There are several potential causes for a high idle on a 1980 Yamaha XS850 Special motorcycle when it's hot. Here are a few possibilities to consider:
Carburetor Issues: The carburetors may require adjustment or cleaning. Over time, deposits and debris can accumulate, leading to improper fuel-air mixture and erratic idling. Make sure the idle mixture screws are properly adjusted and that the carburetors are clean.
Vacuum Leaks: Check for any vacuum leaks in the intake manifold or carburetor boots. Leaks can introduce excess air into the engine, causing a high idle. Inspect the rubber boots connecting the carburetors to the engine for cracks or deterioration.
Throttle Cable Tension: Ensure that the throttle cable is properly adjusted. If it is too tight or has excess slack, it can affect the idle speed. Refer to the motorcycle's service manual for the correct procedure to adjust the throttle cable tension.
Faulty Idle Control System: The XS850 Special might have an idle control system such as an idle speed control valve or an idle air control valve. If this system is malfunctioning, it can result in a high idle. Check the components of the idle control system for any faults or blockages.
Ignition Timing: Incorrect ignition timing can cause issues with idle speed. Ensure that the ignition timing is set correctly according to the manufacturer's specifications. If the timing is off, it can affect the engine's performance, including the idle speed.
Air Filter: A clogged or dirty air filter can restrict airflow to the engine, leading to a high idle. Inspect the air filter and clean or replace it if necessary.
If you're unsure about any of these potential causes or if the issue persists after attempting basic troubleshooting, it's recommended to consult a qualified motorcycle mechanic or Yamaha dealership for a more thorough diagnosis and resolution.