Yamaha dirt bikes employ one of two systems for delivering fuel to the engine. The first is a carburettor that's mounted on the intake manifold. The second is a fuel injection system. Both systems have an idle feature that allows the bike to run at minimum RPM's without throttle input. Carburetted dirt bikes that won't idle may have a clogged idle jet, weak fuel pump, or the throttle cable may need to be adjusted. Fuel-injected bikes that run but won't idle may have a weak fuel pump or a faulty sensor. With either system, troubleshooting begins with throttle cable adjustment and checking the fuel pump output.
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Things you need
- Bike tool kit
- Small cup
- Spray carburettor cleaner
Begin by adjusting the throttle cable. Start the dirt bike and apply enough throttle to keep the engine running. Tighten the tension on the throttle cable using the adjustments at the hand grip and on the carburettor. If the bike idles, the problem has been corrected and you can fine tune the idle setting. If not, proceed to the next step.
Remove the hose clamp that secures the fuel line to the carburettor. Detach the line and hold the open end over a cup. Turn the engine over. If no fuel pumps into the cup, remove and replace the fuel pump. If gasoline pumps into the cup the fuel pump is OK. Proceed to the next step.
Shut off the gasoline supply valve. Loosen the cap on top of the carburettor and slide the cap up the throttle cable and out of the way. Remove the throttle cable from the carburettor.
Loosen the nuts that connect the carburettor to the intake manifold. Remove the carburettor and place it on workbench. Remove the idle adjustment screws from the carburettor body. Remove and replace the air filter cartridge.
Remove the bowl at the underside of the carburettor. Use a screwdriver to remove the main jet and needle jet from inside the bowl. Clean the bowl and float with spray carburettor cleaner. Wipe the excess cleaner with a rag. Clean the jets with spray carburettor cleaner and replace them into the body.
Reassemble the carburettor and install it on the intake manifold. Connect the fuel line and throttle cable. Refer to the owner's manual for idle adjustment settings. Set the idle screws, turn on the fuel supply and start the dirt bike.
Remove the fuel line that connects to the injector. Hold a cup under the end of the line and turn the engine over. If the fuel supply is weak or non-existent remove the fuel pump and replace it. If you have a steady fuel supply the fuel pump is OK and you can proceed to the next step.
Turn off the fuel supply valve. Remove the air filter at the end of the intake. Remove the nuts that hold the intake to the engine head and remove the intake. Remove and replace the intake manifold sensor on the underside of the intake. Remove and replace the fast idle mechanism on the side of the intake.
Reattach the intake to the engine manifold. Refer to the owner's manual for torque specifications and tighten the intake nuts to the specified torque. Reconnect the line from the fuel pump to the injector. Wipe away any residual gasoline with a rag. Start the dirt bike.
If the idle problem is not corrected, have a service technician check the crank angle sensor and the electrical signal to the throttle position sensor.
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