soft compression - wont start

I started my 02 426 last weekend when the bike was cold soaked (30 degrees). It started and I tried to warm it up to quick (stupid-stupid-stupid :thumbsup: ). It ran great for 30 seconds and I wrapped it up and then: It just spooled down like it was out of gas. I went to kick it again and it kicks through without the need for the decompression lever. It has compression in the kick, but only about 60% of what it use to be.

I thought that I messed up the rings. I (quickly) replaced the rings, piston and seals.

But to no correction. It still kicks though with 60% of original compression and no start.

The decompression lever is not sticking. It seems to be working properly.

same thing happened to my 03 wr450. rode for about 3 minutes took off and then died, kind of like it ran out of gas. I tried to kick it over and it had some compression, but just did not feel right. found out there was carbon deposits keeping the valves from sealing properly. got the valves all cleaned up and it ran great until i sold it. now i have an 06 yz450. third or fourth ride (11 hours) i was out on the track, been riding for about 10 minutes, and the same thing, the bike just quit. not instantly, but over the course of about 10 seconds, just started losing power and then died. tried to kick it over and very low compre4ssion. pushed it back to the truck. tried to start it several minutes, but would not fire. i waited about 30 minutes, checked the gas (was full) tried to start it and it fired right up? continued riding and racing, put about 10 more hours over the next couple of months. the other day i go and try to start it , would not start, very low compression, just idles veery low kind of sputtering. toook about 10 minutes to warm up and run right, but now seems to run perfect again? btw, i used yamalube 4 in my wr, and mobil 1 mx4r in the yz so i know the oil is not the problem. anybody have any similar experiences?

Update:

I pulled it behind a tug and after 10 attempts it fired up. I think the carbon build up on the valves was the answer. It runs great at low rpm's, but sputters at high rpm. I'm going to open up the engine and clean the valves.

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