How to Change the Coolant in a 1990 Suzuki RM125: Step-by-Step Guide

Shop parts for a 1990 Suzuki RM125 Dirt Bike.

Maintaining the cooling system of your 1990 Suzuki RM125 is essential for reliable performance, longevity and engine health. The RM125 uses a liquid-cooled, 2-stroke engine that is particularly sensitive to overheating, making a fresh coolant flush a vital part of routine maintenance. Here's a proprietary MotoSport guide for changing the coolant on this classic dirt bike, with extra notes specific to the model year and engine design.

What You'll Need

  • Fresh coolant (premixed or concentrated, formulated for motocross 2-strokes)
  • Distilled water (for flushing the system)
  • 8mm socket or wrench
  • Drain pan or approved container
  • Funnel
  • Shop towels
  • Protective gloves & eye protection

Preparation

  • Place the bike on a stand for stability–a level work surface is essential.
  • Make sure the engine is completely cool before starting to avoid burns.
  • Remove any skid plates or shrouds obstructing access to the radiators or water pump.

Step 1: Draining the Old Coolant

  • Find the radiator cap on the left-hand radiator–remove it to break the system's vacuum and allow for proper drainage.
  • Locate the coolant drain bolt on the water pump cover (bottom right side of the engine).
  • Use your 8mm socket to carefully remove the drain bolt and let the old coolant flow into your drain pan. Tip: Lean the bike gently toward the bolt to maximize drainage.
  • After the initial flow stops, gently squeeze the lower radiator hose or slowly lean the bike back and forth to evacuate as much old coolant as possible.

Step 2: Flushing the Cooling System

  • Reinstall the drain bolt temporarily and pour distilled water into the radiator using a funnel.
  • Reinstall the cap, start the engine, and let it idle for about 30 seconds, then shut it off and drain the water through the drain bolt.
  • Repeat until the draining water is clean–this step is crucial for old 2-strokes to prevent mineral or corrosion buildup.
    Never use tap water, which can leave deposits in the radiators and water jacket.

Step 3: Refilling with New Coolant

  • Tighten the drain bolt (do not overtighten–the threads are soft aluminum).
  • Slowly pour the proper amount of new coolant into the radiator. Use a funnel to prevent spills, and fill just below the base of the filler neck.
  • Lightly tap the radiator hoses and gently rock the bike to release any trapped air bubbles.
  • Leave the radiator cap off and start the engine. Let it idle until you see the coolant begin to circulate or a small drop in coolant level. Top off if needed, and once steady, replace the cap securely.

Step 4: Final Checks

  • With the cap on, let the bike reach normal operating temperature. When cool again, check the level in the radiator and top off one last time if necessary.
  • Inspect for any leaks at the water pump, drain bolt, or radiator seams.
  • Safely dispose of old coolant at an approved facility.

Model-Specific Notes for the 1990 Suzuki RM125

  • This engine lacks an overflow reservoir – check coolant levels directly at the radiator cap.
  • Inspect radiator fins for dirt, bent fins, or blockages, which can reduce cooling on this older, high-revving 2-stroke.
  • The water pump shaft and seals on 1990 models are a weak point. Watch for milky oil or unexplained coolant loss–signs these may need replacement.
  • Keep an eye on radiator hoses for cracking after decades of use and replace as needed.

Why Coolant Change Matters for Your RM125

  • Old coolant can lose its anti-corrosive properties, risking internal rust or pitting in cylinder and head.
    This is especially important if you ride in muddy or wet environments where contaminants can make their way into the system.
  • Fresh coolant ensures proper heat transfer, keeping the engine at peak performance and reducing the risk of heat-related failures.

Regular coolant maintenance guarantees your 1990 Suzuki RM125 stays on the track or trail where it belongs–fast, reliable, and ready for the next ride. If you need replacement parts for your Suzuki RM125's cooling system, remember: Shop parts for a 1990 Suzuki RM125 Dirt Bike.

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Shop Radiator Hoses for a 1990 Suzuki RM125 Dirt Bike.

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.