"Ported" vacuum is accessed just above (not below) the leading edge of the throttle plate. If the port was below, it would be exposed to constant manifold vacuum. If the idle speed screw is adjusted properly, there should be little or no vacuum on that port until the engine speed is increased, then it will build gradually as speed increases. At light throttle cruise speed, it will be very close to full manifold vacuum and the vacuum advance will provide an extra 5-10 degrees of timing advance over what is provided by the centrifugal advance weights in the distributor. If the throttle is opened fully, the vacuum will drop and the spring in the vacuum chamber will retard the spark to avoid detonation during acceleration. As soon as the throttle returns to cruise position and the vacuum rises, the advance cones back in. A very few engines, like the early 1960's 327 V8's, used full manifold vacuum to the advance for some reason. I've never seen that setup on a stovebolt six with factory carburetion.
Jerry
