HELP!!!!!!!! with 1999 foreman

[rev-man]

my dad has a 1999 forman 450ES and it is all stock it still has stock tires but when you get the motor to operating temp. and you get on it and turn around 5000-6000RPM or more and let off completly off the gas it wass come down close to idle 1500-2500 it will release a severe backfire and the muffler tip will release a spark can you help me resolve the problem!!!!!!!

[Francois]

I have see this with sport quads. IT was the jetting that was off. I don't know it it was lean or rich. Too lean i believe. Someone correct me if I'm wrong.