An abnormally high idle speed on a motorcycle when the engine heats up could be caused by a few different factors. Here are a few potential reasons:
Idle control system: Some motorcycles have an idle control system that adjusts the engine idle speed based on factors such as engine temperature. If there is a malfunction in this system, it may cause the idle speed to increase when the engine heats up.
Air intake or fuel delivery issues: Problems with the air intake or fuel delivery system can disrupt the proper air-fuel mixture, leading to an increase in idle speed. For example, a clogged air filter or a malfunctioning fuel injector could affect the engine's performance and cause the idle speed to rise when the engine gets hot.
Throttle cable or linkage: Issues with the throttle cable or linkage can result in a higher idle speed. The cable may become loose or worn over time, causing the throttle to not close fully when released. This can lead to increased engine revs at idle.
Vacuum leak: A vacuum leak occurs when there is an unintended gap or crack in the intake system, allowing extra air to enter the engine. This can cause the engine to run lean and result in a higher idle speed.
Engine tuning or carburetor adjustment: If the motorcycle has a carburetor, improper tuning or adjustment can affect the idle speed. Changes in engine temperature may expose issues with the carburetor's mixture settings, leading to a higher idle speed.
It's important to have a qualified mechanic or technician diagnose the specific cause of the high idle speed in your motorcycle. They can inspect the relevant components and systems to identify and resolve the underlying issue.