Buy a vacuum gauge from an autoparts store, tool section. It should pull plenty, like 19" or better I would think.
Hook it to a vac port, non timed vac port.
The one for the distributor vacuum advance is timed, it is above the throttle blade 'if my my brain is working, still early here in ca.' and only when the throttle blade is opened enough will it see a good vac signal.
Could go off the manifold if you have to.
Take a pic of your carb if you can and show us which 1920 you have