Mechanical slide and constant velocity carburetors used on motorcycles use a needle valve to control fuel flow through the main jet. The tapered needle is fastened in the slider portion of the upper carburetor and it slides down into the main fuel tube. The taper on the needle determines how much fuel can be drawn into the intake stream. The needle position corresponds to throttle setting, all the way into the tube for idle, and sliding most of the way out of the tube for wide-open throttle. Carburetor needles may be adjusted to lean out the main fuel circuit, or make it more rich. Raising the needle in the slider will make the air/fuel mixture more rich, and lowering the needle will lean it out.
Remove the top of the carburetor. Turn the carburetor top mounting bolts counterclockwise until they are removed, then lift the top and slider from the carburetor body.
Remove the needle retainer from the slider. Push the needle up into and then out of the slider. Remove the locator clip on the needle with the pliers.
Replace the locator clip in a lower slot to raise the needle and make the mixture more rich, or in a higher slot to lower the needle and lean the mixture out.
Reinstall the needle in the slider and replace the retainer clip. Guide the needle and slider back into the carburetor body, ensuring that the needle slides easily into the main fuel tube. Re-install and tighten the carburetor top cap bolts to factory specifications for your make and model of bike.
Test-run the engine. Sneezing and popping back through the carburetor at high RPMs indicates a lean mixture and too low a setting on the needle. Excessive black smoke in the exhaust indicates a rich mixture and too high a needle setting. Repeat the steps to correct the needle position.
