How to Change the Coolant in a 1978 Yamaha YZ100
Shop parts for a 1978 Yamaha YZ100 Dirt Bike.
Changing the coolant on your 1978 Yamaha YZ100 is essential for optimal engine performance and longevity. The YZ100 was known for its robust two-stroke engine and air-cooled design, making coolant changes less frequent than modern liquid-cooled machines–but just as important if a previous owner upgraded to a liquid-cooling kit, or if your bike features aftermarket components. Here's a step-by-step guide tailored for MotoSport.com riders and specifically for this classic model:
What Makes This Bike Unique?
- Original YZ100 models are air-cooled. If your YZ100 is stock, it does not have a liquid cooling system. However, some modified YZ100s may be upgraded to liquid cooling via aftermarket kits. The following guide is for bikes equipped with a liquid cooling system. If your bike is air-cooled, routine engine, and radiator fin cleaning are all that is required.
- Always check your engine configuration before attempting a coolant service. If in doubt, refer to your build sheet or consult a MotoSport.com technician.
Supplies You'll Need
- Fresh, motorcycle-specific coolant (not automotive antifreeze)
- Distilled water (if coolant concentrate requires mixing)
- Drain pan
- Socket set or wrench
- Funnel
- Gloves, goggles, and cleaning rags
Draining the Old Coolant
- Ensure the bike is completely cool to prevent burns.
- Place your dirt bike securely on a stand to keep it level.
- Locate the radiator drain bolt (or, if none, carefully loosen the lower radiator hose). Place the drain pan underneath.
- Remove the radiator cap to allow coolant to flow out freely.
- Loosen and remove the drain bolt or hose, allowing the old coolant to fully drain. Dispose of old coolant safely–do not pour it down the drain or on the ground.
Optional: Flushing the System
If your YZ100's system is dirty or maintenance history is unknown, flush with distilled water:
- Reinstall drain bolt/hose. Fill system with distilled water.
- Run the engine briefly to circulate the water, then let cool and drain completely.
- Repeat until drained water runs clear.
Filling with New Coolant
- Reinstall the drain bolt or hose, making sure any sealing washer is in good condition and properly tightened.
- Using a funnel, slowly pour fresh coolant into the radiator up to the bottom of the filler neck. If you are using coolant concentrate, mix it with distilled water as instructed on the packaging.
- Pause while pouring to let air escape the system. Squeeze coolant hoses and gently rock the bike side to side to encourage bubbles out.
Bleeding Air and Heat Cycling
- With the radiator cap still off, start your YZ100 and let it idle for a few minutes. Gently rev the engine or rock the bike side to side to free any remaining air bubbles.
- Shut off the engine. Top off the coolant as needed, then install the radiator cap securely.
- Let the bike fully cool, then recheck the coolant level both at the radiator and the reservoir (if equipped). Top off as needed.
- After your next ride, let the engine cool and check coolant again–this catches any final air release and prevents running low.
Pro Tips for 1978 Yamaha YZ100 Owners
- On original, air-cooled models: Clean engine fins thoroughly to prevent overheating and optimize airflow.
- On liquid-cooled conversions: Inspect hose routing, radiator mounts, and check for leaks after every coolant change.
- Always use motorcycle-grade coolant suitable for aluminum components to avoid corrosion and deposits.
- Coolant intervals are less frequent than oil, but check your coolant level and condition at every major interval or after hard rides.
Final Safety & Environmental Notes
- Never open a hot radiator cap–pressurized, hot coolant can cause burns.
- Dispose of old coolant safely at an auto parts store or recycling center.
Maintaining your 1978 Yamaha YZ100's cooling system preserves its performance and legendary two-stroke snarl for years to come. For all replacement parts and enhancements, trust MotoSport.com to deliver quality and expertise for this classic machine.
Related Shopping Categories
Shop Radiators for a 1978 Yamaha YZ100 Dirt Bike.
Shop Coolant for a 1978 Yamaha YZ100 Dirt Bike.
Shop Temperature Sensors for a 1978 Yamaha YZ100 Dirt Bike.
Shop Tools for a 1978 Yamaha YZ100 Dirt Bike.
Shop Radiator Hoses for a 1978 Yamaha YZ100 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.