KTM 250 DUKE Owner's Manual page 142

15 COOLING SYSTEM
Danger
Danger of poisoning Exhaust gases are toxic and inhaling them may result in
unconsciousness and/or death.
–
–
Start the engine and let it run warm.
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Stop the engine and allow it to cool down.
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When the engine is cool, check the coolant level in the radiator and, if necessary, add
coolant.
–
Remove the cap of the compensating tank
the MAXmarking.
–
Mount the cap of the compensating tank.
K00033-11
Finishing work
–
Fit the front spoiler. (
When running the engine, always make sure there is sufficient ventilation,
and do not start or run the engine in an enclosed space without an effective
exhaust extraction system.
p. 92)

and top up the coolant level up to
140