Possible causes of backfireing and bucking
 

Cold starting I get some backfireing until it warms up. Once warmed up it bucks when decelerating, usually about 2000rpm.
Timing? Mixture?

Mixture. If the popping is in the intake, then a lean mixture is suspected. Popping in exhausts usually suggests a leak in the exhaust system. Bucking also suggests a lean mixture, but I guess I cannot explain how bucking can happen during deceleration (throttle closed). That's a mystery to me.

Is your tach bounching too? Check timing and your point's dwell. How long since a valve adjustment? Hope you have a pop-off valve in the airbox. Cheers, Jim

I would have had blown all the available airboxes in the world if I didn't have a pop of valve. I'm gonna try making the mixture a little richer and see if that helps. Valves are set properly and Timing and dwell is set as well as.