testing compression w/ finger

mnr102 /
Oct 11, 2001 04:20PM

Last night I got my head back together, and tried the plug-the-sparkplug-hole-with-finger test for compression. In the Fred Manual, it says pedal and the compression should force your finger right off. My engine's off the bike, so the only way I could move the piston was to turn the flywheel by hand. It didn't blow my finger off the hole. Either my rings are too old (they look fine, and the bore is blemish free) or you actually need to be turning the piston quickly (pedalling) to get this result. Anyone know which? Turning it slowly by hand I think (I hope) is allowing pressure to "find it's way out" slowly, which pedalling wouldn't allow. I'm going to try to rig up a reverse drill/socket on the flywheel tonight.

THanks.

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You are correct

Fred /
Oct 11, 2001 04:29PM

You have to be turning it fast enough to generate compression.

I don't think turning it slowly with your other hand will give you any useful info... its not fast enough.

The drill might work... or... just wait till you bolt it all in and try it.

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Re: You are correct

mnr102 /
Oct 12, 2001 10:17AM

Fred,

...for future revs of The Manual: turns out that a reverse 1/4 HP drill/socket combo put sufficient RPMs on the piston to force my finger off. Even more suprising, once I put 3/4 cap of 2-stroke oil in the bore, even turning it by hand could force my finger off. I only had it very lightly oiled on the first night where I was losing compression.

Thanks,

Mike

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Re: testing compression w/ finger

Ron Brown /
Oct 12, 2001 11:13AM

Fred is right of course. The trick you pulled with oil in the bore does not tell you much as it will seal bad rings and scoring enough to hide problems. I have not checked Fred's guide lately but the throttle should be wide open too.

Ron

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