Hi, I've also had early 1970's experience and learning and I agree with David. I use ported vacuum because: at idle vacuum is high and you do not need timing advance at idle. You need centrifugal advance as your RPM's increase (low vacuum) so the spark occurs earlier. You need vacuum advance at cruise conditions, where lower steady RPM's ( high engine vacuum) gives high vacuum from above the throttle plates and more timing advance. You should get a good vacuum gauge and learn what it will tell you about your engine, then check out how the engine vacuum changes from a direct vacuum source and a ported vacuum source. I would leave the (inner) unused port on the vacuum advance cannister open. Plugging it may have a damping effect on the diaphram, and just "sitting there" won't hurt anything. Good luck
