This picture shows where the vacuum advance port goes to. It is above the throttle plate, which means that manifold vacuum does not apply to it. As the throttle plates open, the air rushing by creates a vacuum in the carburetor. This is how the fuel is sucked from the bowls, and is how vacuum is created at the spark port. The more air that rushes by, the higher the vacuum.
If this doesn't explain it, I quit.
Take care,
~Chris
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