I was riding my 450 for about an hour. We stopped to take a break for 10-20 mins. When I got on it to ride again I tried roll starting like I sometimes do. It fired but didn't start. When I got to the bottom of the hill I tried kick starting it and it wouldn't kick over. It was like trying to start an older 450 with out the decompression release. After standing on the kick starter it would gradually turn the engine. It never fired back up though. got it home took the plug out and it kicked over fine. So I know the motor isn't locked up. I am thinking valves, or automatic decompress. Anybody ever heard of this????????