The vacuum advance only functions at light-throttle cruise. As soon as the throttle is opened even a little bit to accelerate or pass, pull hills, etc., the vacuum goes away. The timing drops back to the initial + centrifugal advance setting. Try moving the distributor rotor clockwise by hand and see if it moves smoothly and springs back to the same position every time it's moved. The weights and springs have a habit of getting gummed up and sticky and making the centrifugal advance unpredictable. You can access the weights and springs by removing the breaker plate. Do a good cleanup and be sure the springs are in good condition while you're in there.
One degree of over-advance on the timing robs as much horsepower as 5 degrees of retard from the "best-power" timing setting, so err on the side of "not enough" if possible. I have documented this on dyno runs repeatedly over the past 30-something years on all sorts of engines, not just stovebolts. Detonation has a habit of deflecting piston ring lands, breaking rings, and even burning holes in the tops of pistons, not to mention destroying spark plugs. Even a little detonation rattle on a pull is not your friend.
Jerry
