You changed the carb & that (significantly) changed the symptom in question. acts up on part or WOT? (or both). I would go back into that carb & also check for vac leak (carb & elsewhere). I ain't up on 6 paks but I think you can unhook the end carbs & run on the center carb & get it sorted out (esp cuz your changing it out drastically changed how it ran). Might be a vac leak inside/or connected to the outer carbs