If your mixture screws are not doing anything, then your carb primaries are open too far, you are idling in the intermediate circtuit instead of the idle circuit. If you flip your carb over, you will notice that the butterflys are at the slot in the barrel instead of the meater screw hole. You can either open the secondaries up some and close the primaries, or you can drill a small hole in the primary blades. Either will allow more air into engine while allowing you to close the primaries down until the mixture screws work again. It is idling too rich because its idling in the transition circuit, the mixture screws are not doing anything.