I've been working with a Husqvarna 365 chainsaw that came in with a damaged cylinder and piston. It would run fine for just a few minutes, and then it would start to struggle and then finally die. When I couldn't get it started anymore, I measured compression, which was 50 lbs., way too low. When I removed the muffler to see the piston, I saw a lot of scoring on it and on the ring. So I found a cylinder/piston assembly from Stens, and bought it because it was easily the best price around on these parts. I also got some replacement crankshaft seals for it, and got it all back together yesterday evening, and it runs terrible. It will idle for a few seconds and die. I can get it started right back up, but it won't idle for very long before it dies again, 10 seconds at the most. And it won't rev up at all. As soon as I open the throttle, it dies instantly. I tried to mess with the carburetor adjustments, but nothing seemed to make any difference. So then I measured the compression. I was only able to get 90 lbs. out of it (Husqvarna compression gauge, air filter removed, throttle completely open, and 4 brisk pulls on the rope). So I removed the muffler to see if I could see anything. Piston and ring looked just fine to me, as it should have, being brand new. I checked the compression again with the muffler off, and I got 95 PSI. I also verified that the cylinder compression relief valve is working properly; I pushed it in at first, and the compression pushed it back out. But honestly, the compression isn't enough that I even need to push it in.
I'm not nearly as good with 2-stroke engines as I am with the other stuff. But I thought that a minimum of 100 PSI was necessary for one of these engines to run at all. And compression was supposed to be at least about 120 PSI anyway, and probably a little higher. Is there anything other than piston/cylinder that affects this upper end compression that I was testing? I'm starting to think the Stens aftermarket cylinder and piston assembly was a mistake. What do you think is going on here?