MG Car Repair: 1972 mgb, oil filler cap, rubber diaphragm

Question
sputtering/stumbling under acceleration below 2500 rpm and backfiring on
over-run. starts immediately. idles OK. good power above 2500 rpm. just
replaced emission controls, plugs, plug wires, coil, distributor cap and rotor,
fuel filter and air filters. plugs and points gapped correctly. C.O.2 about 3.5%
at idle and 2500 rpm. carbs balanced.  rear muffler deleted. air pump cleaned
and turns freely. gulp valve not replaced. thanks in advance for your help.

Answer
Hi Terry.  Sounds like an air leak which is messing up the slow running.  I would be suspicious of the air gulp valve.  Unclip the top and inspect the rubber diaphragm.  Also, remove the oil filler cap and give it a good clean with gasoline.  It contains an air filter that must be kept clear.