I'm going to assume that the backfiring is happening when you initially step on the pedal and not when you are running at speed. Again, this occurs when the throttle plate is not closing properly, or opening too early. Someone mentioned that the governor may have been adjusted too far. When that happens it allows the engine to over rev leading to a condition called "valve float". The engine is rotating so fast the valves can't keep up and this will cause some popping/backfiring as the valve timing begins to breakdown, but this occurs at top speed. Its also very hard on the engine and will eventually lead to early failure.
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