I agree with just about everything you said.
Ported vacuum means the hole is just a hair above the throttle plate, so it is "open" and active as soon as you open the throttle a tiny bit. This fitting can be virtually anywhere on the outside of the carb. The internal drilled passage is what counts. Same goes for the manifold vacuum fitting. The said Rochester 2-jet I have, has manifold vacuum fitting coming out from under the fuel bowl, WAY above the throttle plate. But the passage goes downwards from there, ending up well below the throttle plates, on the mounting flange.
Here's whats what:
As for ported vacuum being "only" an emissions aid, that I don't agree with. It has the same job as any advance mechanism, providing advance when it is needed, and NOT when it could be harmful. There is no need for "extra" advance at idle once you have the base timing set. (again, in an engine designed to run ported vacuum). It won't run lean at idle if you set the idle mixture correctly. Easily verified on carbed engines with an O2 sensor. All my builds have one. So *idle* vacuum advance has no function. Therefore ported advance does not provide any at idle. It only comes into play when the throttle is opened a small amount. If it created advance at idle too, then you would have no "bump" when you open the throttle, it would stay the same or the difference would be very small. Some engines used a distributor with ONLY vacuum advance. It would then have a stronger vacuum pod, able to retain some advance at bigger throttle openings so you would not try to run the engine solely on base timing alone. Yes ported vacuum. And thus, not only an emissions thing.
The initial bump or spike in vacuum advance as you start cracking the throttle open, does help burn the suddenly momentarily lean mixture when air flow increases rapidly. That's when it leans out. Air moves quicker than fuel, because fuel has much more mass. So it takes a split second for the fuel to get drawn in and balance the mixture out. The quick advance bump helps burn through the lean spot without a stumble. The accelerator jet helps keep the mixture richer as well but it too is momentarily slower than air flowing through the carb.
