How to Adjust a Mikuni Carburetor

by Editorial Team
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motorcycle image by Greg Pickens from

A carburetor is a mechanical device that combines air and fuel in an internal combustion engine. The carburetor controls the flow of air into an engine. The speed of the air entering the engine determines the amount of air pressure in the engine. The air pressure affects the amount of fuel being drawn into the jet stream and ultimately entering the engine. A Mikuni carburetor is a specific type of carburetor that is manufactured in Japan and used in a variety of Japanese motorcycles, including Yamaha, Honda and Kawasaki. It is a good idea to occasionally adjust a Mikuni carburetor to ensure optimal engine performance and to make sure that your bike is not putting unnecessary strain on the engine.

Step 1

Pry the air filter off of its mounting using the flathead screwdriver. Make sure the air filter is clear of debris and pop it back into place. If the air filter is damaged or corroded, it should be replaced.

Step 2

Locate the air screw at the back of the carburetor and adjust the screw using the Phillips-head screwdriver. Turn the screw to the right to lessen the amount of air entering the carburetor and to cause the motorcycle's engine to burn more fuel (since the air-fuel mixture will consist of more fuel). This adjustment will make the engine more powerful but will use fuel more quickly.

Step 3

Adjust the pilot jet, located below the air screw, using the adjustable wrench. The pilot jet regulates the amount of fuel entering the carburetor. Turning the pilot jet to the right lessens the amount of fuel entering the carburetor; turning it to the left increases the amount of fuel entering the carburetor.

Step 4

Adjust the throttle slide valve, which regulates how much of the fuel-air mixture enters the motorcycle's engine through the carburetor. The throttle slide valve has numbered settings, and sliding the valve to a higher-numbered setting means more of the mixture will enter the engine, causing the engine to produce more power.

Step 5

Tighten or loosen the needle jet using the adjustable wrench. The needle jet also controls the flow of the air-fuel mixture exiting the carburetor and should be set to correspond to the same numbered setting as the throttle slide valve.

Step 6

Adjust the carburetor's choke valve by tightening or loosening its screw with the Phillips-head screwdriver. To increase the power of the engine, loosen the screw.

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