The most effective way to troubleshoot Yamaha motorcycle carburettors is by first understanding the basic mechanics of the unit. Yamaha motorcycles use slide valve Mikuni carburettors. Twisting the throttle grip opens a piston valve in the carburettor throat, allowing the fuel/air mixture to be drawn into the intake manifold on the engine. You adjust the fuel/air mixture by using the idle adjustment screw on the carburettor. Minor problems with Yamaha carburettors are related to the fuel supply, the throttle cable or the fuel/air mixture. Other problems are related to the internal components.
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Things you need
- Glass jar
- Metric open-end wrench
Fill the gas tank with unleaded fuel. Turn the fuel petcock at the underside of the tank to the "Closed" position. Trace the path of the fuel line from the petcock to the carburettor and identify the hose clamp that secures the line to the carburettor.
Loosen the hose clamp by turning the clamp screw counterclockwise with a screwdriver. Position a small glass jar under the connection and pull the fuel line off the carburettor.
Hold the end of the fuel line in the jar with one hand and turn the fuel petcock to the "Open" position. If fuel does not flow into the jar there may be debris in the gas tank or a problem with the petcock. Fuel flow indicates the gas tank and petcock are OK.
Close the fuel petcock. Reattach the fuel line to the carburettor and tighten the hose clamp clockwise with the screwdriver.
Trace the path of the throttle cable from the twist grip to the carburettor. Locate the cable adjuster bolt where the end of the throttle cable connects to the carburettor cap.
Stand to one side of the motorcycle where you can reach the twist grip and observe the action of the throttle cable at the same time. Slowly twist the grip until you feel the throttle cable tighten under tension.
Adjust the throttle cable using a metric open-end wrench to turn the cable adjustment bolt at the carburettor cap. Turn the bolt clockwise or counterclockwise as needed to take up any free play in the cable.
Cable tension should be immediate as you turn the twist grip. You must replace the cable if proper adjustment is not possible or the adjustment does not hold over time.
Open the fuel petcock. Start the motorcycle and allow it to warm up at normal idle speed. Locate the brass idle screw on the side of the carburettor, just above the carburettor bowl. The screw has a slotted head and a tension spring that seats against the carburettor.
Turn the idle screw clockwise in 1/4-turn increments until you hear the engine idle begin to slow. Turn the screw counterclockwise in 1/4-turn increments until you hear the idle speed up and begin to slow again.
Continue to turn the screw clockwise or counterclockwise in 1/4-turn increments until the idle is set at the fastest speed.
Tips and warnings
- Replace the air filter and in-line fuel filter for best results.
- Consult a motorcycle mechanic if carburettor problems persist.
- Internal problems are corrected by rebuilding the carburettor. Obtain a carburettor kit if necessary and follow the instructions for rebuilding the unit.
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