This is a close-up picture of the center Weber carb, right side, that feeds #5 cylinder.
The path of the fuel at idle (true idle), is from the bowl, through the idle jet, down the small passageway directly below the idle jet, which turns left and then down past the low range ports to the idle mixture port.
If the engine is idling with all of the idle mixture screws fully closed, the fuel is coming from someplace other than the idle mixture port.
No mystery, the idle speed screw is holding the butterfly's slightly open, allowing fuel to flow through one of the low range ports.
To get the idle mixture screws in the game, back off the idle SPEED screw. This is the one opened at 1/4th turn, according to instructions. Try backing it off completely. It may need to be open slightly if backing it off completely stalls the engine.
One idle speed screw per 3 carb assembly, located at the end of the carbs.
Backing off will close the butterfly's blocking the fuel flow at idle from the low range port.
Do this with all three of the mixture screws open 2 turns. This will keep the engine running. Of course the linkage is dis-connected as well.
At this point you will be able to turn one of the mixture screws in and reduce the fuel to that cylinder. Turn it in until the motor rpm's drop slightly, then open the mixture screw until the rpm's just pickup, about 1/4 turn.
Adjust all of the idle mixture screws, then do as Tyson says with your synchrometer and the idle air screws.
This all assumes the floats and jets are correct.
The key here is to stop the fuel from all but the idle mixture screws, then they have control.
Edit: for the Weber experts out there, you'll notice something missing from the photo. The carbs were on the bench for a photo session.