KTM 125 SX 2021 Owner's Manual page 127

Faults
Engine has too little power
Engine stalls or is popping into
the carburetor
Engine overheats
White smoke emission (steam
in exhaust gas)
Gear oil exits at the vent hose
Water in the gear oil
Possible cause
Exhaust system leaky,
deformed or too little glass
fiber yarn filling in main
silencer
Fault in ignition system
Diaphragm or reed valve hous-
ing damaged
Lack of fuel
Engine takes in bad air
The connector or ignition coil is
loose or oxidized
Too little coolant in cooling sys-
tem
Too little air stream
Radiator fins very dirty
Foam formation in cooling sys-
tem
Damaged cylinder head or
cylinder head gasket
Bent radiator hose
Damaged cylinder head or
cylinder head gasket
Too much gear oil added
Damaged radial shaft seal ring
or water pump
TROUBLESHOOTING 20
Action
–
Check exhaust system for damage.
–
Change the glass fiber yarn filling in
the main silencer. (
–
Check the ignition system.
–
Check the diaphragm and reed valve
housing.
(All SX models)
–
Turn tap handle of the fuel tap to
the ON position.
(125 XC US)
–
Turn tap handle of the fuel tap to
the ON position.
(125 XC US)
–
Turn tap handle of the fuel tap to
the RES position.
–
Refuel. ( p. 34)
–
Check the intake flange and carburetor
for tightness.
–
Clean the connector and treat it with
contact spray.
–
Check the cooling system for leakage.
–
Check the coolant level. (
–
Switch off engine when stationary.
–
Clean the radiator fins.
–
Drain the coolant. (
–
Refill with coolant. (
–
Check the cylinder head and cylinder
head gasket.
–
Change the radiator hose.
–
Check the cylinder head and cylinder
head gasket.
–
Check the gear oil level. (
–
Check the radial shaft seal ring and the
water pump.
p. 66)
p. 104)
p. 105)
p. 106)
p. 117)
125