Did you make sure ALL the old gasket was off the head and manifold?
To check you can run a slow garden hose over it while idling and you will hear a sucky sucky.
Was the gauge vacuum hose a snug fit over the nipple? You should be able to feel it sucking on your finger if you pull the vacuum hose off and cover it with your finger.
Might also be worth doing a compression test, being an unknown motor.