Not all manufacturer's distributors and vacuum advance work the same- with most GM V8's the distributor is at the back of the block and the distributor shaft rotates clockwise. The vacuum advance is on the left side of the distributor and when vacuum is applied, it pulls the pick up coil in a counter-clockwise direction(against spring pressure) and advances the timing. When vacuum is removed from the vacuum advance, spring pressure returns the pick up coil to its original position and timing is returned to initial plus mechanical advance. That is why on a non-computer controlled Chevy V8 you need to use a ported vacuum source- which will be above the throttle blades- for the vacuum advance. Vacuum to operate the advance is created by airflow through the carb- as rpm's increase, ie: you accelerate quickly- the increased airflow through the carb creates vacuum, pulls on the vacuum advance, rotates the pick up coil, and advances the timing. Under load, manifold vacuum decrease. So, if you were to hook up to a manifold vacuum port- a source below the throttle blades- timing would be fully advanced at cruise rpm and as soon as you mashed the throttle, as in to pass someone, timing would retard. Hook a vacuum gauge to different ports on the carb. At idle, a ported vacuum source will have very little to no vacuum. As you increase engine speed, vacuum should increase. A manifold vacuum source at idle, will have full vacuum. As engine speed increases, vacuum will decrease. I don't know which of the other manufacturers distributors work the same; I have heard that some F**ds work differently. I don't own a F**d so I can't say from experience. Hope this helps... Eric
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