Technical early AFB vacuum ports

I am running an early (50s) square bore 400 cfm Carter AFB and there are two vacuum ports, one in front next to large brake port and one to the right side (passenger). Neither is threaded. Which is the correct vacuum advance port? Or does it not matter.

 

Check them with a vacuum gauge with the engine idling. One may be a full manifold vacuum source and should show 15-20 inches. The other may be a ported vacuum source and show 2-5 inches at idle and closer to full manifold vacuum as the idle speed increases.

Something like the choke pull-off needs full manifold vacuum. Ported vacuum may be required for the distributor vacuum advance, or not. Some distributors and tune-ups like full manifold vacuum at the advance, especially for a street driven vehicle. It mostly depends on how much initial timing you can use without having too much total advance.

 

Pretty much what Clay states. Except that at idle, the "ported" (above the butterflys) port normally has no vacuum.
The distributor doesn't care which port you use. you pick, you tune (initial advance) to that combination.

As stated, the decision is the tuners. You can tune the engine to use either, ported or non-ported. It's all in how much vacuum advance and initial (crank shaft advance) advance you tune into the engine.
When accelerating, normally you want as much power as you can get. Ignition timing is power, so the port that provides the most vacuum as you open the throttle is normally the one you want to use. Read that..."ported" vacuum, above the butterflys.

But experimentation is the key to success...experiment with both ports.

Mike

 

This is true but what I've noticed though is many people prefer a fair bit higher idle RPM than the factory spec as it noticeably smooths out and the reason it smooths out is because the vacuum advance starts pulling in very quickly above idle as there is no load. 20° to 30° or more is not unusual. So it ends up being kind of a moot point.

 

Thanks all, I had been suing the side port and it turns out that it is manifold vacuum, now I have to replace the bad vacuum advance!