if it moves it's not "bad" [which would be a shot diaphragm], but things might be gummed up dirty and you may be losing some vacuum to leaks, so it may not be responding "normally" ... the vacuum advance doesn't move the distributor it's full range of travel, it's simply to give a shot of advance while the revs come up and the centrifugal advance inside gets moving .... are you having problems with the way it runs?
Bill
