I had this same problem on my last ride and I thought that I fixed it, but obviously didn't. I can kickstart the bike just fine and it fires right up and runs fine, but within a minute the bike will just stop as if it stalls. Sometimes I can kick it right back up, but it will die again and 1 out of 3 times the kickstarter will absolutely not move when I try to kick it and the wheel wont budge when I try to bump it. When I take a wrench to the flywheel and turn the piston it is pretty tight at first (almost a metal on metal grinding), but then loosens up back to normal and I can kickstart it again just fine, but the cycle starts all over. I'm racing it soon and am so fed up that I'm about to take it to a mechanic and dump a ton of change on this thing if I can't figure out the problem. This is on a 2003 YZ450. Please help!!!