All, I too wondered about the statement "with the idle screw turned all the way in" and thought he must mean the idle STOP screw not mixture screw, as he is in the same sentenance talking RPM.
There are carburetors that enrich the mixture when turning the mixture screw in.The updraft Strombergs we used on Chrysler industrial and many Japanese motorcycle carbs come to mind.Stromberg says the idle needle controls admission of more or less AIR to the idle system.
However my book on the Carter BB updraft says the mixture is made richer by screwing OUT the needle.
