How to Change the Coolant in a 1990 Yamaha YZ125

Shop parts for a 1990 Yamaha YZ125 Dirt Bike.

Maintaining your 1990 Yamaha YZ125 means keeping the cooling system in top shape. Periodic coolant changes protect the engine, especially when riding hard or in varying climates. Below is a step-by-step guide tailored for this classic 2-stroke, with key tips specific to this legendary motocross machine.

What You'll Need

  • Fresh coolant (silicate-free, compatible with aluminum radiators)
  • Drain pan
  • 8mm socket or wrench
  • Funnel
  • Shop rags
  • Protective gloves

Step 1 – Preparation

  • Ensure the engine is completely cool to prevent burns when opening the radiator cap – never attempt this with a warm or hot engine.
  • Secure the bike on a stand or flat, stable surface for easy access.

Step 2 – Remove the Radiator Cap

  • Locate the radiator on the left side of the YZ125.
  • Slowly remove the radiator cap to release any residual pressure and allow air into the system for proper draining.

Step 3 – Drain the Coolant

  • Find the coolant drain bolt; on the 1990 YZ125, it is typically at the bottom of the water pump cover on the right engine case.
  • Place your drain pan under the water pump.
  • Use the 8mm socket to remove the bolt and let all coolant drain out.
  • Inspect the old coolant. If it is discolored, rusty, or contaminated, consider flushing the system with distilled water before refilling.

Step 4 – Reinstall the Drain Bolt

  • After the coolant has completely drained, clean the drain bolt and threads.
  • Reinstall and tighten the bolt securely, but avoid over-tightening to prevent damage to the case.

Step 5 – Add New Coolant

  • Position your funnel in the radiator fill neck.
  • Slowly pour the new coolant in until it sits just below the base of the fill neck.
  • On the YZ125, there is no overflow reservoir; fill the radiator directly and avoid topping off when the engine is completely cold, as expansion will occur during warm-up.

Step 6 – Bleed Air from the System

  • Gently rock the bike side-to-side to help dislodge trapped air bubbles.
  • Start the engine and let it idle for a few minutes, watching for the coolant level to drop as the system purges air.
  • With the engine off and cool, recheck the coolant level and top off if necessary.

Step 7 – Final Checks

  • Once topped off, reinstall the radiator cap securely.
  • Monitor the coolant level after your first ride, and inspect for any signs of leaks around the drain bolt and radiator.

Notes for 1990 YZ125 Owners

  • The 1990 YZ125 features a simple, open cooling circuit. It will not have a coolant recovery tank like modern bikes, so be mindful of expansion – never overfill the radiator.
  • Inspect the radiator shrouds and hoses for aging or cracks; after decades of use, these are common points of failure.
  • Factory-spec coolant mix is vital for optimal cylinder head cooling and longevity, especially during long motos.
  • Change coolant every 40–50 hours of riding or at least once per season for best performance.

Regular coolant changes are a straightforward but vital task for keeping your vintage YZ125 racing at its best. For replacement parts or upgrades, MotoSport has everything you need to keep shredding on the track.

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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.