If you are referring to the threaded port under the right side of the primary fuel bowl, it is a staged or ported vacuum source. Tracing that port should find a hole located above the throttle blades when the throttle is in the idle position.

In the event the staged vacuum hole becomes exposed due to the primary throttle blades being opened too far, then the secondary blades can be opened up in which to close the primary blades. Another option is to drill a small ‘air’ bleed hole in each primary throttle blade which will also allow the primary throttle to be further closed assuming they are open too much. The hole drilling trick is commonly done on the Holley carbs to reduce the amount of fuel transition slot that can be overly exposed by too much throttle blade opening. Putting too small a carb on an engine or going with a much larger camshaft are just two of the reasons a carb may have too much primary throttle opening.