Carburetor Tuning

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Jetting Your Slide Valve Carburetors


All Keihin carburetors are pre-jetted for bolt-on operation. Carburetors are jetted using stock motorcycles and watercrafts. Any major engine modifications like higher compression pistons and racing exhaust systems may require minor jetting adjustments. The following is a guideline for jetting Keihin carburetors. Perform the jetting in the order given below.


1) Correct Float Height

Before changing any jetting parts, check the carburetor floats for correct height. Measure the height from the bottom of the float to the Throttle Opening. carburetor-body gasket surface. Correct height can be found on the chart. When checking the float height, the float should be resting, but not depressing, the spring-loaded float valve pin. This can be done by tilting the carburetor until the float tab just makes contact with the valve pin. If adjustment is needed, bend the metal tab on the float arm until correct height is obtained.--see example #1

Float Height
ALL ALL 28 35-39
9mm 14mm 19mm 16mm
ALL 38 24-38 30-34 36-38
16mm 6.5mm 14mm 20mm 22.5mm

NOTE:(See Jetting instruction #1)
for correct pressure

NOTE: If further information is needed, please contact Carburetor Parts Warehouse
(216) 524-1599

2) Idle

Set idle speed to proper r.p.m, by adjusting the IDLE SPEED SCREW. Turn the IDLE MIXTURE SCREW or the AIR SCREW to for correct procedure, achieve highest speed and best response. The IDLE MIXTURE SCREW (FCR) controls fuel delivery to the idle port and the SCREW is located on the engine side of the carburetor slide. Turning the IDLE MIXTURE SCREW out will make idle and off-idle richer. Turning IDLE MIXTURE SCREW (CR, PWK, PJ, PE) controls the amount of air to the IDLE and SLOW CIRCUIT. This SCREW is located on the air cleaner side of the throttle slide and turning the SCREW out will lean the mixture and turning the SCREW in (clockwise) will richen the mixture.


3) Off Idle To 1/4 Throttle

The SLOW JET and SLOW AIR JET are most effective in this range. When you want a richer mixture in this range, use a larger SLOW JET or a smaller SLOW AIR JET. The opposite holds true for a leaner mixture.


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