Compression PSI

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Dec 6, 2003
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#1
About a year ago after breaking in a new piston I measured the PSI at around 140, now (1 year later) the compression jumped up to 190 and the bike still had monstrous amounts of acceleration and power but once I get into the high RPM's it flails, it chokes and sometimes backfires, I don't know what it could be, any suggestions?

BTW: the bike is a 1994 CR250
 

Studboy

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#2
Your powervalve may be stuck in the closed position. It is unusual for the compression to actually increase over time unless you either have a PV problem or severe deposits in your combustion chamber and exhaust.

Could something clogging the exhaust also cause this? I would check the exhaust also.
 
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#3
Everything checked out fine. I pulled the head and the cyliner is perfect the exhaust is not blocked, power valve works fine. see the pic.

http://xylicon.com/Misc/grooves.jpg
 

darringer

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#4
I would suspect that the 140 psi reading taken a year ago is wrong. It seems very low for a modern 250. 190 psi is near what should be expected for a fresh top end.
 

Studboy

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#6
Those grooves aren't OK. Your piston was either contacting the head or the powervalves. I'll bet it's the powervalves. Has your cylinder been bored oversize?

You should probably clean up and grease those head studs before reassembly, they look a little corroded.