[Gaz64]

A power valve is a load sensing "switch" that adds fuel in parallel with the main jet to the main well.

So while driving, we could have 15 inches at cruising speed, 10 inches at light load, then squeezing the throttle more, the vacuum drops lower.

You might want 6.5 as the opening point, or even lower vacuum at 3.5.

Drive you car with a vacuum gauge fitted to understand what is happening UNDER LOAD.

You are free revving your engine, and that is not how to understand PV operation.

This is some info about your carb:

HR-780-VS 1.375 1.375 1.687 (1-11/16) Down Leg 160 72 83 70 31 Vacuum Secondary / Gas

If you say the engine is rich, how far open are your mixture screws?

All carb engines with cams will exhibit some richness, especially the smell at idle/low speed.

Gary