The throttle plate needs to close above the port for vacuum advance, providing vacuum to that port (engine side of throttle plate) when at an idle.

EDIT I may be wrong above.. the throttle plate should close and cover the vacuum port.. best bet to look at a good carburetor and see how it's done.
I have Zenith that from new (it was NOS) had the throttle shaft installed upside down, causing the throttle plate to close in the wrong place in relation to the vacuum advance port, as a result the vacuum advance didn't work as it should. Reversed the throttle shaft and all is well.

Last edited by Grigg; 05/12/2019 6:34 PM. Reason: oops
•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-