jetting question

just installed k&n power kit i used the jet number they said in the instuctions to use. when i nail it just backfires and dies almost like it hits a rev limiter but it is at a low rpm. By the way its a 2002 400ex. i pulled the plug and it is black.I run only race gas in it. The dynajet numbers dont cross reference withKeihin #on the jets.Im at 600ft and i put a 146 jet in itbecause i put an exhaust on it to.Any help would be appreciated!!!

you probably need a much larger jet,i use a 170 main jet with an exhaust and k&n filter,try a 170 and see how it does:devil