[Marshall V. Daut]

In addition to what the guys have suggested so far, check the exhaust manifold-to-muffler clamp orientation, specifically the gap between the two clamp halves. If the manifold and muffler pipe are not 100% sealed at this juncture, exhaust can blow past and be sucked into the carburetor's throat. That will cause popping and backfiring, especially during deceleration. Loosen the clamp nuts and rotate the clamp so that the gap between halves is not directly facing the carb's throat. This is an overlooked cause of popping/backfiring that is hard to diagnose unless there are big, black ugly splotches of exhaust soot near this juncture. This is not always the case with leakage in this area.
Marshall