Larry
I measured the MAP at 750 rpm at various turns of the mixture screw. I could pull the mixture knob to ICO anytime and the engine stopped. I don't have a primer system. Here are the values at turns of mixture screw from closed to open. I stopped at 5 turns.
Turns / MAP
0 / 15
1 / 14
2 / 13.7
3 / 13.9
4 / 14.3
5 / 13.8
While I have never tried this on an MS carb, you should NOT have a smooth running engine at 750 RPM with the idle screw turned all the way in. The fact that a pull to ICO shuts the engine down means that all of the fuel to run the engine is being provided through the carb. I would definately repeat the test with the FAB removed. A plugged filter or other issues could be causing problems in the bleed side of the idle and main circuits. The most troubling part of your results are the 15" at 0 turns and this is best explained by either a blockage in the filter or high fuel pressure.
You are getting a peak at 2 -3 turns out, which is pretty typical for an MS on a 320. However, I would expect larger jumps in MAP per turn of the screw and, of course, would expect much lower MAP readings. Does your MAP gauge read close to ambient pressure before startup? Have you confirmed there is no leak in the hose/line? For now, leave the screw 2.5 turns out.
I would suggest a problem in the idle circuit or possibly pulling fuel from the main jet. However, the likelihood of seeing this problem on two different carbs with one being freshly overhauled by a pro makes this an unlikely scenario. I am assuming this data was taken from the freshly overhauled carb.
Another possibility is high fuel pressure. That carb can only restrain about 6-7 PSI of fuel pressure before the fuel begins to overfill the bowl. This usually results in most problems at idle and off idle transitions. Do you have a hesitation when accelerating from idle? At some point, we should confirm the accuracy of fuel pressure reading. AC style fuel pumps can and do fail by delivering too much pressure. I had this happen on the chevy small block in my boat. IN that case, I could see the fuel overflowing from the bowl vents, however if yours is only marginally high it may impact idle without overflowing. Have you seen any blue staining around the carb?
With your MAP readings, I would not suspect high fuel pressure, except that it could explain why you get 15" at 0 turns out.
If were trust the rebuilt carb, we need to start looking for a leak somewhere in the path between the carb and each of the four intake bosses. You can get a can of starting fluid and start spraying it everywhere on the intake path (don't spray at the main opening of the carb) while the engine is idling. Any change in sound or RPM, means you just sprayed on an area with a leak. Don't forget that the cylinder heads themselves can crack, as can the sump and tubes, so spray everywhere the air/fuel goes to the cylinder. Intake leaks are most problematic at idle and low RPMs, so this does fit your symptoms and would explain the high MAP readings at idle.
The irregular nature of the MAP readings could be explained by an intake leak. However, they can also be explained by carb problems as well as problems in the valve train.
Larry