Mainer54,
I have been trying for two years to eliminate the same thing. Check your valve lash, ignition timing, and carburetor settings. I lessened the backfiring after sealing a few leaks in the exhaust system (dual carbs and dual exhausts). One thing I have not checked is a leaking carburetor (there is an internal and an external leak - maybe due to a bad float setting).
I have been told that unburned gas in the exhaust and an air leak providing oxygen to the exhaust can result in an ideal condition for backfiring (backfiring out the exhaust, not out of the carburetor).
Good luck,
Tim
