Has anyone else had this problem, and what is the fix? Thanks

 

When does it backfire? On sudden throttle closing? When going on the gas?
Backfire is a sign of lean carb circuits or bad timing. If the motor hasnt been apart, then we can assume its carb related.
If it happens on sudden throttle let off, its the classic sign of a lean idle circuit.To verify that its lean...try pulling on the choke just before the throttle let off, and youll notice the backfire disapears.
Look for intake and exhaust leaks, and if you find none, ...turn the mixture screw about 1/4 turn richer, this will enrichen only the idle circuit., which is what it needs.

 

Thanks a bunch- I will try...