Dist. vacuum advance, retard, ported, manifold?, Anybody know the real story? Options

[JPA914]

I'd like to test the vacuum canister on my 1974 2.0. It just has the one line going to the TB, the other vacuum port is open to the atmosphere. TB has only one port, the other is blocked off for my year model. Now, from all I've heard, this is a retard ONLY setup with no vacuum advance.

However, if I'm understanding correctly how vacuum retard works, I think the canister should be connected to a ported vacuum on the TB. And a ported vacuum source is one that is above the closed throttle plate, so that timing is not advanced at idle (no vacuum above the plate) but advanced at part throttle (source is now below the slightly open plate, creating vacuum).

The problem with this model is that the only vacuum port on the TB is clearly below the closed throttle plate. This would be a manifold vacuum source, usually used for advance only, as the advance would be highest at idle (most vacuum) and less and less as the throttle is opened further. There is no "ported" source. Anybody know if this really is a "retard only" system and, if so, how it's supposed to work? I probably need to know that before I test it (IMG:style_emoticons/default/smile.gif)

[jcd914]

The "Ported" vacuum fitting is just above the throttle plate.
Ported vacuum is used for timing advance since it has no vacuum at idle and get progressively more vacuum as the throttle is opened and the port is exposed. This provide timing advance before the centrifugal advance comes but if the throttle is opened wide (as if under load) the vacuum drops of and you don't get excessive advance to cause pinging.

The "Manifold" vacuum fitting is below the throttle plate.
Manifold vacuum is used for the vacuum retard (as well as other things).
It has the highest vacuum at idle and will retard the vacuum for emission purposes. The manifold vacuum drops off as the throttle is opens and allows the timing to advance or return from the retarded position.

At least some of, if not all of the vacuum canisters have two diaphragms but they are attached together between two metal plates, so they always move together. One can leak while the other does not.

Jim