I would check the float bowls and float needles to see if there is any debris in either one. Or possibly a leak near one on the collectors. Something has to be putting enough extra fuel or oxygen into the exhaust system to produce the backfire. Maybe a rear float set too high and you are getting a splash of fuel into the intake on bumps or decel? A bad power valve will do this but usually that will show up as a super rich mixture and foul plugs.