KTM 125 DUKE 2021 Owner's Manual page 185

The coolant is cooled by the air stream and a radiator fan
which is activated at high temperature.
The lower the speed, the less the cooling effect. Dirty cooling fins
also reduce the cooling effect.
Info
If the cooling system overheats, the maximum engine
speed is limited.
H01904-10
COOLING SYSTEM 16
4
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183