KTM 125 SX 2021 User Manual (English)

Other manuals for KTM 125 SX 2021

Questions and Answers:

A
Alex TaylorAug 19, 2025
What to do if my KTM 125 SX 2021 engine has too little power?
- S
  Sarah CruzAug 19, 2025
  If your KTM Motorcycle engine has insufficient power, begin by checking for any interruptions in the fuel feed, such as a blocked fuel tank breather or a dirty fuel tap. Make sure to check the carburetor components. A very dirty air filter can also cause this issue, so clean the air filter and air filter box. Inspect the exhaust system for leaks or deformation, and replace the glass fiber yarn filling in the main silencer if it's insufficient. Additionally, a fault in the ignition system or damage to the diaphragm or reed valve housing could be responsible, so check these components as well.
J
Juan AnthonyAug 22, 2025
What causes a KTM 125 SX 2021 engine not to speed up?
- Q
  qwrightAug 22, 2025
  If your KTM Motorcycle engine does not speed up, it could be due to the carburetor running over because the float needle is dirty or worn, or the carburetor jets are loose. In both cases, check and set the carburetor components. Also, a fault in the ignition system can cause this issue, so inspect the ignition system.
A
Alexander LambertAug 26, 2025
What to do if my KTM Motorcycle engine cannot be cranked?
- J
  Juan HumphreyAug 26, 2025
  If the engine of your KTM Motorcycle fails to crank, several factors could be at play. First, ensure the start procedure is correctly followed. If that's not the issue, check the 12-V battery to see if it's discharged and charge it if necessary. Also, inspect the main fuse and replace it if blown. The starter relay or the starter motor itself could be faulty, so these should be checked as well.
J
Jacob NielsenAug 28, 2025
What to do if my KTM Motorcycle engine overheats?
- D
  David PriceAug 28, 2025
  If your KTM Motorcycle engine is overheating, check the coolant level and look for leaks in the cooling system. Ensure there's sufficient air stream and switch off the engine when stationary. Clean the radiator fins if they are very dirty. If there's foam formation in the cooling system, drain and refill the coolant. Also, inspect the cylinder head and cylinder head gasket for damage and replace any bent radiator hoses.
J
Jerry MoraAug 31, 2025
Why KTM Motorcycle engine stalls or is popping into the carburetor?
- T
  Tiffany MccormickAug 31, 2025
  If your KTM Motorcycle engine stalls or pops into the carburetor, it might be due to a lack of fuel. For all SX models and 125 XC US, ensure the fuel tap is in the ON position, or RES position if needed, and refuel if necessary. Another cause could be the engine taking in bad air; check the intake flange and carburetor for tightness. Also, inspect the connector or ignition coil for looseness or oxidation, and clean the connector with contact spray.
J
Jenna WalkerSep 3, 2025
How to fix KTM Motorcycle engine with no idle speed?
- C
  christina70Sep 3, 2025
  If your KTM Motorcycle engine has no idle speed, the idling jet might be blocked, so check the carburetor components. The adjusting screws on the carburetor could be distorted, so adjust the idle speed. A defective spark plug can also cause this, so replace the spark plug. Finally, a defective ignition system could be the problem, so check the ignition coil and spark plug connector.
W
William HughesSep 6, 2025
Why is there white smoke emission (steam in exhaust gas) from my KTM 125 SX 2021 Motorcycle?
- H
  Hailey HarrisonSep 6, 2025
  White smoke emission (steam in exhaust gas) from your KTM Motorcycle could be due to a damaged cylinder head or cylinder head gasket. In this case, check the cylinder head and cylinder head gasket.
J
Jacqueline AndersonSep 9, 2025
Why is gear oil exiting at the vent hose of my KTM 125 SX 2021 Motorcycle?
- L
  Lori ParksSep 9, 2025
  Gear oil exiting at the vent hose of your KTM Motorcycle is likely because too much gear oil was added. Check the gear oil level.

