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.

Use a vacuum gauge, adj. to highest reading. 

"FREE SAMPLE"

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.

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.

I believe that I have it set pretty good. I followed the tips you guys wrote and it does idle nice and smooth. It will start to stumble and die when I turn the screws in to about 1/2 turn out. Idles really smooth between 1 to 1 1/4 turn out on each screw. I was not able to take the car out due to the weather today, but it is supposed to clear up tomorrow. If I have to adjust, I will only adjust in 1/8 turn amounts.
Thanks for the tips!

Right, but you were talking initially about 1 and a half to 2 and a half turn increments. If you have a vacuum gauge or AFR meter that might help to see just how finely calibrated the idle mixture screws really are. Just a smidge maybe 1/16 or so - will move the AFR a whole point. You'll probably never able to hear this or detect any difference in the way it idles, but it might fail a smog test

If engine starts to labor and stumble with idle mixture screws 1/2 turn out, that's pretty typical and what you want to see. Just slightly richer from that point consistent with a smooth idle - that is the optimal point per the manual. Then leave it alone. Richen up the "pump shot" from the overtravel lever arm holes if necessary.

There are other seasonal or "climactic" adjustments to make on the carburetor depending on temperature, and like you mention every engine is a little bit different. That's why they have those idle mixture screws instead of fixed jets in the first place. It might be necessary to make a slight adjustment to them for altitude or season.



But it's not uncommon in the auto forums sometimes to hear things like "My engine won't even idle without the mixture screws turned 4.5 turns out, they are almost falling out", things like that, this is obviously some tuning defect that is affecting the carb operating point.

Well I was able to take the car for a ride yesterday, it runs like a champ! No more stumbling from 5 - 35 MPH. It cruises along great at 20 MPH and accelerates without a cough or stumble. It stays running when I put it into gear, and I have the idle RPM lowered to where it seems it should be, but I don't have an RPM gauge. I ordered a vacuum gauge tester and will use that when that arrives. I am interested to see how much I will still need to adjust. This new ultrasonic cleaner just paid for itself with how well it did on this carb! 
Thank you for the tips. 

Notice every Tune-Up and Shop manual always has the vacuum test as the second diagnostic procedure (after compression) and keep in mind the latter is primarily just to determine whether the engine in question is mechanically suitable for performing a Tune-Up in the first place. I bought a mechanic's vacuum gauge years ago and eventually (I'm slow) figured out it was one of the most useful tools. They can help diagnose an amazing number of engine derangements. All that, and a fuel pump tester too!

I went to a GM tune up and carburetor school many years ago.  We were taught to start with a warm engine below factory idle speed.  start with the mixture screws 1 1/2 turns out from totally closed.  then, using a tachometer, adjust the screws in or out 1/4 turn at a time.  both screws should be turned the same amount.  you adjust until you get the highest idle with the screws and after each increase in RPM, adjust the idle down to the starting point.  Once you have reached the point where any adjustment diminishes the RPM you are done.  Then idle to factory spec and you are done.

Here is my update on my 56 Mercury 312..  I used my new vacuum gauge and my new timing light. the vacuum is 19-19.5! From everything I have read I believe that is a good reading and it is on the upper end of the "normal" range on the tester, so I am happy with that.  The timing is what I thought too far advanced, but after reading through a lot of the timing discussions on here, I guess that it is not that unusual. It is about 30 degrees BTDC at idle(650-670). It is advancing nicely as the RPM rises. I can not seem to get the idle down to the 450-550 as the manual says. My idle is about 650-670 according to the timing light RPM reading. It seems/sounds like a nice low idle to me, and it sure idles nice and smooth.  
We put 100 miles on the car today, and when I refilled the tank it worked out to 14.25 MPG. That was 40 percent town, 60 percent highway driving. I was happy with that.
I know it is not matching the manual, but I am thinking I am going to leave it as it is now being it seems to run so good, nice and smooth at all RPM, smooth idle, smooth acceleration, smooth cruising at low speed and highway speed. Starts and shuts off normally.
So here is my big question of the day:  with it running so well, do I leave it as is or do I continue to try to get it to match the manual for timing and idle speed?  
I am thankful for all the topics on here, most all of my questions are answered by reading through the questions/answers that are already on here. 
Any comments are appreciated. Thanks.