Sync the carbs, and back off the airscrews to 2 1/2 turns out as a start point. No body mentioned it, but you could have eyeballed the butterflies adjustment for similar opening (usually still close enough, unless you moved the screw) before you installed the carbs to the head. Also, be checking for air leaks on the intake side as that will cause backfiring.

Last edited by Ryk; 08/15/2017 1:15 AM.