I was wondering if this would come up-
Hey Mr. C,
It apparently came up today.

So if we're going to have this discussion, and avoid any ambiguities, we should agree to use the Shop Manual Terminology, if THAT'S alright with you and Mr. Comechero. OK?
As far as i know- everything pertaining to the idle circuit is on the front half of the carb.
"EVERYTHING"?
That's where the idle air screws are,
The 1964 Ford & Mercury Shop Manual calls these components "Idle Mixture Screws". What you are calling "idle air screws", I shall henceforth call "Idle Mixture Screws". OK?
The reason I question your term about "EVERYTHING pertaining to the Idle Circuit is on the front half of the carb" is because in Group 10, Part 10-4, Page 10-39, under the Sub-Head: IDLE FUEL SYSTEM, the Shop Manual says:
"The difference in pressure between the fuel bowls and the idle discharge ports forces fuel through the primary and secondary stage idle fuel systems."
and when you adjust the idle speed screw, you're tweaking the front butterflies, not the rear.
Alright.
The 1964 Ford & Mercury Shop Manual calls these components "Primary Throttle Plates" and "Secondary Throttle Plates". What you are calling "front butterflies" and "rear butterflies", I shall henceforth call "Primary Throttle Plates" and "Secondary Throttle Plates". OK?
At this point permit me to mention that in Group 10, Part 10-1, Page 10-6, under the Sub-Head: SECONDARY THROTTLE PLATE ADJUSTMENT - 4-BARREL CARBURETOR, the Shop Manual says:
" 1. Hold the secondary throttle plates closed.
2. turn the secondary throttle shaft lever adjusting screw out (counterclockwise) until the secondary throttle plates stick in the throttle bores.
3. Turn the screw in (clockwise) until the screw just contacts the secondary lever.
4. Finally, turn the screw 3/4 turn clockwise for the correct setting."
that's not to say there isn't some sort of base setting on the rear butterflies, but i don't think it pertains to the idle, because there's no way to meter the idle fuel back there.
The above quote out of the Shop Manual refers to the "base setting" of the Secondary Throttle Plates. And the opening of the Secondary Throttle Plates using the Secondary Throttle Plate Adjusting Screw, provides a small amount of air and fuel to go into the Intake Manifold.
It has been my personal experience to, after having replaced the Secondary Throttle Plated Adjusting Screw with a Hex Headed Bolt, to while the Engine is running, increase and decrease the Idle Speed of the Engine with the Hot Engine Idle Speed Screw completely off of its pad, which renders the Primary Throttle Plates COMPLETELY closed, and the Fast Idle Screw not in contact with the Cam in the Choke Mechanism, which also renders the Primary Throttle Plates COMPLETELY closed.
This is NOT theoretical. This is an ACTUAL fact. I have personally done it. The idle speed of an Engine with an Autolite 4100 C4AF-9510-DG Carburetor can be increased and decreased using the Secondary Throttle Plate Adjusting BOLT.
This is not to say that the Engine's idle speed cannot also be increased and decreased using not only the Idle Mixture Screws, the Hot Engine Idle Speed Adjusting Screw, AND the Fast Idle Screw, but if you think that the Secondary Throttle Plates cannot, by adjusting the Secondary Throttle Plate Adjusting Bolt acting against the Secondary Throttle Lever, increase and decrease the Engine's idle speed, you are mistaken.
And my ORIGINAL comment was about making the adjusting of the Secondary Throttle Plates a simple operation by installing a Hex Headed BOLT that could be accessed while the Carburetor was on the Intake Manifold AND the Engine running, rather than having to take the Carburetor off of the Intake Manifold to adjust the Secondary Throttle Plate Adjusting SCREW which CANNOT be done while the Carburetor is installed on the Intake Manifold.
Anyway, was kind of taken aback by the accusatory tone of Mr. Comechero's response to my post. Would like to ask you: Do you think I need a Defense Attorney?
Looking forward to YOUR response, and to Mr. Comechero's reaction.
Be well.
JC