Add to all the above ... any change in the fuel mixture or a weak spark will make ign more difficult within the cylinder. On the overrun there is extreme suction sucking through a closed throttle plate and drawing in air from wherever it can, [air is more easily sucked through than fuel] making the mixture less than ideal. So add to the all above suggestions.... a weak or partially blocked idle mix, or an airleak through the threads of the idle jet ( I always smear a very small amount of hylomar blue on these threads).. Some back popple ( backfire, after-fire or whatever you want to call it ) on rapid down shifts or hard overrun is, I feel, inevitable with the primitive carburation of the Ford A; it is in the nature of the beast. I am sure I am not the only one who remembers the racing 7 litre Ford Galaxies (engineered and tuned to perfection) on the European (and no doubt USA) circuits in the 1960s giving an almighty backfire ( and flames as well) as they changed down and into overun from over 100 mph to less than 30 mph when approaching hairpin corners . The magnificent sound and visual image has lived with me for ever
