What is the purpose of the thermostat in a liquid-cooled motorcycle engine, and what happens if it gets stuck closed?

1. It controls coolant circulation to maintain optimal engine operating temperature; if stuck closed, it restricts coolant flow to the radiator, causing the engine to overheat.
2. It measures the battery temperature; if stuck closed, it prevents the starter motor from cranking.
3. It controls the fuel flow into the carburetor; if stuck closed, it causes the engine to run lean.
4. It regulates the fan speed; if stuck closed, the cooling fan will run continuously, draining the battery.