there are 2 types of Vacuum , ported & manifold
Mainfold is below the throttle plates , it is highest when idling or when decelerating with the throttle closed , as the throttle is opened the vacuum reading will drop but it will create higher vacuum when the throttle is closed at higher RPM which is why the brakes will generally work the first time & maybe the second time .
Ported vacuun is above the throttle plates in the carb Venturi , this wirks opposite to the manifold vac, ported increases with air flow through the carb so it should be zero at idle & rises with RPM
Long duration cams & large plenum single plane intakes can cause very low vacuum readings at idle , this causes poor response from the carb & bad idle , , advancing the timing, reducing the air bleed size , drilling holes in the throttle plates help to correct idle problems , , long duration cams love intial timing in the 16-22* range , drilling holes in the throttle plates just creases an air leak while aloowing the throttle plates to remain closed aloowing the idle transfer slots to remain covered to aid in pulling fuel in for the idle circuit , reducing the air bleed size also help to allow the low manifold vacuum to pull more fuel into the air stream .
the other adjustment that needs to be made is to lower the powervalve / metering rod spring pressure to the point where the valve stays closed or the rods stay down in the idle position
