1982 Honda cb900 custom and it backfires when you let of on the throttle what causes this?

I have the same bike, same year. First I'd say, is it really backfiring (small explosions in the tailpipes) or more "popping" when you let off the gas? I think backfires might be more common on giving an engine gas, not letting up...

Mine was popping (which I originally referred to as 'backfiring', as well), and the diagnoses was that the engine was running lean, and when I had the carbs cleaned, it went away completely.