Pulled the carburetor and spacer off of a friend's wife's 68 Mustang. The gasket underneath the spacer plate, at the manifold, was cooked. Pulling air like crazy and wouldn't keep running. This gasket looked like a diaphragm type of material. Very thin. Anyway I checked the top and bottom of the spacer with a straight edge from a square and it appeard to be flat along with the carburetor base. Replaced the gasket and it still had a big time vaccum leak. I had to keep spraying carb cleaner at the base to keep it running. Is the spacer gasket, at the manifold, a special gasket material or can I use a regular gasket. I may just try thicker gaskets or have the spacer plate milled if necessary. Any thoughts?

 

It's thicker then most other gaskets. You could try putting two gaskets if using one thicker one doesn't work.

 

Place the plate or carb on 600 grit paper placed on a piece of glass, give it a quick swirl, that'll tell you if it's warped.

IIRC, some 68 4300 carbs have a large manifold vacuum tube sticking out the back. Needs to be plugged if open.