84 yz250 low compression

Well, A while back I got this bike and it was in pieces, I rebuilt everything, replaced the piston and a whole bunch else.Then I fryed the engine because the power valve wasn't turning, I replaced the piston and the power valve and the head gasket. It started up once but it had really low compression, about 25-50 psi. I recently took it apart and the cylinder looks great and the piston has a great seal. The reed valves are all good, but one thing was that the power valve was getting hung up on one part, I pulled it out and cleaned it all really well, an it now turns but before I put it back together, I wanted to know if the power valve not turning all the way or getting stuck could cause my problems? Also the chambers for the antifreeze had a bunch of whitish gunk all in them, any ideas. I cleaned it all out, but I am just curious about why it is there. Sorry for the long post but I figure any info will help. Thanks everyone.

Well, A while back I got this bike and it was in pieces, I rebuilt everything, replaced the piston and a whole bunch else.Then I fryed the engine because the power valve wasn't turning, I replaced the piston and the power valve and the head gasket. It started up once but it had really low compression, about 25-50 psi. I recently took it apart and the cylinder looks great and the piston has a great seal. The reed valves are all good, but one thing was that the power valve was getting hung up on one part, I pulled it out and cleaned it all really well, an it now turns but before I put it back together, I wanted to know if the power valve not turning all the way or getting stuck could cause my problems?

Is this on an '84 or '94? I had an '85 CR and all it had was a butterfly in the expansion chamber. If it really has a power valve, it should not interfere at all with the piston whether it is working correctly or not. The power valve just adjusts the exhaust timing in relation to rpm...when you have a higher rpm, the valves open up allowing the exhaust port to be open sooner in the stroke bla, bla, bla. Low compression; you have another issue be-it the crank seal or base gasket seal, rings, etc. The power valve is not going to affect compression. Describe how you "fried the engine."

Also the chambers for the antifreeze had a bunch of whitish gunk all in them, any ideas. I cleaned it all out, but I am just curious about why it is there. Sorry for the long post but I figure any info will help. Thanks everyone.

The whitish gunk is calcium from tap water. If you mix antifreeze with tap water, the calcium (hardness) in the water plates out on the hot surfaces of the cylinder impeding heat transfer. This is why the container will tell you to use distilled water to dilute the antifreeze. Distilled water has 0 hardness; or is completely soft water. Wash what you can see out of the water jacket and rinse it out well. I've never used it so read the label well but you might be able to use CLR to clean it out?

Well, I think the old power valve just wasn't turning and it just blew then engine. The piston was all messed up and so was the power valve. Anyway i replaced them both.

Has the cylinder been bored out?

No, but it looks perfect.

Is this on an '84 or '94? I had an '85 CR and all it had was a butterfly in the expansion chamber.

Yup, they got the YPVS in 80 i beleive

According to Wiseco any piston over stock size for that year needs the power valve to be modified for .040 or larger. I have an '83 and may be doing a new top end myself.

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