The mixture screw on the Keihin carb is located in a vertical position on the forward exterior of the main body. This pilot mixture screw adjusts the amount of fuel flow running through the carburettor during idle conditions. By changing the amount of fuel flow through the carb, you'll adjust the air/fuel mixture of the carb at idle, ensuring that the 1990 Honda CBR1000F idles smoothly. This adjustment process is done with the bike running and the carb fully mounted, allowing you to receive instant adjustment feedback.
Turn the pilot screw clockwise until it bottoms out, indicating that the screw has reached its seat. Turn the screw counterclockwise exactly two turns to establish a base setting.
Start the bike and allow it to idle and warm up.
Turn the screw counterclockwise until the engine idle begins to reduce. Note the total number of turns, including the two base setting turns, it takes to affect the engine's idle.
Turn the idle screw clockwise, listening as the idle increases, peaks, then begins to decrease again. Note the number of turns it took before the idle began to decrease with the clockwise turns.
Calculate the number of counterclockwise turns it will take to reach the midpoint between the farthest out and in on the screw. Turn the screw the designated number of turns and you've properly set the mixture screw on the 1990 Honda CBR1000F Keihin carb.