[rotorwrench]

A back fire is where the fire comes back through the intake plenum and carburetor. An after fire is where fuel gets past an exhaust valve and ignites in the exhaust with a loud report. A sticky valve or more is generally the cause. Engines don't like to set for too long. MMO down the carb with some in the fuel might do the trick.

Back fire is generally caused by a lean condition in the carburetor. Occasionally a carbon tracked or cracked distributor cap can cause a cross fire to the wrong cylinder at the wrong time. A bad condenser generally causes a miss fire of one or more cylinders with poor or no idle.