Popping through the carburetor or the exhaust?
There are numerous potential causes to a back fire.
Ignition secondary cross talk (arcing in the cap).
Lean miss,
Sticking valve,
Mis. adjusted valves,
Exhaust leak,
Vacuum leak,
Ignition primary issue,
Bent valve,
Cracked valve seat,
Blown head gasket between cylinders,
First thing to, which has zero costs associated, is to do a compression check, be sure this is done with the carburetor full open, no spark.
Second thing to do is a complete ignition system check, plug gap to cap condition.
Third thing is to run it and do a spray test with some carb cleaner looking for a vacuum leak.
Fourth is look for a cracked/leaking header pipe,
Beyond that, I got little/nothing.
GT
