Stihl 031 revs up and down without touching throttle trigger.

eldubya54 · Jul 28, 2013

Hey! Here's a new poser even for me and I'v managed to resurrect a few "yardsale" Stihls. This 031AV starts with just a few pulls and runs with power, but after opening the choke it "idles" from idle to full throttle and back without ever touching the trigger. I have it torn down to "looking at the crankcase", but I see no leaks in the intake boot or impulse line. Any ideas how this could rev 0-100 and back, over and over, without touching the accelerator? It doesn't stall, just hunts back and forth.

Scooby074 · Jul 28, 2013

I still think air leak.

Did you test the crank seals?

LD1 · Jul 29, 2013

Have you opened up the carb and possibly give it a cleaning or rebuild.

Does/did it run under power? and the issue was only while idling?? Or would it not rev at all with the trigger pulled?

Could be a plugged low jet in the carb, or just out of tune.

Gale Hawkins · Jul 29, 2013

Scooby074 said:
I still think air leak.

2x here.

adirondackmtnman · Jul 29, 2013

Scooby074 said:
I still think air leak.

Did you test the crank seals?

X3. Check the boot between the carb & jug as well

Sent from my LGL35G using TractorByNet

Treemonkey1000 · Jul 29, 2013

Get it running and spray some carb cleaner or WD around the carb boot and crank seals to see if that affects it at all. Also while you are fiddling make sure the filter in the tank is cleaned as well as airfilter. Take the airfilter out and wash it with soap and water then dry it.

eldubya54 · Aug 2, 2013

First, Thanks to all members who have weighed in on this problem. Your replies are encouraging and appreciated. It turns out to be sucking air, after all. The pinch clamp around the cylinder end of the intake boot seems to be weak. My other 031 has a screw clamp in this place. After I remedy this, I will report to you all how it turns out. Thank you all again.

Gale Hawkins · Aug 2, 2013

I have had pin holes develop on each the air and fuel intake hoses that let a 2 cycle engine go lean on fuel and rev up.

eldubya54 · Aug 3, 2013

I've ordered a screw clamp for intake boot; supposed to arrive Tuesday. Hoping this is the cure. I would, however, be thankful for info on how to "test" crank seals.