vacuum advance '67 bug

I have found conflicting info on how to test the vacuum advance. One says suck on the hose and if the points move it is ok. Another says suck on it and plug the hose if the points move AND THEN DO NOT move back, it is ok.

The difference is that the second option requires the vacuum to be held.

On mine I can move the points and keep them from moving back by sucking constantly. In other words the vacuum is bleeding out. If I plug the hose the points move back.

Is this vacuum advance good?

Per the shop manual max advance occurs with only about 2.4" of vacuum.

 

The vacuum advance advances the ignition timing to improve fuel economy when you are cruising down the road with high vacuum, very little load on the engine. The advance should work as vacuum is applied and if the advance slowly falls off when the vacuum is held in the unit it is a sign there is a leak in the hose or the advance unit itself. Just suck on the hose and then pinch the hose off with pliers and see if it holds vacuum.

 

If it is not holding vacuum, the diaphragm is going and / or the hose is dried and allowing air to seep around it, where it connects to the advance. I would replace them both.