Two ways that advance timing (distributor turns counter-clockwise) automatically...
Centrifugal advance - there are weights and springs inside the dist. housing that turn it more with increasing rpm.
Vacuum advance - connected to the vacuum port of the carb. Turns the dist. more with increasing manifold vacuum.
The vacuum adv. turns the dist. at idle and cruising speed, when you have high vacuum.
With increasing rpm and lower vacuum, the centrifugal adv. takes over.
There are different distributors and vacuum advances over the years.
For best fuel economy on a mostly stock and street engine, get the matching set for your engine and year.
Check the shop manual for specs.
A vacuum gauge is a must tool to have. It can help diagnosing the engine for various symptoms...
http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htmIf you t-ed this tool to a rubber hose connected to the vacuum advance and suck on it, you know if the vaccum advance has leaks, when it starts to work and when it has reached max. pull.
