Backfiring is usually caused by the throttle linkage out of adjustment or the idle screw adjusted to where it is holding the carb throttle plate open a little. What is happening is the throttle plate is open and it sucks gas in when coasting, then when you hit the gas pedal again it starts igniting the fuel causing a backfire. Suggest taking it to someone who knows what they are doing. It has nothing to do with the fuel you have in it.
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