If possible, the vacuum advance should be connected to "ported" vacuum, which is usually tapped off somewhere near the middle of the carburetor. There will be little or no vacuum present at idle, and it will increase as the RPM is raised by opening the throttle. A quick snap of the throttle will kill the vacuum, allowing the advance to retract and minimize detonation during acceleration. Only use full manifold vacuum if no source of ported vacuum is available. If the advance has full vacuum at idle, it will be very difficult to get a good initial timing setting.
Jerry
