It happens when I slow down.Any one have any suggestions on what causes it.

 

My 300 had the same problem. i was told that AC jets the carbs to run rich so i took out my jet and put in the next size smaller. it helped allot with the backfiring but i still get a pop out of it once in awhile. before you do this make sure it is running rich to begine with. if it is to lean then it could burn a hole in the top of the piston if it is jetted wrong.