I've got a 4160 (0-80551) marine carburetor on a Ford 351W engine (PCM marine conversion). The carburetor is new (not rebuilt). The engine is a 1994. Ignition timing is set per OEM specs at 10 degrees BTDC at idle. The carburetor and engine seem to working well. Power is strong and transitions are good. I am at 4,000 feet of elevation, so I replaced the main jets going from the stock #66 jets down one size to #65 jets. Additionally, I had to deal with a hesitating / stumbling problem when going from idle to wide-open-throttle. I changed out the accelerator pump nozzle from a number 31 to a number 35. Hesitation / stumble is now gone. Everything seems to be OK. Here is my question:

When setting up the carburetor initially, I set the idle mixture screws using the following procedure:

Initial idle mixture screw setting started at 1 1/2 turns out (counter-clockwise) from lightly seated.
750 RPMs.
No vacuum leaks (that I could determine).
Transmission in gear, idling forward (on boat trailer, in the water).
Vacuum gauge hooked up to PCV hose connected to carb/manifold spacer (should be true manifold vacuum).

Going with 1/4 turn increments, I could not get the vacuum gauge to read much higher than 12 inHg.

I went up to 5 turns out and the vacuum only decreased by 2 inHg down to 10. The best vacuum reading I got was at the starting point of 1 1/2 turns out.

Then I tried something...I turned the idle mixture screws all that way closed (clockwise) to "lightly seated". The engine did not quit -- and didn't appear to be surging or sputtering, which is what I was expecting.

Is this normal? Does this indicate a problem? Is this due to the fact that even though the carb is in the idle circuit, is already starting to transition to the main circuit?

Thanks for your input.