How to Change the Coolant in a 1985 KAWASAKI KX125

Shop parts for a 1985 KAWASAKI KX125 Dirt Bike.

The 1985 Kawasaki KX125 is a classic race-ready two-stroke that features a water-cooled engine – meaning proper coolant maintenance is essential to keep it running strong and prevent overheating. Follow this MotoSport.com guide for a smooth and mess-free coolant change that protects your engine for seasons to come.

What You Need

  • Fresh coolant suitable for dirt bikes (silicate-free, aluminum safe)
  • Drain pan
  • 8mm & 10mm socket or wrench
  • Phillips screwdriver (if required for radiator cap)
  • Funnel
  • Shop towels
  • (Optional) Distilled water for flushing
  • (Optional) Air nozzle/compressed air

Step-by-Step Coolant Change

  1. Prep the Bike
    Ensure the engine is cool – hot coolant can be dangerous. Prop the KX125 safely on a stand for easy access.
  2. Remove the Radiator Cap
    Open your radiator cap to allow air in and help coolant drain smoothly. Some tank shrouds may require removal for access.
  3. Locate the Drain Bolt
    On the KX125, the coolant drains from a small bolt on the bottom of the water pump cover (right side of the engine). Place your drain pan below before loosening the bolt.
    Tip: Use the 8mm socket/wrench. Be prepared for coolant to flow out quickly once loosened.
  4. Drain the Coolant
    Allow all coolant to fully drain from the system. For faster draining, gently rock the bike side to side.
  5. Flush for Best Results (Optional but recommended)
    Reinstall the drain bolt temporarily and fill the radiator with distilled water. Run the engine briefly (or kick it over gently without starting, plug removed) to circulate, then drain again.
    For a thorough flush, disconnect the lower radiator hose and use compressed air to blow out remaining fluid.
  6. Reinstall the Drain Bolt
  7. Add Fresh Coolant
    Using a funnel, fill the radiator to the base of the neck with your chosen dirt bike coolant. Avoid overfilling.
  8. Bleed Air from the System
    Gently tilt the bike side to side or slowly squeeze upper radiator hoses to help air escape. Check and top off coolant level if needed.
  9. Reinstall the Radiator Cap
  10. Run the Engine & Recheck Coolant
    Start the KX125 and let it idle for a few minutes with the radiator cap on. Shut down and allow it to cool, then recheck and top off as needed.

Special Notes for 1985 KX125 Owners

  • The 1985 KX125 uses early-generation water cooling. Factory radiators are robust, but be aware of potential aging or leaks and inspect hoses for brittleness.
  • Factory spec calls for the use of a silicate-free coolant to protect aluminum components. Never use automotive coolant with silicates or phosphates.
  • If coolant appears milky or contaminated with oil, inspect the head gasket and water pump seal for leaks – these are common aging issues in vintage machines.
  • Replace the radiator cap if it shows signs of wear, rust, or doesn't hold pressure – as this can prevent overheating on the track.

Coolant Change Interval

It's best to change the coolant every one to two years or after heavy muddy rides. If you race or ride in extreme temperatures, inspect and change coolant more frequently to keep your classic KX125 running cool under pressure.

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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.