Join Date: Sep 2000
Location: Washington, Michigan USA
Vacuum is not adjustable, and has nothing to do with the weights in the distributor. Vacuum is a function of the engine design, intake manifold, and the camshaft profile. The weights and springs in the distributor determine the starting point and the rate of increase in centrifugal spark advance, which is totally independent of manifold vacuum.
You can learn more about how your engine works by buying a book like "How To Rebuild Your Small-Block Chevy" by David Vizard - published by HP Books.
'69 Z28 Fathom Green