the stop screws for the butterfly, but you need to go the same amount on both sides to keep the carbies in sync.

Yes, the stop screws will adjust the idle speed, but if it's not idling well you may also need to adjust the mixture. Slowly turn the screw on one carb until the engine starts to stumble (you'll hear it go "off" a bit,) then back it out 1/2-3/4 turn (experiment a little - learning to tune by ear can take a while, but you'll end up with the best result.) Then do the same with the other carb and take it for a spin. If it stumbles off idle it may be the accelerater pump or a dizzy prob, or you may need to overhaul the carbs (or they may be still unbalanced.)