Sean,

There are two basic types of low speed mixture adjustment, air bleed screws and needle valve.

An air bleed carb does not have a needle valve for the low speed mixture adjustment, it has an air bleed hole whose size is modified by an air bleed adjustment screw. Turning the air bleed screw one way or the other changes the amount of air that is allowed to bypass the throttle plate, thus changing the mixture, more air (bigger hole) is a leaner mixture.

HTH