For an explanation of vacuum advance, think of the vacuum advance as a "load sensor" for the ignition system. As the load on the engine increases, the amount of timing that it requires is decreased. At light loads the engine can tolerate more spark advance, so the vacuum advance is added to the initial + whatever mechanical advance you have. The vacuum advance system is what allows you to run this extra spark advance at light loads. As the load increases, the vacuum advance provides less and less advance. So, you can think of the vacuum advance as a "vacuum un-advance" in that it un-advances the timing as the load increases.
Hope this helps.
MIke
Hope this helps.
MIke