67 Mustang 390 vacuum advance

My engine seemed like it could perform better(gets about 8 mpg) and in investigating the dual advance distributor I gave the vacuum advance the "suck" test and there was no leak but no movement of the distributor advance plate,so off to the parts store to get a new vacuum advance diaphragm.After installing the new one I could only get the engine to idle decently with the distributor in the most retarded position. The engine would not run decently past idle and sounded like it was suffering from excessive advance.The vacuum takeoff for the diaphragm seems to be metered and is a tube on the front side of the Holley carb.The engine runs OK with the vacuum advance tube removed and the distributor rotated to a more advanced position. Could it be a wrong or defective vacuum advance diaphragm? or two much centrifugal advance? or the vacuum takeoff on the Holley??Did I throw away a perfectly good diaphram??Sorry to be so qualitative on this subject as I haven't observed the situation with a timing light yet.Any suggestions???Thanks KenG

"kenG2" wrote in news: snipped-for-privacy@comcast.com:

The new vac pot had both ports? I've only seen replacement pots with one.

Sounds to me like the vacuum retard port is hooked to the carb. That would be the one pointing upward. In that case, the timing is being retarded rather than advanced. The vacuum retard port was always routed via a vacuum sensor screwed into the thermostat housing.

It's also possible (but unlikely) that your breaker plate is seized.

There is a great deal of confusion about these ports. Vacuum advance is always ported off the carb. I've had people argue me blue in the face claiming that vacuum advance is taken from the intake manifold. Sorry, that does exactly the opposite of what's intended.

It's also possible that your timing chain has jumped a link, and you are adjusting the distributor to compensate. I've known engines to run quite well like this until the timing chain jumps another link. BTW, 8 MPG is not unlikely with a 390, depending on carb and other options. It's one of the most inefficient engines ever made. It can be made efficient, but it takes careful planning as far as carb, intake manifold, heads, valves, cam, and CR.

IMO, the timing light should have been your first move.