Distributor Vacuum Advance?

Vacuum advance only operates at light throttle cruising, it's purpose is to improve efficiency and economy. With any significant throttle opening there is not enough manifold vacuum for the vacuum advance to add timing.

The vacuum advance is normally hooked to the timed port that is half way up the side of the primary metering block. This port is exposed to the venturi above the throttle blades when they are closed so there will not be vacuum on the port at idle. Connecting it to a port exposed directly to the manifold will cause full vacuum advance at idle which can cause off idle drive-ability problems.