Yamaha Outboard Motors Factory Service Manual F6 and F8, Page 244Get this manual
THERMOSTAT, TIMING BELT AND GEARS CHECKING THE THERMOSTAT
Measure: Valve operating temperature Valve lift Out of specification ReplaceWater temperature Below 60 C(140 F) Above 70 C(158 F) Valve lift mm (0 in) Min.3 mm (0.12 in)
Measuring steps: (1)Suspend the thermostat in container filled with water(2)Place thermostat in the water(3)Slowly heat the water(4)While stirring the water, check that the thermometer opens at the specified temperature