dgc333 said

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.

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moper said

Adding to what Dave said... the vacuum advance adds timing when the engine doesn't mind it. This gives (in most cases) a decent jump in gas milage, keeps the spark plugs cleaner longer, and means the engine is more powerful at partial throttle. It has no affect on idle quality, nor does it do anything at wide open throttle. If you have a race car, or typically don't spend much time at part throttle like on the street, or have an engine combination that doesn't have a vacuum signal in the range of the vacuum pod's adjustment, then running without one makes sense. Otherwise, hook it up and set it up and it will only help.

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