A needle valve is a small metal pin in a lawnmower carburetor's float assembly. The valve sets the level for the float when providing fuel to the engine. Over time, the needle valve can wear down or break, which requires replacing the valve. Lawnmower manufacturers and various retailers sell needle valves.
Access the lawnmower's engine. On many walk-behind mowers, the engine is already accessible. For many riding mowers, however, accessing the engine may require removing the engine's top cover by taking out the screws that connect the cover to the mower.
Insert a flat-head screwdriver into the float bowl cap, and press it down to pry off the cap. The float bowl cap is at the bottom of the engine's carburetor.
Pull out the hinge pin from the float assembly, located underneath where the cap was positioned. Use needle-nose pliers for that task. The needle valve is right below the float assembly.
Lift the needle valve out of the float assembly with the needle-nose pliers. Set the needle valve aside.
Insert the replacement needle valve in the pin hole of the float assembly. Reinsert the hinge pin right above the needle valve.
Press the float bowl cap back onto the float assembly. Place the engine cover on the mower, and, if necessary, reinsert screws in the cover.