What is the function of the adjustment screw/stop on the throttle linkage that touches the plate connected to the choke? How should it be adjusted?
The fast idle screw only activates when the choke is applied, increasing engine speed during warm-up.
I can’t quite identify which version of the jet that is. You can search for “4MC .pdf” and “M4MC .pdf” on Google, which will lead you to two different factory service manuals.
Feel free to ask me most questions—I’m familiar with most Quadrajet versions inside and out.
That looks to me like a square bore carburetor on a '64 Impala with a 327 engine.
That’s definitely a 4GC. Try Googling “4GC .pdf” if you can’t locate it. Let me know if you need help.
If it’s a Carter, it would be a WCFB.
I think it might be a Carter AFB or AVS. Turn the screw to increase or decrease the gap between the throttle linkage and the plate, setting the desired fast idle RPM.
Hello. I believe that also controls the base idle speed once the choke is disengaged. The steps control the fast idle speed when the choke valve is closed. If it’s too high, adjust the rod to lower it, rather than the screw, so you don’t lose the base idle setting. Hope that makes sense. Also, that’s a 4GC carburetor—it looks kind of square when viewed from the top.