OK here it goes. I was riding my cousins YFZ 450 (2nd) ride.. OK i know what i was doing. I was riding a bowl. 1 st gear speed. Combination of sliding, sideways riding. (knobbys)Suddenly, took the YFZ to its limits on knobbys, then i rolled the YFZ like 2 times, rolled on top of me! then checked for damages. no damages(not even a cracked flag).. OK then my cousin rides it. Raced up comp hill 6+ times... Then he gets ready to line up again. Stalls.. Thats when the problems start coming. It'll take a while for it to start. It'll fore up. then he rode it for 3-5 mins. starts backfiring. stalls out. Started goes bad.... (lugging). Then just towed it back to camp... ?????

 

That is definately weird, check if a line got kinked or something. Even your carb, maybe something got into it.

..other then that I am stumped without being able to see it or hear it.

 

Change the spark plug, clean the air filter and put in fresh gas. It may be that a few grains of sand got into your carb and its plugging a jet. If you were able to run Comp Hill 6 times then it most likely isn't from the roll over. Just do a good cleaning and let us know what happens.