I think the idea generally with these is always to turn them in (clockwise) and lean out the idle mixture as far as possible, consistent with a smooth idle. There will be a definite noticeable point maybe 1/2 turn to 3/4 out where the engine will start to labor or stumble. Slightly richer from there will be highest vacuum. It's termed the "idle circuit", but it is very important to make this adjustment closely as it definitely affects just off idle acceleration and driveability, where much street driving is done. The "transition slot" is dependent on this. The idle circuit feeds some fuel up to around 35 mph or so.

If the engine won't idle or run reasonable without turning the screws to some extreme, particularly beyond the bench setting it's a safe bet something is screwy somewhere in the setup - obstructed idle air bleeds or passages, vacuum leaks, maybe ignition timing defects, incorrect float height, or combination of all of the above. It's practically a diagnostic if something like the idle mixture won't adjust or respond normally to factory spec it always indicates a fault that lays somewhere else. Carburetors are funny that way. The Autolite (Holley?) with annular booster discharge clusters is considered by many to be the finest ever design.

One thing I noticed messing around with a wideband AFR sensor is just how precise those mixture screws actually are, we tend not to think of a carburetor as a precision metering device but they are nothing short of amazing considering the technology of the time. Just a very slight turn makes a major change in the idle AFR, the spark plugs will start to "load up" and begin to foul within minutes if too rich. Recall that just towards the end of the carburetor era the mixture screws themselves were pretty much disabled or crippled from the factories to try and prevent any (mis)adjustment.

You'll find if you get things really dialed close seasonal temperature changes are definitely noticeable right away. I think that's why people tend to run with "pig rich year round" as a general rule as the transition into wintertime and back into summer can be made without any attention.

Does it start to stall when you turn them in? If they do, 1 at a time,  your butterflies are pretty close on their transition. If not, there are passages not open or clean enough. Lord is correct use a vacuum gauge. For an automatic car start in neutral and get as close as possible, then put in gear. 

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Use a vacuum gauge, adj. to highest reading. 

"FREE SAMPLE"

My 56 Mercury with the 312, carb is an Autolite 4100. I have it all cleaned and rebuilt. Everything I see/read says to set the two adjustment screws 1 1/2-2 turns out as a good starting point. My question is which way do they commonly go from there? I realize every engine will be slightly different but I don't really notice a difference from 1 1/2 to 2 1/2 turns out. I have held at different RPM and adjusted, but still don't notice a difference. I have not had it out of the garage yet, I am guessing I may notice difference when the engine is under a load? Is this simply going to be a trial and error method?
Any tips/suggestions will be appreciated.