A motorcycle may idle high on startup due to a few reasons:
Cold engine: When a motorcycle engine is cold, it requires more fuel to operate efficiently. To compensate for this, the engine control unit (ECU) or carburetor may increase the idle speed to provide extra fuel and ensure smooth operation until the engine reaches its optimal operating temperature.
Fast idle mechanism: Some motorcycles have a fast idle mechanism that engages when the engine is cold. This mechanism increases the idle speed automatically to help warm up the engine more quickly. It's often controlled by a temperature sensor or a choke mechanism.
Idle adjustment: If the idle speed is consistently high even when the engine is warm, it could indicate a misadjusted idle screw or throttle stop screw. These screws control the amount of air flowing into the engine at idle. If they are set too high, it can cause the engine to idle at a higher speed than necessary.
Vacuum leak: A vacuum leak in the intake system can cause the engine to idle high. When unmetered air enters the engine through a leak, it disrupts the air-fuel mixture and can lead to higher idle speeds.
Faulty throttle position sensor (TPS): The TPS is responsible for relaying the throttle position to the ECU, which in turn adjusts the fuel and air mixture. If the TPS is malfunctioning or sending incorrect signals, it can cause the engine to idle at higher speeds.
If your motorcycle consistently idles high on startup or during normal operation, it's advisable to have it checked by a qualified mechanic. They can diagnose the specific cause of the high idle and perform any necessary adjustments or repairs to ensure the engine operates correctly.