I have a 2002 250ex, I recently got a HMF full exhaust, When i rejetted I used a 106 main jet and I believe there is a 38/40 pilot jet. I'm still getting a little backfire on decel. Should I fiddle with the a/f mixture screw or go up another jet size. I am running with a UNI filter with the air box lid on.

 

usually when it backfires its the main jet

 

have you checked the spark plug to see whether it's running rich or lean?