How to Change the Coolant in a 1978 Suzuki RM125

Shop parts for a 1978 Suzuki RM125 Dirt Bike.

Keeping your 1978 Suzuki RM125 running at peak performance involves routine cooling system maintenance. Changing the coolant is an essential, straightforward process that improves engine longevity, especially for a classic dirt bike like the RM125, which features an air-cooled engine in most regions but may have aftermarket or competition liquid-cooled configurations. Here is MotoSport's definitive guide for changing the coolant and specific tips for this legendary two-stroke motocross machine.

Before You Start: Important Notes on the 1978 RM125

  • The 1978 Suzuki RM125 is typically air-cooled from the factory. However, if your bike has an aftermarket liquid-cooled conversion or a very late production run with a rare factory water-cooled kit, these instructions apply. If your bike is air-cooled, it will not have a conventional coolant reservoir or radiator–no coolant change is needed. Verify your engine type before you begin.
  • For original water-cooled models or conversions, only use an anti-freeze & summer engine coolant compatible with aluminum radiators, mixed at a 50:50 ratio with distilled water–never use tap water to avoid mineral deposits. 

Tools & Supplies You Need

  • Socket and ratchet set (typically 8mm or 10mm for drain bolt)
  • Phillips screwdriver for radiator cap or bleed screw
  • Drain pan for old coolant
  • Rags for cleanup
  • Fresh coolant (aluminum-safe)
  • Distilled water
  • Gloves and eye protection

Step-by-Step Coolant Change Procedure

  1. Ensure the engine is completely cool. Never open the radiator cap on a hot engine.
  2. Remove the seat and side panels if access is restricted by plastics or fuel tank. This step may not be necessary on all builds.
  3. Place a drain pan under the water pump area. Locate the drain bolt–usually found at the base of the water pump cover or radiator (8mm or 10mm bolt).
  4. Open the radiator cap to allow air in, which promotes complete draining.
  5. Loosen and remove the drain bolt. Allow all coolant to fully drain.
  6. For a thorough flush, pour distilled water into the radiator and allow it to exit the drain hole. This removes any lingering old coolant or contaminants. Repeat until the water runs clear.
  7. Reinstall and torque the drain bolt snugly. Do not overtighten to avoid stripping the threads.
  8. Mix your coolant and distilled water 50:50 (unless using pre-mixed coolant).
  9. Slowly fill the radiator with your mixed coolant. Tap the hoses and gently rock the bike from side to side to release any air pockets trapped inside.
  10. If equipped, use the radiator bleed screw to purge air as you fill until liquid escapes, then tighten the screw.
  11. Replace and secure the radiator cap. Wipe up any spilled coolant.
  12. Start the engine and let it idle for a minute, then turn it off. After cooling, double-check coolant level and top off as needed.
  13. Dispose of old coolant safely–never dump it into the ground or drains.

Classic RM125 Owner Tips

  • The 1978 RM125 in stock form does not require coolant maintenance. Procedural steps above are solely for liquid-cooled variants or units with modern conversion kits.
  • Original air-cooled engines may benefit from cylinder fin cleaning and regular inspection to ensure unrestricted airflow, as these serve the same cooling purpose.
  • The RM125 is celebrated for its high-revving two-stroke power and light chassis–keep up on regular maintenance for best results on classic tracks.
  • Seals and gaskets on older bikes may be brittle. If you see leaks after service, inspect and replace as necessary using quality replacement kits.

Final Check

  • Always verify coolant level after the first ride. Air pockets may resolve only after the engine cycles hot and cold once.
  • If engine temperature warning signs or coolant loss persist, investigate the cooling system for leaks or blockages. Consult your owner's manual for model-specific details.

Trust MotoSport for all your 1978 Suzuki RM125 maintenance parts and upgrades, so you can focus on riding and preserving your classic machine's legacy.

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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.