Hi, ported vacuum is used for a vacuum advance distributor. The vacuum source is above the throttle plate. There is no vacuum "signal" at idle, only when the throttle is partially open. Put a vacuum gauge on your hose and open the throttle. You will see the vacuum drop (no vacuum advance, only centrifugal advance) as the throttle opens. The vacuum advance kicks in during steady part throttle cruise high vacuum conditions. Now you know. By the way I have a feeling the shaft to body seal is defective in your distributor. See if the shaft is smooth and tight in the bushing, no wobble.