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How do I Change the Needle Seat in a 40MM Keihin Carb

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 back on the carburetor.

Instructions

1

Set the carburetor, top-side up, on a bed of rags on a stable surface. Remove the four screws that hold the top on the carburetor with a flat-head screwdriver.

2

Remove the top, the small collar and the long spring. Lift out the vacuum piston from the middle of the carburetor along with the small parts inside.

3

Remove the needle and the spring seat from the vacuum piston.

4

Reassemble by placing the needle through the center hole in the vacuum piston. Place the spring seat over the top of the needle.

5

Reinsert the vacuum piston into the carburetor body so the vacuum piston fits in the slide track grooves. If the piston does not fit, turn it 180 degrees. Ensure the diaphragm seats evenly in the groove on top of the carburetor body.

6

Place the spring over the spring seat, and carefully lower the top. Hold the top down with one hand.

7

Insert one finger from your other hand into the venturi, and raise and lower the vacuum piston to ensure the spring is not binding.

8

Screw the top back on the carburetor with a flat-head screwdriver.