Clean the throttle body

Home mechanic (basic tools) · Time: 30 to 60 min (estimate) (estimate)

The throttle body is the injected bike's air valve: a round bore with a flat throttle plate (butterfly) that the throttle cable opens. Next to it sit the throttle position sensor, often an intake pressure and temperature sensor, and an idle air passage with a small idle control motor (stepper motor) that the ECU uses to set idle. Oil vapour from the crankcase breather and dust leave a black film around the plate and in the idle passage. The engine then idles badly, stalls when you close the throttle, or hangs.

Safety

• Engine off and cold. Key OFF before you unplug anything.
• Cleaner vapour is flammable and harmful. Work outdoors, no flames, wear gloves and glasses.

You need

• A cleaner labelled safe for throttle bodies and sensors. Spray it onto a cloth or brush where the sensor is close.
• A new O-ring if you remove the throttle body from the manifold (K-Light 125 manual: new throttle body O-rings on refit).

Steps

1. Remove the side cover, seat or tank as needed to reach the intake side of the throttle body.
2. Loosen the clip and pull the air box hose off the throttle body inlet. Usually the throttle body can stay on the bike.
3. Look into the bore with a torch. A black ring around the plate edge confirms the problem.
4. Hold the throttle open with the twist grip, never by pushing the plate with a tool. The K-Light 125 manual warns against propping the plate open.
5. Spray cleaner onto a cloth or soft brush and wipe the bore and both faces of the plate. Work the throttle so you can reach all round.
6. Clean the idle bypass opening in the bore if you can see it, without pushing dirt further in. Do not let dirt block it (K-Light 125 manual, PDF page 259).
7. Do not spray directly into the throttle position sensor, the stepper motor or any connector.
8. Do not turn the idle stop screw. It is factory set; idle is controlled by the ECU.
9. Wipe out loose dirt and let the cleaner evaporate for a few minutes.
10. Refit the air box hose and clip. Check the air filter while you are there: Clean or replace the air filter.
11. Turn the key ON for a few seconds before starting so the ECU can set the idle motor.
12. Start the engine and let it warm up fully without touching the throttle.

Check your work

• Steady idle when warm, and the engine returns to idle cleanly when you close the throttle.
• If idle is unstable after cleaning, the ECU may need to relearn the cleaner airflow. Ride normally for a while, or see Reset the idle after throttle body work.

How often

K-Light 125 manual: clean the throttle body every 10,000 km or once a year in normal use. More often if the air filter has been neglected or the breather is putting oil into the intake.

If it did not fix the problem

• Still unstable: Find an intake air leak, then Read engine fault codes.
• The K-Light 125 manual also lists wrong valve clearance as a cause of unstable idle and stalling on deceleration: Check and adjust valve clearance.

Reference values (K-Light 125)