Dirt Rider Magazine

Hey guys, new here. Ive got an 08 rmz 250, with a built motor. Problem is , I was at My friends track practicing for ax. And after A Couple hours of Riding, i was pulling Out Of a corner and She felt low On power. Shifted All The Way down to first, and Still wouldnt go. Stopped immediatly, And Shut it off. Had fresh oil, and Plenty Of coolant. Seemed the Bike was kinda hot, but May have been from riding. Now it wont Start. Every 5-7 kicks just One Loud backfire. Has spark, clean air filter, great compression, timing is On, valves are In perfect check. Air/fuel mixture screw was good and Play In the hot start. We tried fresh fuel, and different Plug. Still Nothing. Please help guys!! Thank you In advance!