How to Change the Coolant in a 1990 KTM 125EXC

Shop parts for a 1990 KTM 125EXC Dirt Bike.

Routine coolant changes are critical maintenance for your 1990 KTM 125EXC–helping ensure proper engine cooling, longevity, and peak performance on every ride. This guide from MotoSport covers the steps unique to your KTM and details any quirks for this model you should know before starting.

Why Change Coolant?

The coolant breaks down over time, losing its ability to protect against corrosion and overheating. Old coolant can also clog radiator passages—a particular concern with KTM's high-revving 125cc two-stroke engine.

What You Need

  • Premixed motorcycle coolant (use a type compatible with aluminum components)
  • Socket set or appropriate wrenches
  • Drain pan
  • Funnel
  • Protective gloves and eyewear

Step 1: Prepare the KTM 125EXC

  • Make sure the engine is completely cool to avoid burns and pressure in the cooling system.
  • Position the motorcycle upright and secure it safely—a bike stand works well.

Step 2: Drain the Old Coolant

  • Locate the lower radiator hose or the water pump drain bolt (on most 1990 KTM 125EXCs, you will find the drain at the bottom of the water pump housing—check for a small bolt).
  • Place your drain pan directly below the drain point.
  • Remove the radiator cap to allow the coolant to flow freely.
  • Loosen (do not fully remove the first turn) the drain bolt or hose clamp, then fully remove it and let the coolant drain completely into the pan.

Step 3: Flush the System (Recommended)

  • If the coolant is especially dirty, you can flush the system by pouring distilled water through the radiator and letting it drain. Repeat until the water runs clear.
  • Reinstall the drain bolt or reattach the hose securely after draining.

Step 4: Refill with Coolant

  • With the motorcycle upright, slowly fill the radiator with fresh coolant until it reaches just below the filler neck. KTM recommends a 40 percent antifreeze to 60 percent water mix; most premixed coolants meet or exceed this spec.
  • Check the manual for the exact capacity, but usually, it's a little under one liter for this EXC model.
  • Replace and tighten the radiator cap.

Step 5: Bleed the Cooling System (Model Quirks)

  • Start the bike and let it idle for several minutes. This will help purge air from the system—check the coolant level as air bubbles are replaced by coolant. Shut off the bike and top off the radiator if the level drops.
  • Repeat once or twice until the coolant level remains steady. Watch closely, as early 1990s KTMs don't always have a bleed screw, so proper burping is essential to prevent overheating.

Final Checks

  • Inspect all connections you disturbed for leaks.
  • Double-check the coolant level after the first full cool–down cycle.
  • Dispose of old coolant responsibly—never pour it down drains or onto the ground.

Special Notes for 1990 KTM 125EXC Owners

  • This model's radiator and hose layout is tight, so take care when loosening or tightening hardware to avoid damaging the aluminum fittings.
  • Use only coolant that's safe for aluminum parts, as the EXC's components are sensitive to improper chemicals.
  • Check the cap and hoses for signs of age or deterioration during service—these parts are now several decades old and could be ready for replacement.

With these steps, your 1990 KTM 125EXC's cooling system will be ready to handle whatever the trail throws at it—letting you focus on the ride instead of the temperature gauge.

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Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.