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Q & A: Cold Backfiring

I have a 74 MGB roadster with a set of HS 4s that appear to be giving me trouble but I’m not positive they are the problem.

Background: Installed new points, cap, wires, rotor, Set points as specified; set the timing as you have suggested; Watched timing with timing light and noticed the timing does jump from time to time but generally it holds steady and advances when accelerated.

Issue: When cold, pull choke, fire motor, run smooth but fast due to choke. Back off choke as she warms. When carb snapped to accelerate sharply, the rear carb backfires on acceleration, backfires, then accelerates fairly smooth as RPMs climb. When the engine warms to operating temp, the carb runs smoother but does backfire from time to time on a hard acceleration. No problem with front carb. Both carbs have had new throttle shafts installed, floats adjusted and jet set at prescribed flats +/- a flat.

You should have HIF carbs on your 1974 MGB -- the SUs with the float bowls on the bottom. What I would do here is to double check the timing. You know, you might try 15 BTDC at 1500 rpm vac disconnected and see how much of a difference that makes... read more