How to Change the Coolant in a 1991 Honda CT70

Shop parts for a 1991 Honda CT70 Dirt Bike.

The 1991 Honda CT70, an iconic and compact trail bike, features a simple cooling system since this model is air-cooled – it does not utilize a liquid-based radiator system like modern liquid-cooled motorcycles. However, many enthusiasts upgrade classic CT70 bikes with aftermarket liquid-cooling kits or engine swaps. If yours still uses the stock air-cooled engine, there is no factory coolant to change – routine maintenance would focus on oil and air filter changes.

If your 1991 CT70 has been upgraded with a liquid-cooled cylinder head or engine conversion, follow these proprietary MotoSport steps for draining and replacing engine coolant:

  • Gather Supplies: You will need compatible ethylene glycol-based coolant suitable for aluminum parts, a drain pan, funnel, basic hand tools, and safety gloves. 
  • Ensure the Engine is Cool: Never open the radiator or coolant reservoir when the engine is hot. 
  • Access the Cooling System: Remove any necessary covers to reach your radiator cap and drain plug or tap. Place the drain pan beneath the lowest point. 
  • Open the Radiator Cap: Slowly relieve pressure by opening the radiator cap or reserve tank cap. 
  • Drain the Old Coolant: Locate and remove the drain plug on the radiator or water pump housing. Let all coolant flow out completely. Inspect the drained coolant for signs of corrosion or contamination.
  • Flush (Optional): For best results, briefly fill the system with distilled water, run the engine with the cap off (if safe) until it reaches operating temperature, then drain again to remove any residue. 
  • Inspect Washers & Seals: Replace crush washers on drain plugs to prevent leaks during reassembly. 
  • Refill with Coolant: Insert the drain plug securely. Use a funnel to carefully fill the system with fresh coolant up to the specified level in the radiator and the reservoir.
  • Bleed the System: Start the engine with the radiator cap off to allow air bubbles to escape. Watch the coolant level as it circulates; top off as needed. Once bubbling stops and the level stabilizes, reinstall the cap.*
  • Check Levels After Cycling: After a full heat cycle, allow the bike to fully cool. Recheck and top off coolant in both radiator and reservoir. 
  • Dispose of Old Coolant Properly: Recycle used coolant according to local regulations.

Key Notes for the 1991 CT70 Owner:

  • This model is originally air-cooled. Only follow coolant instructions if you have confirmed a liquid-cooling conversion.
  • Check for aftermarket radiators or heads before starting – procedures and coolant capacity may differ between kits.
  • Always use coolant safe for aluminum components to prevent internal corrosion.
  • Inspect hoses, clamps, and seals during the process, as these may have aged or degraded on classic bikes. 

Maintaining an upgraded 1991 CT70 is rewarding, but always reference your specific kit’s instructions and verify the type of cooling system before beginning this service for your peace of mind and the longevity of your ride.

*Never allow the coolant to overflow – replace the cap as the engine heats up to avoid spills and burns.

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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.