Hello,
My Chilton's guide to small engine repair lists these causes for backfires.
!. The carb is adjusted too lean. (Is the air filter relative clean?)
2. Ignition is not timed correctly. (Spark plug look OK?)
3. The valves are sticking. (Have the valve lash ever been adjusted?)
Regards
Jeff
Recheck the exhaust valve gap.
Didn't work still backfires. Runs great thoughValves were very loose. Will report on outcome.
Does it have a solenoid valve on the carb