From what I've read, the AVS is an air valve similar to the quadrajet set-up. This set-up provides a much smoother transition when opening, and is much more accurate, being controlled by airflow. AFB's used a counterweighted air valve, as I remember wasn't as easy to adjust. Holley uses a vacuum secondary on some carbs..this vacuum is flow generated at a venturi, not manifold vacuum.
On my 302's w/450 cfm Holleys, I get 4200 rpms wide open. At 3000 rpm cruise, vacuum is about 6". This drops rapidly as the props get dirty. If the secondaries open, it really drops. I suspect a quadrajet with the small primaries may get slightly higher cruise vacuum. Quadrajets use a power piston system with metering rods, which is held closed by vacuum against a spring. I think AFB's use a similar set-up. They're also useful for idle set-up, but I always fall back on the "lean 'em until the engine stumbles, then fatten a 1/2 turn. This is substantiated by the vacuum readings. Interestingly, my 2 engines have about 3" different vacuum at idle, probably due to differences in cam timing and connecting rod offset.
Quadrajets have a similar leaky well plug that causes dry starts. There's enough fuel in the accelerator pump to light off....once, briefly.
Because of the cold nature of marine engines, and the fact that often the throttle is "firewalled" rather than opened gradually, I still prefer a maximum accelerator pump shot.
We haven't even discussed inlet air temperature or the effect of flame arrestors on breathing. Or timing.
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