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What's good compression?

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I find compression numbers relatively useless for single cylinder dirt bikes, just because there is so much variability between gauges, and even test procedures.  I find gauges much more useful for checking the health of multi-cylinder engines where you can look for consistency across cylinders.

 

For two stroke dirt bikes, pull off the pipe and shine a light up through the exhaust port. Look for blow by, scratches, etc.  If the piston looks good, and the bike runs well (without a lot of piston slap), RIDE IT!

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What would be good compression for a 99 yz 250?

To expand on what rpt50 said...comp checks are good for comparison, yes. So if you know what your fresh top end comp is or was, then a comparison can be made at 10 hours, 20 hours, 30 hours etc. Some guys use a 10% drop in comp as a teardown and measure signal. I don't really subscribe to that because I've found rings beyond service limits by measuring ring end gap when there was little drop in compression.

Looking at the piston/rings through the exhaust port (blow by) is good way to know when the rings are absolutely warn out. But again, I believe the rings are beyond service limits before significant blow by is evident on the piston.

The only way to really fend off a hooked port and broken ring is to teardown and measure every so often on a schedule that you develope based on your riding style, oil used, etc. A teardown and measure costs a base gasket and about 2-4 hours of time if you know what you're doing. Could potentially save you a 200-300$ damaged cylinder. I say it's worth 7$ base gasket and a few hours to check the ring end gap.

My .02

-E

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