The high-end and low-end needle screws are the mechanisms for making adjustments on a Walbro carburetor. By tightening or loosening these screws with a specialized screwdriver, users tune the carburetor to provide the optimal mixture of fuel and air to the engine for peak performance.
The needle screws are typically located on the side of the carburetor, perpendicular to the carburetor throat. Carburetor needle screws have different shaped screw heads requiring specialized tools to properly make adjustments. The types of carburetor adjustment tools are splined, double D, single D and Pacman.
The high-end needle adjusts the fuel and air mixture for when the engine is operating at medium to full power. The low-end needle adjusts the mixture for engine idling up to medium power.
To begin adjustments, turn both screws clockwise until they are tight. Then, unscrew both screws roughly two complete revolutions.
In order to precisely set the screws, start the engine and allow it to warm up for a few minutes. To set the high-end needle, squeeze the throttle all the way open. Then, screw or unscrew the needle until the engine is running at peak revolutions per minute. To set the low-end needle, slowly let off the throttle until the engine begins to "four-cycle". Adjust the low-end needle until the four-cycle sound ceases. Repeat the process of letting off the throttle and adjusting the needle until the engine is running at idle speed without four-cycling.