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Jetting Keihin Slide 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
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.
4) 1/4 TO 3/4 Throttle
The JET NEEDLE is the most effective component in the range. Changing the STRAIGHT DIAMETER (D) will change the calibration in the transition range from the SLOW circuit to the MAIN circuit (1/8 to 1/4) throttle. A smaller diameter will make this range richer and a larger diameter will lean this range. TAPER (A) changes are only made if there is a problem balancing the calibration between 1/4 and 3/4 throttle. If the mixture is rich at 1/4 throttle and lean at 3/4 throttle, a JET NEEDLE with a larger taper is needed. If mixture is lean at 1/4 throttle and rich at 3/4 throttle, change to smaller taper. If the calibration is lean from 1/4 to 3/4 throttle, raise the JET NEEDLE by lowering clip position, or use JET NEEDLE with shorter length (L1). If the calibration is rich, lower the JET NEEDLE with a longer (L1).
5) Wide Open Throttle
Changing the MAIN JET affects this range. Select the size of MAIN JET which offers the best W.O.T. performance, then install one size larger MAIN JET for ideal engine durability.
6) Accelerator Pump
In normal applications, the ACCELERATOR PUMP should not require any adjustment. If a rich stumble occurs as the throttles are opened, the ACCELERATOR PUMP timing can be delayed by widening the gap on the ACCELERATOR PUMP linkage where it makes contact with the plastic lever. Reducing the gap will cause the ACCELERATOR PUMP to deliver fuel earlier.
7) Throttle Valve Cut-Away
(FOR CR, PWK, PJ, & PE) THROTTLE VALVE CUT-AWAY will influence the calibration in the area of 1/8 to 1/4 throttle. What the CUT-AWAY does is change the air velocity over the MAIN NEEDLE JET which changes when the MAIN SYSTEM begins delivering fuel. If the transition is lean change the throttle valve to a valve which has less CUT-AWAY (lower number). If this range is rich use a throttle valve with more valve CUT-AWAY (higher number).
Below listed are .PDF adobe files on carburetor tuning. However these might be for specific carburetors we highly suggest you read them regardless of make or model of carburetor. These files contain more helpful TUNING techniques.
Jetting CDK II Diaphragm
Set the idle speed to proper r.p.m, by adjusting the IDLE SPEED SCREW. Next, turn the IDLE MIXTURE SCREW to get the highest speed, then turn out the IDLE MIXTURE SCREW 4) approximately 1/4 to achieve best response. The IDLE MIXTURE SCREW controls fuel delivery to the idle port and also contributes fuel to the SLOW CIRCUIT.
2) Off Idle To 1/4 Throttle
3) Wide Open Throttle
Changing the MAIN JET or HIGH SPEED SCREW adjustment affects this range. Start adjusting by turning the HIGH SPEED SCREW until the best performance is achieved. Then, check number of turns on the screw. If the screw is adjusted all the way in, install a smaller MAIN JET. If the screw is out more than three turns, install a larger MAIN JET. Continue until optimum performance is achieved and the HIGH SPEED SCREW is at approximately 1 to 1 1/2 turns. This will allow some adjustment range on the HIGH SPEED SCREW to perform minor adjustments or to compensate for temperature changes.
The SLOW JET is the most effective in this range. When you want a richer mixture in the range, use a larger SLOW JET. The opposite holds true for achieving a leaner mixture.
In most applications it will not be necessary to change the POP-OFF PRESSURE. If the engine is modified or a larger carburetor is used, changing POP-OFF to a lower pressure may benefit the calibration. If engine exhibits poor acceleration or is lean at W.O.T., try lowering the POP-OFF pressure by using a lighter spring or a larger valve.
Needle Valve Size
Calibration Chart (for CDK II)
Jetting Mikuni Slide
Below listed are .PDF adobe files on carburetor tuning and parts. However these might be for specific carburetors we highly suggest you read them regardless of make or model of carburetor. These files contain more helpful TUNING techniques.