Yz426 problems

I have read a lot on TT but can't find an instance like mine. I recently put new rings on the bike, long story, but I decided to check everything while I was in there and everything was within spec. After I got it all together it ran great then the next day it was like I had a fouled plug or it was out of time, popping and backfiring with a bad bog, idles fine though. After an hour or two it runs great again. A week later and it is screwed up again, this has gone on for quite a while back and forth and I have checked and rechecked the timing gone through several plugs, cleaned carburetor a couple of times and still this stupid problem. This morning I went to go riding and it didn't work, after cleaning the carb one more time it worked, until I got there and it acted up again. Any help would be appreciated and sorry for the long post.

Sorry it's an 2001 yz426 f all stock

I wondered if this may be the problem, bout the only place I haven't gone with this bike, another thing though, now I can kick through on occation without decomp lever, basically like my valves are stuck this just happened today for the first time just after I posted, and I was wondering if trying to "ride through the problem" could have done some serious damage? Thanks for the info gray.

If carbon builds up near the edge of the valve, it can sometimes hold a valve off its seat enough to reduce the cranking compression quite a bit. Not that unusual, and usually temporary.

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