Carburetor Tuning

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Jetting Your CDK II Diaphragm Carburetors
1) Idle

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

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. 

3) Pop-Off Pressure

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.

4) 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.


Pop-Off Pressure
Needle Valve Size
1.6(STD 1.8 2.0
42mm 70GR.(std) 11mm 1290-947-2000 24psi 21psi 18psi
40gr. 9.17mm 1290-966-2000 15psi 13psi 11psi
mid range
& 42mm
70gr. (std) 8.7mm 1290-961-2000 N/A N/A N/A
40gr. 7.4mm 1290-992-2000 N/A N/A N/A


Calibration Chart (for CDK II)




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