by Vince Knauf
Most often a backfire is the result of an air leak in the exhaust system. For a moment when you up shift and often when you let off the gas, there is a temorary "over rich" condition with too much gas in the cylinders. This blows into the exhaust and if there is oxygen (air leak) then it explodes in the hot exhaust pipe. Without the air leak, it is cooled enough not to explode by the time it reaches the atmosphere in a tight exhaust.
Look at the exhaust system and repair any problems. Also make sure that the carb is adjusted for good running and fuel economy otherwise.