KTM 125 SX 2021 - Troubleshooting

20 TROUBLESHOOTING
124
Faults Possible cause Action
The engine cannot be cranked
(starter motor)
(125 XC US)
Operating error – Carry out start procedure. ( p. 30)
12-V battery discharged –
Charge the 12-V battery. ( p. 99)
– Check the charging voltage.
– Check the closed current.
– Check the alternator.
Main fuse blown – Change the main fuse. ( p. 100)
Starter relay faulty – Check the starter relay.
Starter motor faulty – Check the starter motor.
Engine turns but does not start Operating error – Carry out start procedure. ( p. 30)
The motorcycle has been in dis-
use for an extended period and
old fuel is in the float chamber
– Empty the carburetor float chamber.
( p. 116)
Fuel feed interrupted – Check the fuel tank breather.
– Clean the fuel tap.
– Check/set the carburetor components.
Spark plug oily or wet – Clean and dry the spark plug, or
change it if necessary.
Plug gap of spark plug too wide – Adjust plug gap.
Guideline
Spark plug electrode gap
0.60 mm (0.0236 in)
Fault in ignition system – Check the ignition system.
Short-circuit cable in wiring
harness frayed, switch-off but-
ton faulty
– Check the switch-off button.
The connector or ignition coil is
loose or oxidized
– Clean the connector and treat it with
contact spray.
Water in carburetor or jets
blocked
– Check/set the carburetor components.
Engine has no idle speed Idling jet blocked – Check/set the carburetor components.
Adjusting screws on carburetor
distorted
– Carburetor - adjust the idle speed.
( p. 113)
Spark plug defective – Change the spark plug.
Ignition system defective – Check the ignition coil.
– Check the spark plug connector.
Engine does not speed up Carburetor running over
because float needle dirty or
worn
– Check/set the carburetor components.
Loose carburetor jets – Check/set the carburetor components.
Fault in ignition system – Check the ignition system.
Engine has too little power Fuel feed interrupted – Check the fuel tank breather.
– Clean the fuel tap.
– Check/set the carburetor components.
Air filter very dirty – Clean the air filter and air filter box.
( p. 64)