Chainsaw Repair

Sorry if this is a double post, tried to reply this morning but it has not shown up.

Thanks for the good ideas. If they aren't the cause they will be good to think about next time.Here are more facts I should have given, been working on it off and on as weahter permits so didn't remember all of it when I posted.

Clutch: Bar and chain removed, no effect
No scoring: I opened the case.Didin't know about ring position pins. One new ring cost me $18 at Stihl, now know I can get an aftermarket piston kit for same price as one ring.

Muffler and screen: Very clean

Vent: I started it several times and it would run as long as I held the throttle to a high speed. When I did remove gas line it spurted. Full tank warmed up in sun. Still shouldn't have pressure on it. Tried running with check valve/vent removed. No help.

Air leak: Haven't checked that but have old manifold gauge, tubing, tees, and 60cc catheter syringe. Do I need to block ports or just position the piston to do that and apply pressure/vacuum to the impulse port? How much pressure/vacuum and how long?

I suppose dunking and pressuring carb is called for too but I replaced all gaskets.

I was sure it was the diaphragm since the problem started suddenly but that has been replaced too.

Thanks,
jh