09' KX250f - low compression

badfish
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badfish
Edited Date/Time 1/26/2012 6:50pm
Trying to help out a internetless friend. He has a 09 kx250f with very low compression....as in like 50psi. He tore the motor down and noticed his exhaust valves were open.....had a lot of carbon build up around them. He said his piston/cylinder (new at end of last season, not even ridden this season) looked to be in very good shape...no lines, gouges, nicks, etc. He cleaned everything up and put it back together and thats when he got the 50psi reading. I'm thinking it's the valves or springs that are worn and thats why the compression is so low. What do you guys think?? All help is much appreciated! BTW: I'm 4stroke illiterate!
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mxtech1
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mxtech1 8/31/2011 5:52am
8/31/2011 5:52am
need to cut new valve seats and replace the valves. That's the only way he's going to ever get a good seal again between the valve and seat. You should probably do all 4 valve at the same time if he can afford it. if not, at least the 2 exhaust. Always replace together, or at least in like pairs to promote even wear patterns. don't forget to check for valve clearance and re-shim if necessary as this can also hold valves open if they're to tight.
mxtech1
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mxtech1 8/31/2011 5:54am
8/31/2011 5:54am
BTW where are you at in IL? I do service work at pretty affordable rates (alot cheaper then the shops). I'm in Galesburg if you need anyone to do some work for you.
motofab36
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motofab36 8/31/2011 6:00am
8/31/2011 6:00am
There is a automatic decompressor (compression release) on the exhaust cam, you will never get true comp. readings with this installed.
badfish
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badfish 8/31/2011 6:42am
8/31/2011 6:42am
mxtech1 wrote:
BTW where are you at in IL? I do service work at pretty affordable rates (alot cheaper then the shops). I'm in Galesburg if you need...
BTW where are you at in IL? I do service work at pretty affordable rates (alot cheaper then the shops). I'm in Galesburg if you need anyone to do some work for you.
We are just outside of Rockford.

The Shop

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mxtech1
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mxtech1 8/31/2011 11:44am
8/31/2011 11:44am
motofab36 wrote:
There is a automatic decompressor (compression release) on the exhaust cam, you will never get true comp. readings with this installed.
True, but even with the auto decompression 50 psi is too low. a normal reading would be no less than 100 psi.

The only true way to do it is with a leak down tester at TDC
kripple428
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kripple428 9/1/2011 7:30am
9/1/2011 7:30am
could need a new cam chain that happened to my 450 but I had too much compression the cam chain got worn out and it jumped timing and you couldnt even get the kicker to move we thought it was siezed up im sure it could have the reverse affect also

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