The 40 mm, constant velocity, Keihan Carb is probably the most common carburetor on motorcycles. It has been standard equipment on BMWs, Harleys and on several Japanese bikes as well. In a nutshell, you change the needle and seat by removing the top of the carburetor, pull out the vacuum piston and the three small parts inside, reassemble the vacuum piston with a new needle, and screw the top
