Could also be floating valves…Ike others have said do a leak down test and see what your cylinders look like. I’d the carb is running lean or rich, it could also cause it.. set your floats – check for proper tuning (lots of videos on YouTube) set idle, set idle mix to best idle, reset idle rpm, set idle mix screws to best idle again. Check to see if your backfire ceases.

You should post a video not pictures