2.5, 3.5, 4.5 power valves the culprit. a holley carb must run rich for light throttle cruising imo. everybody leaning them out when they foul cold plugs street cruising. put a 6.5 pv in mine 73-83 jets and backed timing up 30* total. hotter plugs rc12yc. cruising like a new yorker now. and sounds great at traffic lights. change plugs when using nitrous. car was breaking up like crazy before light throttle cruising, fine wide open. now can cruise or race, just have to change plugs for nos.