What would be good compression for a 99 yz 250?
Fresh top end about 220-230 psi at sea level. That's ideal, but around 200 is still good.
When it breaks your leg haha
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!
Fwiw, my low hours 05 just tested 225.
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.
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