I run an old 12mm Bing on my Allstates. If they are the same as mine, there is no fuel/air mixture screw, as that's what the jet does. There IS a screw there though, that adjusts the idle. It pushes up or down on the slide, but ya only want to adjust it inward with some throttle open; otherwise they dig into the side of the slide.
Not sure if that's the same with yours. I do have a blow-up diagram of my carburetor. If ya need, I could post it up. it's on the other computer.