Worth checking the diaphragm is intact by sucking on the vacuum port. More often than not, they're buggered, so there's no vacuum advance anyway, and air will be drawn in during idle. If it's leaking, you're better off plugging the hole in the manifold, and leave it disconnected until you can find another distributer or good vac unit. Only really advances during the overrun and part throttle cruising when manifold vacuum is high anyway.....