Retarded timing will get you backfiring on deceleration, no doubt. You should put a timing light on it to know exactly what your timing- both initial and total- is set at.Mertz said:The timing is correct as far as I know. There is no indication of valve problems based on the vacuum gauge. Could this be lean conditon?
Check the plugs after a run to see what they tell you AFA the carb being lean.
Be aware that some non-diagnostic type vacuum gages are damped to keep the needle from fluctuating- exactly what you DON'T want when diagnosing an engine. Be sure your gage is one meant for the purpose. Because a valve sealing or cam lobe problem could be at the root of it all.