:rofl:
I was just going to ask if You were using a handheld tune-up tach, or one already in the vehicle, & to check the setting/hookup......some old tachs also have an issue with HEI signals depending on the hookup.
Is it backfiring as in spitting/coughing out the carb, or is it combustion of unburnt/excess fuel in the exhaust pipes......or both?