So how much effect does adjusting the idle mixture screws have? At what point can you get the engine to stumble/die? Is it only rich at idle, or is it rich at part throttle and going down track? Trying to get it narrowed down as to where it's fat. My guess is something is blocked or stuck open. While you have it apart, make sure to blow out EVERY passage in the carb with carb cleaner and blow it dry with air.