How to Tune a Carburetor on a Motorcycle

by Don Davis
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Motorcycle carburetors take in air and mix it with fuel. Numerous factors can throw a motorcycle carburetor out of tune. Typically, motorcycle carburetors are shaken out of tune by vibration. Manufacturers like Mikuni, Keihin and S & S sell original equipment and aftermarket carburetors for numerous motorcycles. Details vary but tuning principles are the same for all these devices. Here, specifically, is how to tune the carburetor Harley-Davidson has used since 1989.

Step 1

Remove the air cleaner cover with an Allen wrench. Loosen the air cleaner with a Torx wrench.

Step 2

Disconnect the two breather tubes from the top of the air cleaner with your hands. Remove the air cleaner. Remove the air cleaner backing plate with a socket wrench.

Step 3

Put the motorcycle in neutral. Start the motorcycle and let it run until the engine is hot. Leave the engine running during the following procedures.

Step 4

Spray short bursts of aerosol carburetor cleaner into the venturi (the open mouth) of the carburetor. Allow the engine to stumble and recover. Then repeat.

Step 5

Turn the idle speed adjuster screw next to the throttle cam with a flat head screwdriver until the engine idles smoothly without stumbling at the lowest possible engine revolutions per minute. Turn the screw clockwise to reduce idle speed and counterclockwise to increase idle speed.

Step 6

Turn the low speed, fuel-air mixture screw on the bottom of the carburetor 1/8 clockwise. Wait five seconds for the idle to stabilize.

Step 7

Continue to turn the low-speed screw clockwise 1/8 turn at a time with a five-second pause between turns until the engine stumbles and drops revolutions per minute. When the engine stumbles, turn the low-speed screw counterclockwise 1/8 turn.

Step 8

Wait five seconds, then open the throttle. The engine should idle cleanly and respond without stumbling when you open the throttle.

Step 9

Repeat adjustments to the idle speed adjuster screw and the low-speed, fuel-air mixture screw as needed until the engine idles cleanly at low speed and responds without stumbling when you open the throttle.

Step 10

Turn off the motorcycle. Reinstall the air cleaner backing plate, the air cleaner, the breather tubes and the air cleaner cover.

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