The "idle circuit" is the fuel emulsion path within the carburetor passages that provide the correct fuel/air mix for the engine to both start and idle.
As engine rpm/load changes, there are several individual circuits designed into the carb that each provide a different mix as needed, and as the engine transitions in rpm/load, the circuits work together and overlap to provide seamless performance across the power band.
The idle speed (curb idle) adjustment is a simple mechanical stop screw that keeps the engine idle speed constant, that has nothing to do with adjusting the idle mixture, even though the two items are used together since the idle mixture can't be adjusted unless the idle speed is corrected and visa-versa.
When the circuits are not working in harmony is when we have issues that performing a rebuild corrects...if the correct parts are used and not some cobbled up mess of boneyard carb parts. In the last several years, I've delt with only one 19xx carb that was a total virgin and that was just a few weeks ago. I've done more than 30 of these just in calendar year 2010 and each of them was a mess to begin with due to previous efforts at rebuilding a crap carb that had been thrown together out of junkyard parts.
Today...I started building a virgin 1908 (hot air choke and visi-flow fuel bowl) for my personal use/collection. That is a Ford carb that will be slightly adapted internally to my IH engines. I've been waiting for one of these to come along for years, I finally have one. And this one will have an electric choke cap installed, not hot air. But since it's designed for use with a "spark control valve" and a loadomatic Ford distributor, that feature will be eliminated by using a different throttle body and installing the Ford throttle shaft/lever.
I can't really advise you as to "what" carb/distributor combination your particular engine is supposed to have. And...in my world that stuff only means something if I'm doing a complete restoration, not just building a driver. I don't care what something is "supposed" to be as long as the combination of fuel system and ignition system I'm using do work in harmony. Mechanics know how to do stuff like that based upon years of experience, hobbyists and diy'rs generally don't deal with all the variations and subtleties of this stuff to ever become experienced ina hands-on fashion, neither do they have an extensive supply of parts and variations to bring this stuff together. In the case of the 1904, I do have the background and the parts, otherwise I would not be dealing with this stuff on this forum! But I do not have the direct experience needed for the Ford stuff as I've simply never had an interest in anything other than the v8 stuff of the muscle era.
Only you can do an analysis as to what distributor package goes with what carb on which Ford platform based upon the exact design level of the engine you are working with. A perfect example is that your engine is a road draft version but it's been adapted to pcv in an incorrect manner. And I suspect that there has been a whole lotta cooks involved in making that happen that are stirring pots on a whole lotta "Ford" enthusiast forums that do not fully understand the nuances and variations of these vehicles spread over the many years of production as the basic engine systems evolved.
Since you have the Ford shop manual, the ignition and fuel system section in that manual is a very detailed analysis with outstanding line art that explains the functionality of this subject carburetor in great detail, as well as the operation of the various distributors. That manual section was written by Holley service engineering and handed to Ford (as well as ihc and chrysler), that stuff was not developed by Ford. That is the way that supplier contracts for these parts work. That is an excellent reference for these carbs, there is nothing better anywhere. Some of that information has also been posted on I-net sites such as "the old car manual project". Same for the various distributors which were joint venture components done by Holley/Ford and Holley/prestolite.
