Do you need to hook up vacuum advance?
You want to plug the line to the vacuum advance when setting the timing.
After setting the timing, hook the vacuum line back up and adjust your idle speed if necessary.
When checking you mechanical (centrifugal) advance, the vacuum line must be plugged as well..
How does a vacuum advance work?
Vacuum advance works by using a manifold vacuum source to advance the timing at low to mid engine load conditions by rotating the position sensor (contact points, hall effect or optical sensor, reluctor stator, etc.) mounting plate in the distributor with respect to the distributor shaft.
How much vacuum advance is too much?
The maximum is about 14 degrees of vacuum advance. If too much advance is added, the engine will either start to knock or ping or perhaps it may surge slightly at very light throttle opening with high vacuum.
How do I adjust my vacuum advance?
Disconnect the vacuum pump and insert a 3/32 Allen wrench into the port on the vacuum advance where the hose connects. There is a small adjusting screw in the vacuum advance. Turn the screw clockwise to decrease the vacuum advance and counterclockwise to increase the advance.
When should vacuum advance start?
Carbureted engines with long-duration cams will benefit the most from vacuum advance. If your engine idles at 10 inches Hg or less, you can start the tuning process by setting the initial timing to at least 15 degrees BTDC.