Tuners are often confused when it comes to being able to identify whether a particular adjusting screw on a carb is a fuel screw (that adjusts low speed fuel delivery) or an air screw (that adjusts low speed air delivery). Carb accessory guru Lonn Peterson at Thunder Products tells us that generally, if the adjustment screw is on the engine side of the carb slide it is a fuel screw and thus can be used to fine tune low speed fuel delivery. If the adjustment screw is on the airbox side of the carb, then it is (generally) an air screw and is used to fine tune the low speed air delivery. On most carbs found on late model sleds, this seems to be fairly consistent.
This is helpful to know, as (generally) turning an airscrew out adds more air (a leaner mixture) and turning a fuel screw out adds more fuel (richer mixture). Knowing which is which and what happens when you crank it this way or that aids in off-throttle performance (as the engine comes off idle) and low speed operation, as well as idle stability. Many riders use this adjustment to maintain a crisp throttle response, especially as riding conditions (elevation, temperature, barometer) vary.