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Frostbite

Compression loss question for the techys

8 posts in this topic

Does anybody think it’s possible for a running but flooding engine to wash the cylinder walls down enough to lose enough compression to quit?

On 2 separate occasions under full load my engine has lost power and quit. When I tried to restart it felt like the sparkplug was out, there’s no difference between the decomp lever in or out?

After sitting and kicking occasionally for ½ to 1 hour, the compression gradually comes back and the engine fires. It runs terrible for about 5 minutes, backfiring and gotta keep it revving high to stay running. Then it clears up and is perfectly normal. The second time it happened I noticed that snow packed near the crankcase vent tube was black like there had been lots of blow by. The snow is there 6 months of the year and has never been black like this before. After removing the black snow and restarting, the new snow stayed clean after hours of hard riding.

I recently installed a 430 kit so the rings probably aren’t fully seated yet. Would a dry cylinder lose that much compression and allow that much blow by?

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The backfiring makes it sound lean to me, particularly given the cold weather, which tends to require richer jetting anyway. It could also be an ignition problem. Richness causes a regular miss, like the engine skips every other power-stroke ( in a two-stroke, we used to call it "four-stroking") accompanied by black smoke. Misfires caused by lean mixture are irregular and are accompanied by random exhaust backfire. You probably have some water or something solid bobbing around in your carb.

It would be very difficult for an engine that was either so rich or so low on compression that it quit under a load to restart at all. I saw a Honda 90 once with a ring set broken in at least 152 pieces that still ran if you pushed it fast enough.

Loss of compression on a hot engine that returns when cooled down sounds like valve clearances.

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Loss of compression on a hot engine that returns when cooled down sounds like valve clearances.

That was my original thought, but the valves are bang on, and a tight valve shouldn't cause the extra blow by. :naughty:

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Have you considered ice in your fuel? Even ice crystals running through the carb would cause problems.

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I have experienced this same problem once or twice over the years on my 426, it disappears and never happens enough to diagnose. I have always suspected a sticking valve. It runs awesome

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I have experienced this same problem once or twice over the years on my 426, it disappears and never happens enough to diagnose. I have always suspected a sticking valve. It runs awesome

Interesting. :naughty:

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This same thing happened to me trail rideing over the weekend, I thought my decomp lever may have been sticking. But it fired up and ran fine, no sign of it since.

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Update. I rode the bike for a few hours last week with no problems and it was running fine when I parked it. I tried to start it on the weekend, lots of compression but it wouldn’t run. Finally it started, ran for 15 seconds and then died. No compression when I tried to restart. This is the first time I’ve lost compression in a cold engine. I’m leaning toward a sticky valve now.

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