Do you mean when you connect the vacuum line to the vacuum advance canister (looks like a metal biscuit, sort of)? If so, then take a length of rubber vacuum tubing, connect one end to the advance canister pipe and suck on the other end. You should be able to know if the advance is working. It won't take a lot of vacuum, either. If on the other hand you get no resistance when you suck on that tube, then it tells you the vacuum diaphragm is ruptured (common...happens when they're assembled and you see it pretty often today because they're made in a country with 5 letters beginning with C which isn't Chile or Czech). If you get no resistance, then you know your carburetor is getting open air back to it, and if your distributor was meant to work on manifold vacuum (as opposed to ported vacuum), then yes...with an open vacuum line your engine will die.
As I recall there were a couple of different versions of the 250 and one had ported vacuum while the other had manifold vacuum. If you have manifold vacuum and it is around 17 or so, then hooking that up to the distributor might cause it to go fully advanced immediately. If you have ported vacuum, there probably will be little to no vacuum at idle.
Let's start there. Can you tell us if your vacuum advance line connects to the base of the carburetor above or below where the throttle valve will be?
