I had just purchased an 02 WR426. I have been doing some reading on mods for this bike, and I have done just a few of them. I cut down the throttle stop, cut open the air cleaner lid and put a FMF powercore 4 exhaust on. Now the problem. The bike is street legal and I have not had a chance to take it out in the dirt as of yet, but I did ride it around town. So far it starts fine, even when warm, but if you back off of throttle it stats a little back firing and at idle if you snap throttle it will try to die. If you have RPM's up in nuetral and left off throttle it will cough and die. This is my first four stroke, so my first thought is air/fuel mixture. adjust air screw first before playing with any jetting. If this is correct, any suggestions as to where to start with screw turns. Thank you all. P.S. I live in the Arizona Desert 300 Ft. A.S.L. if this should help.