KTM 125 SX 2021 Specifications

General

Engine Type	1-cylinder, 2-stroke engine
Displacement	124.8 cm³
Bore x Stroke	54 mm x 54.5 mm
Starter	Kickstarter
Transmission	6-speed
Front Suspension	WP XACT-USD, Ø 48 mm
Rear Suspension	WP XACT Monoshock with linkage
Cooling	Liquid cooled
Front Tire	80/100-21"
Rear Tire	100/90-19"
Seat Height	950 mm
Ground Clearance	375 mm
Frame	Central tube frame made of chrome molybdenum steel
Front Brake	Disc brake
Rear Brake	Disc brake
Fuel Tank Capacity	7.5 liters
Fuel System	Mikuni TMX carburetor

Summary

Safety Advice

Use Definition - Intended Use

Defines the intended use of the vehicle and compliance with motorsports regulations.

Misuse

Safety Advice

Degrees of Risk and Symbols

Explains the risk levels (Danger, Warning, Caution, Note) and associated symbols.

Tampering Warning

Safe Operation

Provides safety instructions regarding rider fitness and exhaust gas toxicity.

Protective Clothing

Controls

Opening the Fuel Tank Filler Cap

Preparing for Use

Advice on Preparing for First Use

Provides essential advice on rider fitness, protective clothing, and initial vehicle checks.

Running in the Engine

Details the engine running-in procedure, specifying performance limits and throttle use.

Preparing the Vehicle for Difficult Operating Conditions

Covers preparations for challenging conditions like sand, mud, high/low temperatures.

Preparing the Vehicle for Riding on Dry Sand

Preparing the Vehicle for Riding on Wet Sand

Preparing the Vehicle for Riding on Wet and Muddy Circuits

Preparing Vehicle for High Temperatures or Slow Riding

Preparing the Vehicle for Low Temperatures or Snow

Riding Instructions

Checks and Maintenance Measures When Preparing for Use

Lists essential pre-ride checks covering fluids, brakes, tires, and controls.

Starting

Provides instructions and safety warnings for starting the engine.

Starting Off

Explains how to start moving from a standstill, including clutch and throttle control.

Shifting, Riding

Guides on gear shifting, riding techniques, throttle control, and engine braking.

Applying the Brakes

Details proper braking techniques, warnings about brake system issues, and surface conditions.

Stopping, Parking

Provides instructions and safety warnings for stopping and parking the motorcycle.

Transporting

Refueling

Details safe refueling procedures, fuel specifications, and hazard warnings.

Service Schedule

Required Work

Lists mandatory maintenance tasks and their required intervals (operating hours, races).

Recommended Work

Lists recommended maintenance tasks and their required intervals (operating hours, months).

Tuning the Chassis

Checking the Basic Chassis Setting with Rider's Weight

Explains how rider weight affects chassis setting and the need for suspension adjustment.

Air Suspension XACT 5448

Compression Damping of the Shock Absorber

Explains high-speed and low-speed compression damping and their adjustment.

Adjusting the Low-Speed Compression Damping of the Shock Absorber

Provides step-by-step instructions for adjusting low-speed compression damping.

Adjusting the High-Speed Compression Damping of the Shock Absorber

Details the procedure for adjusting high-speed compression damping on the shock absorber.

Adjusting the Rebound Damping of the Shock Absorber

Guides on how to adjust the rebound damping of the rear shock absorber.

Measuring the Rear Wheel Dimension Unloaded

Checking the Static Sag of the Shock Absorber

Details how to check the static sag of the rear shock absorber and its adjustment.

Checking the Riding Sag of the Shock Absorber

Provides steps for checking the riding sag of the rear shock absorber and its adjustment.

Adjusting the Spring Preload of the Shock Absorber

Describes how to adjust the spring preload on the rear shock absorber for proper sag.

Adjusting the Riding Sag

Guides on how to adjust the riding sag by selecting and mounting a suitable spring.

