How to Change the Coolant in a 1979 KAWASAKI KX125: A MotoSport Guide

Shop parts for a 1979 KAWASAKI KX125 Dirt Bike.

Changing the coolant in your 1979 KAWASAKI KX125 ensures lasting engine performance and prevents overheating. This classic dirt bike features a liquid-cooled system that, despite its vintage, benefits immensely from routine maintenance. Below, you’ll find a step-by-step, MotoSport proprietary procedure crafted specifically for your MMY, along with model-specific notes every owner should know.

What You’ll Need

  • Socket wrench set (8 mm – 10 mm recommended)
  • Screwdriver
  • Pliers
  • Drain pan
  • Distilled water
  • Funnel
  • New coolant (performance motorcycle coolant is recommended)
  • Gloves and shop rag

Preparation

  • Let the engine cool completely. Draining coolant from a hot KX125 can cause severe burns and will affect proper draining.
  • Position your bike upright, preferably using a stand, on a solid, level surface.

Step-by-Step Coolant Change for the 1979 KX125

  1. Remove the Radiator Cap:
    Locate the radiator cap at the top of your left radiator. Press down and twist to release. This relieves pressure in the system and maximizes coolant flow during draining.
  2. Locate the Coolant Drain Bolt:
    On the right side of the cylinder or at the base of the water pump, find the coolant drain bolt (typically 8 mm). Place your drain pan directly beneath before loosening.
  3. Drain the Old Coolant:
    Slowly loosen and remove the drain bolt. Allow all coolant to drain fully into the pan. For more thorough drainage, slightly tilt the bike away from the side stand or its natural lean. If your KX125 is equipped with a bleeder screw on the opposite radiator, open it briefly with a Phillips screwdriver to vent the system and speed up draining.
  4. Flush the System (Recommended):
    Reinstall the drain bolt. Pour distilled water into the radiator, replace the cap, and gently shake or tilt the bike to circulate. Remove the cap and drain bolt to empty the rinse water. Repeat once more if the previous coolant appeared dirty or neglected.
  5. Inspect the Radiator and Hoses:
    Now is the perfect time to check for clogged or bent radiator fins, inspect hoses for cracks, and ensure all clamps are secure. The 1979 KX125’s radiators are robust, but age can lead to pinhole leaks or corroded hose connections.
  6. Fill with New Coolant:
    With the drain bolt securely in place, insert a funnel into the radiator mouth and slowly fill with fresh coolant. Allow bubbles to escape and fill until coolant reaches the very top of the fill neck.
  7. Bleed Air from the System:
    Leave the radiator cap off and briefly start the engine (20–30 seconds maximum). This circulates the coolant and releases trapped air. Watch for bubbling; once it subsides, top off coolant as needed, then shut off the engine.
  8. Reinstall the Radiator Cap:
    Firmly press and twist the cap closed. Check under the bike and around the radiator for leaks.
  9. Dispose of Old Coolant Responsibly:
    Used coolant is toxic – always dispose of it at an appropriate recycling facility. Never pour it down drains or onto the ground.

Model-Specific Notes: 1979 KAWASAKI KX125

  • The original KX125 radiator lacks a modern overflow reservoir – monitor coolant level frequently, especially after the first ride post-change.
  • Radiator caps are prone to wear over decades. If yours does not seat securely or the rubber seal is brittle, replace it to maintain proper pressure (stock relief pressure: 14–18 psi).
  • Check carefully for pinched, cracked, or hardened hoses, since replacements may be needed due to age-induced brittleness.
  • If your bike overheats or loses coolant despite a proper fill, inspect the water pump for shaft play or seal failure.
  • Always use motorcycle-specific coolant; automotive coolants may be chemically incompatible with legacy magnesium engine components found in period KX bikes.

Routine Maintenance Tips

  • Change your coolant every one to two seasons, or immediately if it appears dirty, rusty, or clouded.
  • If you ride in harsh or muddy conditions, check cooling system integrity more often, as debris can clog vintage radiators more readily.
  • After coolant replacement, monitor engine temperature and check for coolant loss in the days immediately following.

Keeping your 1979 KAWASAKI KX125’s cooling system in top condition lets you enjoy legendary two-stroke performance ride after ride. Regularly changing coolant and inspecting the system ensures your classic KX is always ready for action on the track or trail.

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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.