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

Well,
Do you understand what advance and retard are going to do for you with regards to how the motor runs?

Yes, I have some idea of why both were implemented and the different effects they have on the engine. I just don't know how my car is configured, which is what my original post is asking.

Well, my choice is always hook up the vacuum advance and never hook up the retard.

Testing, as you asked in your post, would be simple enough with a timing light while you move the vacuum line from one side to the other of th canister to show you what is up.

There are several throttle bodies with both one and two ports.
There are several canisters with one or two ports.
Makes it hard to know which one you left the factory with.

Rich

The lines are different sizes, so no chance of mixing them up. I guess I could rig up something for the test you describe, but first I'd like to see if my canister even holds vacuum. Any idea how best to do this?

BTW, I tend to agree that vacuum advance is preferred in general, as vacuum retard was (according to some) a band aid solution ushered in by the first wave of pollution controls. At the same time, though, I'd want to proceed with caution when second-guessing the engineers. Probably could be done, but it might require a different static timing. Not being an expert, I'd be nervous trying that

Cap off one side, pull a small vacuum on,the other side, see if it holds.

Any idea if the two ports are working on a single diaphragm? Or is it the canister more complex? Forgive me for all the detailed questions, just trying to get a picture of how it works.

One diaphram

Simple test...
Stick finger over one port
Suck on other port
Does your tongue stick to the port?

If yes, it holds a vacuum, move on to another test.

Your base timing is a mechanical setting, your overall timing full advance is also a mechanical setting.
So, you can run the motor with no vacuum advance at all and still end up with the same timing settings, nothing will change.

Vacuum Advance ramps up the advance sooner so you,do not need to rev it as high to get into the torque band.
Vacuum retard reduces the advance when you take your foot off the throttle, the clutch is in, and you are slowing down.
The VR was to reduce emissions due to over run of the motor.

In the end, full advance is achieved due to centrifugal force, not vacuum.

So, if you are getting bad behavior the number one issue I look for is a clean distributor with new grease in between the,advance plates so they move freely.

The OEM grease is now 40 years old and when it gets hot, which it does, it turns into glue versus grease...and it holds the advance plates in all sorts of positions...
Super hard to diagnose.

So, a good cleaning is how you might want to start.

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.

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

Yup, and if you take a look at the advance plates you will see that total advance is never affected by th vacuum advance, it's not physically possible to add more advance to create pinging in our distributors....maybe others, but our are fixed by a mechanical limit.