Thermostat Inspection

Make sure that air bleed valve (1) of thermostat is clean.
Should this valve be clogged, engine would tend to overheat.
Check to make sure that valve seat (2) is free from foreign matters which would prevent valve from seating tight.
Check thermostat O-ring (3) for breakage, deterioration or any other damage.

Fig 1: Identifying Air Bleed Valve, O-Ring & Bypass Valve
Courtesy of SUZUKI OF AMERICA CORP.
Check thermostatic movement of wax pellet as follows:
1. Immerse thermostat (1) in water, and heat water gradually.
2. Check that valve starts to open at specific temperature.
  Temperature at which valve begins to open
  
  80 - 84 °C (176 - 183 °F)
  
  Temperature at which valve become fully open
  
  95 - 97 °C (203 °F)
  
  Valve lift
  
  More than 8 mm at 95 °C (203 °F)
  
  If valve starts to open at a temperature substantially below or above specific temperature, thermostat unit should be replaced with a new one. Such a unit, if reused, will bring about over cooling or overheating tendency.
  
  Fig 2: Checking Thermostat Valve Opening Temperature
  Courtesy of SUZUKI OF AMERICA CORP.