How to Adjust Carburetor Needle Valve

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How to Adjust Carburetor Needle Valve

 your efforts to keep your equipment running seamlessly, you will often find that you need to service the Carburetor needle valve .

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 center bolt from the float chamber. This is often a 7/16” or a ½” size. Tug gently on the float chamber and remove it.

Pull the pin out that holds the float in position. Notice that the clip holding the needle valve and the needle valve come out with the float. The position of the clip is important. It should have its open side toward the air intake side of the carb.

The Carburetor seat valve is up inside the hole between the float mounting points and the center casting of the carb. I use a special tool that looks like a darning needle on one end to pull the old valve seat out. I have also used compressed air into the gas inlet. If you use this method, put a piece of shop towel over the hole. This keeps the rubber piece from coming out like a rubber bullet. Trust me on this one. Another method I have used is to carefully thread a course screw into the rubber and pull it out that way.