Weak compression diagnostic help? -- I think I done broke my chineesium!

[ajc4]

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.

 

[L34]

Looks to have serious scuffing on intake side of piston.

 

[ajc4]

Looks to have serious scuffing on intake side of piston.

Yeah. Some. I don't have a lot of experience here. Would you consider that abnormal? Would it cause low compression?

 

[L34]

Abnormal yes.
Low comp may be some scores In cylinder hard to tell from pics on my phone.

 

[Bama Raised]

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 use brake parts cleaner, a brass brush, and sometimes a razor blade (very carefully) to remove old gasket material. Once you hit it with the brake parts cleaner it should come off pretty easily.

 

[ajc4]

I see a little chipping in the chrome near the top of the intake port. Maybe the cylinder bore and/or piston was out of round or off concentric or something. Aside from a few spots of chipped chrome within 1/16" of the intake port the wear pattern in the cylinder is mostly concentric. I don't see any vertical marks at all.

 

[Bama Raised]

As far as the saw itself goes, from what I can see in the pics the scoring doesn't look bad enough to lose that much compression. It looks like maybe some debris is making it past your filter to cause that intake side scoring. I have also read that a bad bearing (lower rod or main) can cause lower intake side scuffing, but I suppose that is more uncommon.

 

[TBS]

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.

Get rid of that decompress valve or switch it to oem. You can take it apart and plug it with jb weld. Also check the intake boot. Some of that casting slag in the transfers may come loose too.

 

[ajc4]

I have also read that a bad bearing (lower rod or main) can cause lower intake side scuffing.

Uht oh! They don't "feel" loose but I probably don't have feeling precision down to the tens of thousanths that would matter. I could reassemble and check to see if the squish has changed.

 

[Bama Raised]

Uht oh! They don't "feel" loose but I probably don't have feeling precision down to the tens of thousanths that would matter. I could reassemble and check to see if the squish has changed.

Hopefully it's just the decompression valve as the guy above mentioned, that's best case scenario. lol

 

[TBS]

I see a little chipping in the chrome near the top of the intake port. Maybe the cylinder bore and/or piston was out of round or off concentric or something. Aside from a few spots of chipped chrome within 1/16" of the intake port the wear pattern in the cylinder is mostly concentric. I don't see any vertical marks at all.

If the chrome is chipped I'd replace the cylinder even if that isn't what caused the lower compression.

Another thing with these kit saws is the bearing many times need to be cleaned up but with 15 tanks through it I'd say they were fine and if one failed you'd have a completely trashed piston and cylinder usually.

 

[RED-85-Z51]

Crosshatch looks too shallow to me...rings also look rough like they were chattering

Sent from my LM-G820 using Tapatalk

 

[ajc4]

Thanks BrushSlasher. Signs are that your on the right track and I might luck out yet. I put a dab of Motoseal on the seat of the decompression valve and bolted the cylinder back down. That brought back the hard thumpy pull at the top of the stroke I'm used to. I can't plug it. I'm too wimpy to start this without decompression. Even if I was strong enough I'd probably bust the recoil. I'll get an OEM decompressoin valve and a decent compression tester on the way. If that fixes it up enough to give me confidence that I don't have any bottom end problems a I'll move toward a better quality topend without chrome chips on the intake port.