Temorary Loss of Compression in '02 YZ426

[polybash]

This issue has happened twice to me so far. The first time was right after a short warm-up loop. I got back to camp and was sitting waiting for the rest of the group. I revved the engine up little bit and then it just died. I didn't think anything of it since it will sometimes do this when it is still a little cold out. I went to kick to find TDC and it pushed all the way through. There was still compression, but not enough to get to the hard spot 426's have. It sat for probably 30 min. or so I tried kicking it again, it fired up. Shut it off and it had full compression again. The second time I was going up a small washed out creek side, got stuck in neutral, the engine revved up, and it died. This time it was able to start back up again relatively quick, and I putted back to the trucks. When it happened the second time, I noticed the lever was very loose, like the little valve that releases compression was stuck open or something.

I don't know what could cause this. After reading through some previous posts with similar issues, it sounds like it could be a sticky valve, sticky decompression cable, or wrong timing. But none of the other posts stated they could start their bikes, I could. Once it got started I had full compression again. Any ideas? I have a race in 2 weeks, and I would like to figure out the problem before then so I don't do any damage to the bike. Sorry for the novel of a post, I wanted to provide as much info as possible to try and diagnose the problem. Thanks for your help.

[grayracer513]

You might want to see if the return spring is correctly placed and working on the compression release lever on the front of the head, given your comment on the lever feel, and you should check your valve clearances, but it's possible that it's just a matter of a valve hanging open due to carbon on the stems.

[usaf_yz426]

i am also having the exact problems with my bike. i am thinking my valves may need re-shimming. do you think the valvescould stretch when it warms up, and then shrink when it cools down?

[polybash]

Thanks for the replies. I had a similar thought with the spring being out of whack. I had a devil of a time with it when I had the top of a year or so ago. If there is carbon build-up, is there a way to clean it without pulling out the valves (some kind of cleaner you pour in)? Thanks for the help, and I will look into the spring issue.