03 yz450 backfiring just traded for it

im new to fourstrokes i just traded my banshee for a 03 450 with a fmf header/pipe. i dont know what all is dont to it other then that but it does have about of little add on's, it deff was a race bike the number plates have say 155 with Huber under them. so the kid i got it from didnt know much about it he traded for it to. so here i sit knowing nothing about the bike other then whats on the outside it fires up but back fires very very bad and will grow header with only a couple secs after starting it. i cleaned the carb to make sure it wasnt just plugged up which it wasnt, today i pulled the main jet out and its a 170 im in ny at about 2000 feet above sea level. if anyone is in upstate near cobleskill i could really use some help from someone that knows something about this thanks

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