[AzBob]

A distributor with vacuum advance is primarily controlled by the centrifugal advance bob weight governor. Low vacuum (during acceleration or hill climbing) retards the points plate. When cruising, the vacuum diaphragm advances the timing for improved fuel economy and performance. In other words, a centrifugal advance distributor enhanced by vacuum which is an indicator of engine load.

As Al in NY correctly pointed out, the diaphragm uses a vacuum port above the throttle plates as in the case of my downdraft Weber 32/36. The vacuum diaphragm does not see full manifold vacuum at idle.

The Lucas 25D distributor that Dave of MN modified is a mechanical / vacuum advance type of distributor and works as described above.