So I've had this issue for over a year. The bike is bored over with fresh piston and jug. I've done numerous things to try to remedy this issue with no success. I've cleaned all the jets in the carb and adjusted the fuel/air mixture with no luck. The spark plug seems to be burning good with good color. I changed the spark plug along with trying to go hotter and colder with no luck. There is no exhaust leak at all. Checked this numerous times. I then decided to purchase a new aftermarket carb with no luck as well. The aftermarket carb does not have an adjustment for the fuel air mixture. Where the adjustment screw would be there is something blocking it, but can not adjust. It has two small holes which I tried to remove and couldn't. I put the choke on to see if I wasn't getting enough fuel in which the backfiring increased a lot. My guess is that I'm getting too much fuel. Any advice as to what my next plan of attack is? Thanks

 

I had one the same year but it would try to stop running like the exaust was cloged and it would back fire and run fine i know this is no help but ...

 

Have you checked valve clearances and valve timing? Some engines are prone to backfiring, the Honda 450 Foreman was, they would blow the baffles out of the silencer.