This is a tough one. The symtems are classic for the plug in the discharge passage in a Model B carb falling out. But trying 3 different carbs & getting the same results rules that out. What about a vacuum leak in the plastic spacer under the carb. It needs a gasket on both sides & sometimes can have a crack in the spacer. Its possible a stuck intake valve is letting compression back up into the intake manifold.
I agree a vacuum gauge is a poor way to set timing. You will generally wind up way too much advanced.
