Often a hole or leaky joint in the exhaust system as above. Closely inspect it for any tell tale black sooty streak marks. Heat exchangers to silencers / headers is a common leaky point. Had this a few times in the past, I got to the point I could almost time the backfire exactly to make pedestrians jump especially when coming down hill. (Young & foolish days !)
Once you've ruled this easy one out then look deeper into things like distributors / carbs.
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