I am not familiar with the weber, but I'd bet your initial idle is high - 750rpmish - so that when decel with engine brake the throttleblades are closed and the vacuum high. The transfer slots will be exposed and the highvacuum will call for fuel, in brief fuel is raining into the engine via both idle and transfer slots and this raw fuel is causing the backfireing and general poor running.
It looks like Elmer is referring to the Holley-Weber
Ford 740 Weber DFT32 sold by Langdons (new carbs - progressive 2-barrel; originally used as the single carb on 1600cc engines). Mine would easily and smoothly idle at 400rpm.
