Gutless Yamaha 350 Warrior?

Help!I have a 89' Yamaha 350 Warrior and it barely has enough power to move itself. It starts and runs fine, but if you snap the throttle open, it will die, and if you slowly open it up, the engine revs up fine. The engine doesnt make and noise, it sounds tight, but it backfires through the carb when the engine is reving down and the problem is getting worse. Now if you start it up and rev it up the exhaust header gets red hot within a minute, I've never seen anything like it. Carb and valve timing is good. Thanks.