The proper vacuum feed to the distributor vacuum advance is almost always taken off the carburetors "controlled" vacuum port. This vacuum port can usually be identified as being slightly below the throttle plate cross shaft. Basic operation:
1) At idle the throttle plate is nearly closed, cracked just enough to allow "some" air to pass through the venturi to mix with the low speed/idle port fuel mixture. There's very little vacuum present below the throttle plate so NO vacuum is passed to the distributor's vacuum advance. You don't need timing advanced at idle.
2) As the throttle is opened, vacuum is generated under the throttle plate which is then applied to the distributors vacuum advance which activates it (advances timing)
3) Vacuum will continue with increasing RPM until the throttle plate hits a certain degree of open when the vacuum under the plate ceases to be enough to operate the distributors vacuum advance. This is OK as the mechanical advance has taken over by then.
Vacuum advance is off at idle, begins to advance timing at low speeds to mid speed then drops away at higher RPM and/or acceleration.
The vacuum for your distributor should come off of the control port on the dist and not from the manifold as it probably provides vacuum at idle (not needed) and at higher RPM when the mechanical advance should have timing advance control.
Easy way to determine which is the control port is by placing your finger over the port, at idle there should be no vacuum (suction), open the throttle a little and you should feel vacuum (suction)..
