This prob and the backfire indicate significant problems with how the carb is adjusted. The choke may not be opening at the right time, the bowl(s) may be overfilled, the idle mix may be rich, or the accelerator pump may be overactive. Whatever the cause, it definitely sounds like a rich condition. Check the Haynes troubleshooting chapter. Also, the timing may be late (retarded), or the vacuum advance may be disconnected.

Steve
83 Custom w/95 4.9L EFI, 78 NP435, 83 NP208, 83 D44IFS 3.07, 87 Ford 8.8ABS 3.08, Michelin LTX AT 31x10.50R15