The thermostat is so designed that it remains in the closed position when it is in a normal cold
condition. If the thermostat malfunctions, it will remain closed even when the engine reaches normal working temperature.
The flow of coolant will be impeded so that it does not pass through the radiator for cooling and consequently the temperature
will rise abnormally, causing boiling over.
If the performance of the thermostat is suspect, remove it from the machine as follows and test it
for correct operation. Remove the fuel tank and drain the coolant as described in Section 2. Remove its two retaining
screws and withdraw the thermostat cover, noting how the sealing 0-ring is fitted. Withdraw the thermostat. On refitting,
note that the sealing 0-ring is fitted around the cover inner boss, so that it fits inside the housing when the cover is
installed; a smear of grease or similar lubricant will help the 0-ring to slide into the housing.
Examine the thermostat visually before carrying out tests. If it remains in the open position at room
temperature, it should be discarded. Suspend the thermostat by a piece of wire in a pan of cold water. Place a thermometer
in the water so that the bulb is close to the thermostat. Heat the water, noting when the thermostat opens and the
temperature at which the thermostat is fully open. If the performance is different from that specified, the thermostat
should be renewed.
If the thermostat is faulty it can be removed and the machine used without it as an emergency measure
only. Take care when starting the engine from cold as the warm-up will take much longer than usual, and ensure that a new
unit is fitted as soon as possible.