It would appear that you are getting full manifold vacuum applied to your vacuum advance, which is not a good thing.
I would, first of all, make sure that you have the vacuum advance hooked to the correct port.
After that, it is possible that there are leaks in the carb that are allowing full manifold vacuum to reach the vacuum advance port. There are two possible causes: 1. The carburetor was rebuilt with the wrong gaskets the last time it was apart, or 2. there is sufficient wear on the various parts (throttle shaft, etc) to allow full manifold vacuum to the vacuum advance port.
A vacumm advance should never be hooked to the manifold vacuum, by the way. You can also check the advance canister by using a vacuum tool to check whether it is leaking.
