I've been looking at my spare Sprint distributor today. I've had to dismantle it to check the vacuum pod and it did seem to be quite stiff. I couldn't move it by trying to rotate the baseplate.
However I filled up the back of the vacuum pod with Plus-Gas and connected the vacuum line to SWK ( an 1850 at idle).
I was expecting the operating rod to move at idle but it didn't ! It only moved when I revved the engine up to about 2,000 rpm. ( i.e when the engine vacuum was lower.)
However the good news was that after a few minutes of revving the engine and dropping back to idle the operating rod moved O.K.
But it was in the direction of retarding the ignition.
Is this correct please ?
I too thought it was manifold vacuum operated and would advance at idle, etc. But I was wrong.
Assuming your 1850 is on SUs, this from sucarb.co.uk/technical-su-carburetters applies:
"On many cars it is found desirable to use vacuum operated ignition advance to obtain optimum part throttle consumption figures. The take-off point for this vacuum is arranged slightly to the air intake side of the butterfly, and in such a position that opening the butterfly allows the throttle disc to pass over the vacuum take-off point so that it then communicates with the manifold depression. By this means the vacuum is small at the distributor during idling and full throttle conditions, and is large at part throttle, being at a maximum when the throttle is open a few degrees. "
So, as the vacuum port it's shut off by the butterfly at idle, ignition timing is going to retard between mid rpm to idle. Is that what you see?
But, wrt checking the diaphragm operation, what's wrong with sucking at the business end and watching the points plate rotate towards you. If it don't move that way, then it's a fault finding exercise seeing if its the VU that's duff or the plate that's jammed, e.g. by leaving off the washer under the cheese head slotted screw that holds the points.