There is one other possibility, and that is the advance weights inside the distributor. These weights spin with the distributor shaft and move outward by centrifugal force as engine speed increases. If the springs that control the movement of the weights are broken, or if the advance mechanism is frozen, the timing will not advance properly and the engine will be hesitant and sluggish when accelerating, with possible backfiring.

Also, the Zenith progressive carbs suck. They have vacuum dashpot-actuated secondary barrels which frequently fail to open properly, also causing the symptoms you describe. If the problem is narrowed down to the carbs, I would definitely switch to twin Weber 32/36 carbs, which have positive, mechanically-actuated secondaries. These carbs idle smoothly, are trouble-free, and will increase mileage and performance slightly over the Zeniths. I have a single Weber of this type on my BMW 2002, and it runs beautifully and reliably. They run about $225 apiece, and I think they will bolt right onto your stock manifold. Also, when you eliminate all of the vacuum plumbing associated with the Zeniths, your engine compartment will be a lot tidier, too.
Hope this helps
Colin