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)

[Dave_Darling]

Most of the 74+ 914s were set up with vacuum retard only. The single fitting on the throttle body will be a bit downstream of the closed position of the throttle plate. When the throttle is closed, there should be some vacuum on that fitting, which would be appropriate for vacuum retard.

The ported vacuum would come from the other side of the throttle body, and the fitting would be almost even with the closed position of the throttle plate. That way you'd get the ported vacuum when the throttle plate was open just a very little bit.

The advance and retard both work on the same diaphragm in the dashpot. Note that the advance and retard fittings are on opposite sides of the dashpot.

--DD

[JPA914]

Most of the 74+ 914s were set up with vacuum retard only. The single fitting on the throttle body will be a bit downstream of the closed position of the throttle plate. When the throttle is closed, there should be some vacuum on that fitting, which would be appropriate for vacuum retard.

The ported vacuum would come from the other side of the throttle body, and the fitting would be almost even with the closed position of the throttle plate. That way you'd get the ported vacuum when the throttle plate was open just a very little bit.

The advance and retard both work on the same diaphragm in the dashpot. Note that the advance and retard fittings are on opposite sides of the dashpot.

--DD

Yeah, after reading a bit more, it seems these 2-way (advance and retard) vacuum canisters work sort of backwards from what I'm familiar with. Usually, the "downstream" port (past the the throttle plate) is used for vacuum advance and the "ported" fitting would be hooked up for retard. At least on some other cars. Oh well, always more to learn!

In doing more research, I also am becoming convinced that these vacuum canisters have two diaphragms instead of one. Some members here have found the advance side is holding vacuum but not the retard and vice versa. Moral of the story: if you're testing the canister, be sure and pull a vacuum on both fittings.