Checking the Basic Setting of the Fork

Adjusting the Fork Air Pressure

Details the procedure for adjusting the air pressure in the front fork for optimal performance.

Adjusting the Compression Damping of the Fork

Explains how to adjust the compression damping on the front fork for desired handling.

Adjusting the Rebound Damping of the Fork

Provides instructions for adjusting the rebound damping on the front fork.

Service Work on the Chassis

Raising the Motorcycle with a Lift Stand

Removing the Fork Legs

Installing the Fork Legs

Checking Steering Head Bearing Play

Explains how to check for play in the steering head bearing and potential damage.

Adjusting Steering Head Bearing Play

Details the procedure for adjusting steering head bearing play to eliminate looseness or detent.

Removing the Shock Absorber

Step-by-step guide to removing the rear shock absorber, including related components.

Installing the Shock Absorber

Provides instructions for installing the rear shock absorber and tightening specifications.

Cleaning the Air Filter and Air Filter Box

Guides on cleaning the air filter and air filter box, including proper drying and oiling.

Changing the Glass Fiber Yarn Filling in the Main Silencer

Removing the Fuel Tank

Provides step-by-step instructions for removing the fuel tank, including safety warnings.

Installing the Fuel Tank

Guides on installing the fuel tank, including fitting spoilers and connecting hoses.

Cleaning the Chain

Checking the Chain Tension

Details how to check chain tension and the consequences of incorrect tension.

Adjusting the Chain Tension

Provides step-by-step instructions for adjusting the chain tension and wheel alignment.

Checking the Chain, Rear Sprocket, Engine Sprocket, and Chain Guide

Guides on checking wear of chain, sprockets, and chain guide, and when to replace them.

Checking Throttle Cable Routing

Checking/Correcting the Fluid Level of the Hydraulic Clutch

Explains how to check and correct the hydraulic clutch fluid level.

Changing the Hydraulic Clutch Fluid

Provides instructions and safety information for changing the hydraulic clutch fluid.

Brake System

Checking the Free Travel of the Hand Brake Lever

Details how to check the free travel of the hand brake lever and adjust it.

Adjusting the Basic Position of the Hand Brake Lever

Checking the Brake Discs

Explains how to check brake disc thickness, damage, and deformation.

Checking the Front Brake Fluid Level

Details how to check the front brake fluid level and add fluid if necessary.

Adding Front Brake Fluid

Provides instructions and safety information for adding front brake fluid.

Checking the Front Brake Linings

Guides on checking front brake lining thickness, damage, and cracking.

Changing the Front Brake Linings

Provides instructions and warnings for changing the front brake linings.

Checking the Free Travel of Foot Brake Lever

Details how to check the free travel of the foot brake lever and adjust it.

Adjusting the Basic Position of the Foot Brake Lever

Checking the Rear Brake Fluid Level

Details how to check the rear brake fluid level and add fluid if necessary.

Adding Rear Brake Fluid

Provides instructions and safety information for adding rear brake fluid.

Checking the Brake Linings of the Rear Brake

Guides on checking rear brake lining thickness, damage, and cracking.

Changing the Rear Brake Linings

Provides instructions and warnings for changing the rear brake linings.

Wheels, Tires

Removing the Front Wheel

Step-by-step guide to safely removing the front wheel.

Installing the Front Wheel

Provides instructions for correctly installing the front wheel and tightening specifications.

Removing the Rear Wheel

Details the procedure for safely removing the rear wheel.

Installing the Rear Wheel

Guides on correctly installing the rear wheel, chain, and tightening specifications.

Checking the Tire Condition

Checking Tire Pressure

Details how to check and correct tire pressure for optimal performance and safety.

Checking Spoke Tension

Electrical System

Charging the 12-V Battery (125 XC US)

Details the importance of proper charging for battery life and procedures.

Changing Main Fuse (125 XC US)

Cooling System

Checking the Antifreeze and Coolant Level

Details safety warnings and procedures for checking antifreeze and coolant levels.

