Spread Bore vs Straight Bore Carbs

Vacuum secondaries are not 'held shut' by manifold vacuum, they are pulled open by primary venturi vacuum. There is a tiny hole in one of the primary venturi, I think the passenger side, that connects to the secondary spring/vacuum diaphragm. Your vacuum secondaries will only open if you have enough airflow through the primary venturi to overcome the spring pressure. Switch to a heavier spring if they're opening up too early.

Holley 2300C, 4160, 4160C carbs