Popping through exhaust during deceleration

Sounds like a fairly serious build, and would be perfectly normal. A cam in the .660 lift range probably has some decent duration and overlap. When you cut the throttle the manifold vacuum spikes and some fuel is still being pulled in. With the overlap and high vacuum, port reversion will inevitably dump fuel into the exhaust. If you pay attention at just about any short track, the cars are always running flames going into the corners. This was even common in NASCAR before they switched to fuel injection, and I can guarantee you those guys know how to tune a carb better than anyone on the planet.