Checking the Coolant Level

Draining the Coolant

Refilling with Coolant

Changing the Coolant

Tuning the Engine

Checking Throttle Cable Play

Adjusting Throttle Cable Play

Carburetor Tuning

Carburetor - Adjusting the Idle Speed

Details the procedure for adjusting idle speed and idle mixture for optimal starting and idling.

Plug-in Connector of Ignition Timing Map Adjustment

Changing the Ignition Timing Map

Service Work on the Engine

Emptying the Carburetor Float Chamber

Checking the Gear Oil Level

Changing the Gear Oil

Adding the Gear Oil

Storage

Troubleshooting

Technical Data

Engine

Engine Tightening Torques

Carburetor

Details carburetor types, settings, and tuning charts for different models and conditions.

Chassis

Fork

125 XC US

Shock absorber

Substances

Super Unleaded (98 Octane) Mixed with 2-stroke Engine Oil (1:40)

Auxiliary Substances

Related product manuals

KTM 125 SX 2021 User Manual

Table of Contents

Other manuals for KTM 125 SX 2021

Questions and Answers:

KTM 125 SX 2021 Specifications

Summary

Safety Advice

Use Definition - Intended Use

Misuse

Safety Advice

Degrees of Risk and Symbols

Tampering Warning

Safe Operation

Protective Clothing

Controls

Opening the Fuel Tank Filler Cap

Preparing for Use

Advice on Preparing for First Use

Running in the Engine

Preparing the Vehicle for Difficult Operating Conditions

Preparing the Vehicle for Riding on Dry Sand

Preparing the Vehicle for Riding on Wet Sand

Preparing the Vehicle for Riding on Wet and Muddy Circuits

Preparing Vehicle for High Temperatures or Slow Riding

Preparing the Vehicle for Low Temperatures or Snow

Riding Instructions

Checks and Maintenance Measures When Preparing for Use

Starting

Starting Off

Shifting, Riding

Applying the Brakes

Stopping, Parking

Transporting

Refueling

Service Schedule

Required Work

Recommended Work

Tuning the Chassis

Checking the Basic Chassis Setting with Rider's Weight

Air Suspension XACT 5448

Compression Damping of the Shock Absorber

Adjusting the Low-Speed Compression Damping of the Shock Absorber

Adjusting the High-Speed Compression Damping of the Shock Absorber

Adjusting the Rebound Damping of the Shock Absorber

Measuring the Rear Wheel Dimension Unloaded

Checking the Static Sag of the Shock Absorber

Checking the Riding Sag of the Shock Absorber

Adjusting the Spring Preload of the Shock Absorber

Adjusting the Riding Sag

Checking the Basic Setting of the Fork

Adjusting the Fork Air Pressure

Adjusting the Compression Damping of the Fork

Adjusting the Rebound Damping of the Fork

Service Work on the Chassis

Raising the Motorcycle with a Lift Stand

Removing the Fork Legs

Installing the Fork Legs

Checking Steering Head Bearing Play

Adjusting Steering Head Bearing Play

Removing the Shock Absorber

Installing the Shock Absorber

Cleaning the Air Filter and Air Filter Box

Changing the Glass Fiber Yarn Filling in the Main Silencer

Removing the Fuel Tank

Installing the Fuel Tank

Cleaning the Chain

Checking the Chain Tension

Adjusting the Chain Tension

Checking the Chain, Rear Sprocket, Engine Sprocket, and Chain Guide

Checking Throttle Cable Routing

Checking/Correcting the Fluid Level of the Hydraulic Clutch

Changing the Hydraulic Clutch Fluid

Brake System

Checking the Free Travel of the Hand Brake Lever

Adjusting the Basic Position of the Hand Brake Lever

Checking the Brake Discs

Checking the Front Brake Fluid Level

Adding Front Brake Fluid

Checking the Front Brake Linings

Changing the Front Brake Linings