Start with Cleaning the carb and airfilter, and give it a new sparkplug.
And tell os more about the bike, has it sat for a while, did it just suddently start backfiring, or has it been faulty for youre entire ownership? Pretty important information.
F.x It doenst help to adjust the air/fuelscrew, if the carb is filthy and clogged up, or if theres no airflow trough the airfilter ect.
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