If not already done, would assure that the two carb vent hoses are serviceable and that both of them exit to open air.
The vents are needed to allow supply of incoming fuel into the float bowls.
so are the vents you speak of between 1/2 and 3/4, are they "T's"?
This is an old thread but the tees between the carbs are fuel inlets.
Vents are the brass tubes on carbs 2 and 4.
