So it just started doing it all the sudden? If it isn't gummed up carbs you probably have unseated carbs. I got my bike after it sat in police impound for two years (previous owner died on it) and the carbs popped loose during the first start up. This creates a lean condition and gets worse with throttle (engine vibration). I've seen it on every hawk I've worked on. Lean conditions do cause backfires but stock hawk jetting should handle bolt on aftermarket parts without backfiring even at sea level not to say you shouldn't go richer with mods.