Yamaha Marine Outboards F4A/F4 Factory Service Manual, Page 170Get this manual
THERMOSTAT
SERVICE POINTSThermostat inspection 1Measure: 9Valve operating temperature 9Valve lift Out of specification Replace the thermostat Water temperature Below 5862C (136.4143.6F) Above 70C (158F) Valve lift mm (0 in) mm (0.12 in)
Measuring steps: Suspend the thermostat in container filled with water8 Slowly heat the water8 Place thermometer in the water8 While stirring the water, observe the thermometer's indicated temperature