I messed with a carb similar on a jeep, are you sure the big plug on top is vacuum,,???it could be a bowl vent,,I plugged one once, it created all kinds of havoc trying to fill the bowl, pull from the bowl etc,, I always wondered where one of the lines from the carbon canister went,,after the carb swap,,,I wrapped a hose around from the carb to the fire wall and put a differential vent on it,,,
