I put together a 660 kit back in Jaunuary. The build is all chineesium except for an OEM chain tensioner. I only have very occasional use for it to noodle up really big rounds. I mostly got it because I always wanted to see what a big saw felt like even though I still prefer my 025 90% of the time. I have to say, when I've got really big wood to break down, it feels good - a whole lot better than small saws on those rare occasions! I have maybe 15 tanks of fuel through it. I always use Stihl ultra 40:1. The last time I ran it I thought it felt easier to pull on start. Break-in settling in? Problem? All in my head? I wasn't sure? Today I was noodling up a 35" oak trunk, running it pretty hard. I stopped to gas & sharpen. On restart it felt really weak compression and took way more cranking than usual to fire. Something felt off but it finally started and I continued. Half way through the next cut it died in the cut and the cranking to try to restart it was really, obviously way soft. My compression tester seems to be busted but the right arm compression test reads way, way down.
I'm no engine mechanic. Anytime I've torn down a low compression engine in the past it has been obvious what's wrong. This time not so much. It was running hard when it cut off. The spark plug was in tight and the ceramic looks cCarhart brown so I don't think I was too lean. I'm positive I mixed the gas up right, the fuel in the tank is dark and the piston surface and crank looks wet. The wrist pin and crank bearings feel pretty tight. The ring gap looks tight when I squeeze them down. I'm posting some pictures below. Any guesses what might be the problem? My only guess at this point is maybe the decompression valve somehow became leaky. Is there anything in particular to look for on those.
I used the 1mm paper gasket from the kit. Squish came out in the mid thirty thousands. I think I'll use the thinner metal gasket on reassembly this time. Any advice on how to best scrape off the Motoseal and old gasket without buggering the mating surfaces up? Will brass scratch magnesium or aluminum?
Piston and cylinder pics below. I'd be thankful for any tips anybody can share.
I'm no engine mechanic. Anytime I've torn down a low compression engine in the past it has been obvious what's wrong. This time not so much. It was running hard when it cut off. The spark plug was in tight and the ceramic looks cCarhart brown so I don't think I was too lean. I'm positive I mixed the gas up right, the fuel in the tank is dark and the piston surface and crank looks wet. The wrist pin and crank bearings feel pretty tight. The ring gap looks tight when I squeeze them down. I'm posting some pictures below. Any guesses what might be the problem? My only guess at this point is maybe the decompression valve somehow became leaky. Is there anything in particular to look for on those.
I used the 1mm paper gasket from the kit. Squish came out in the mid thirty thousands. I think I'll use the thinner metal gasket on reassembly this time. Any advice on how to best scrape off the Motoseal and old gasket without buggering the mating surfaces up? Will brass scratch magnesium or aluminum?
Piston and cylinder pics below. I'd be thankful for any tips anybody can share.