An air screw (on a two-stroke) and a fuel screw (on a four-stroke) are like the nostrils on a race horse. They can be adjusted to control how the incoming air and fuel are mixed. When the carburetor's slide is closed to a quarter throttle or less, it doesn't atomize fuel effectively. For these quarter throttle or less settings, a carburetor uses bypasses to efficiently meter fuel to the engine. These passages channel air into the pilot circuit (where it mixes with gasoline). This circuit controls how the bike runs from idle to the midrange. Both the air screw and fuel screw are tapered needles that effect the mixture of gasoline and air. Both are adjusted with a screwdriver.

WHAT IS THE DIFFERENCE BETWEEN AN AIR SCREW AND A FUEL SCREW?
There are two ways to control the amount of fuel and air metered by the pilot circuit, and they differ by engine type.

Air screw (two-stroke): The externally adjusted air screw controls the amount of air that is fed into the pilot circuit. It is adjusted by turning it with a screwdriver.

Fuel screw (four-stroke): This externally adjusted needle meters how much of the fuel and air that was mixed by the pilot circuit flows into the engine

HOW DO YOU ADJUST A FUEL MIXTURE SCREW?
Set the bike to a high idle. Turn the fuel mixture screw in until the engine almost dies. Count the turns out until the engine rpm peaks. If you turn the fuel mixture screw too far out, the exhaust note will become dull and lumpy.

WHAT DOES LUMPY AND DULL SOUND LIKE?
It is the opposite of crisp. When engine rpm peaks, the exhaust note is sharp and crisp, but if you go too far, the exhaust note will become choked and will lose that snappy sound.