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compression loss


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I searched other threads on this, but didn't see anything written down that dealt with my setup, so here it is.

2001 XR250. Head was rebuilt with Kibblewhite valves and 277 kit was installed. I have about 40 hours on that. This year a head bolt pulled the threads in the jug causing the head gasket to blow and slightly warp the head. I've got a really good mechanic here that builds professional hill climb motors, so he helicoiled the jug for me, planed the head (it wasn't too bad), put in new head gasket, and installed a stage 1 hot cam.

This is my sons bike and he said that the motor really woke up after that. Since then we did the National Enduro here in Wyoming, did three trail rides on the mountain of about 40 miles each, and taught a cousins son how to ride in the front yard. The bike then sat for two weeks until yesterday when we went to the mountain. No compression! Tried to bump start and it wouldn't go. I've read that the auto decompressor can cause this, but I'm also under the impression that with the Hot Cam this is removed.

Any ideas? I'm not much of a mechanic, so I'm looking for some simpler things to try before I take it back down to the shop and drop more money into it. I'm planning on getting a different bike for my son next year so I don't want to have to put a lot of money into this one. It's been a great bike, but with my son approaching 200 lbs (he's turns 16 on Tuesday so he's not done growing), the more open terrain that we race here, and the loss of power due to elevation, he needs something with a little more grunt than this bike can provide.

Any ideas would be appreciated.

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same thing happened to me a few months back,after my bike had stood for a

couple of weeks,ive got a '88 280,

i put a post up about it,think it was titled mysterious loss of compression,

suggestions were,decompression acting up,but they are both cable operated

on my 88 and disconecting them made no difference,

valve clearance's which i checked and they were ok,

and the piston being dry of oil so the rings were not sealing properly,

dont know what caused it ,but finally got it going with a push,been ok since,

coincidently i just had my head off last week to change the blowing gasket,

when tightening it back down one of the 10mm bolts pulled the thread out of

the barrell,so had to pull it down again and remove barrell as well to do a helicoil,­čśĆ

nothing like doing a job twice hey

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have you put a decent piston in the bike?? if not then it might have worn out the rings due to crap metal in rings, also did you rejett the bike when you put the hot cam in, becouse the bike could have run lean and burnt out the rings.

another thing you could try is to take out the plug and pour a small caps worth of engine oil down the plug hole. i used to do this to bost compression on my 85 xr250 and it really worked well!!!

hope this helps a little bit...

cheers.

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1st, I would check the rocker arm clearances to make sure that all the valves are seating. If that is OK, try pouring a small amount of oil into the spark plug hole, the rings might be dry from the bike sitting. Make sure to blow all the crud away from the spark plug base before removing it. You are correct, the hot cam does not have a provision for the auto decompressor, so that should not be the problem.

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I did put a Wiseco kit in, so I don't think that would be the problem. I did change the jetting and actually made it a little rich to keep it cool for the enduro we did.

I will check the valve lash when I get home tonight and see if something changed there, and try the oil thing if that isn't the problem. Thanks for the responses.

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You've got to love ThumperTalk. Responses from England, New Zealand, and Illinois. Creeky wins the cookie though. I had no idea that the valve lash would change that quickly. I could go a year or two without touching the valves, so it never dawned on me that I would have to check them this soon after a new cam was installed. Thanks for all the help guys, I'm back in business.

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