Paul
I ran Webers on my big block prior to going to an EightStack system. It's really difficult to get rid of the backfire on decel. Hard to cut the fuel supply with the throttles closed, they still leak slightly around the blades and you get oxygen via reversion, coming backup the pipes. I could never dial it out completely. Some days were worse than others, I am guessing due to air density but don't know for sure.
I learned to keep the blades open slightly when throttling down, that seemed to help a little bit. Also, make sure you're not running too much timing, that is also a factor.
Look on the bright side - it really looks cool at night seeing the flames pop out of the side pipes.
Paul