching the crankshaft. There's a lot less room to work on the comparatively small saw crank. If you've had success, that's encouraging.