"Idle mixture functioning."
Are you saying that as you turn the idle mixture screw all the way clockwise the RPM goes down until the engine wants to die?
Are also saying that as you turn the idle mixture screw all the way counter clockwise the RPM also goes down and the engine stumbles?
And that you then adjust the idle mixture screw until you reach the highest RPM?
The reason I ask is that when there is a vacuum leak large enough to prevent lowering the idle speed with the the idle screw (as you have indicated), one generally finds that one does not have the ability to get much to happen to the RPM by adjusting the idle mixture screw.
Do you have a throttle cable connected to the carb? Is the throttle cable stop preventing the throttle plate from closing all the way with the idle screw all the way out?
If that is not the problem, I would check to see if there is any daylight between the throttle plate and the throttle bore with the idle screw all the way out. You may need to take the carb off the engine and shine a light from the bottom of the carb. This can be corrected by loosening the throttle plate screws slightly, allowing the throttle plate to self center, then tighten the screws.
