How to Change the Coolant in a 1981 Yamaha YZ100
Keeping the cooling system in peak condition is essential for your 1981 Yamaha YZ100. Over decades of hard riding, dirt, and age, coolant breaks down – replacing it protects your air-cooled engine from overheating, corrosion, or scale buildup. While the YZ100 is a two-stroke, air-cooled model (meaning it doesn't have a conventional liquid cooling system or radiator), some riders have added aftermarket liquid cooling kits for specialized racing applications. If you have such a setup, or are working on an engine with custom cooling, here’s a comprehensive, MotoSport-exclusive guide.
Before you start
- Verify your model: The stock 1981 Yamaha YZ100 is air-cooled. If you have no radiator or external coolant tank, this procedure does not apply.
- If your YZ100 is modified with a liquid cooling system, check the aftermarket manufacturer’s instructions for the exact coolant capacity & service intervals. Most custom kits follow general dirt bike coolant maintenance best practices.
Tools & Supplies Needed
- Ready-mixed motorcycle coolant or coolant concentrate plus distilled water (do not use tap water)
- Drain pan
- Socket/wrench set
- Clean funnel
- New radiator cap (recommended for older systems)
- Protective gloves & eye wear
- Service manual (for custom-fitted systems, if available)
Step-by-Step Coolant Change for a Custom-Liquid Cooled YZ100
- Preparation:
- Allow the engine to cool completely before starting to avoid burns.
- Place the bike on a level stand.
- Locate the Radiator & Draining Bolt:
- Find the lowest bolt on the radiator or the coolant drain plug on the water pump (if present).
- Remove the radiator cap to allow air in as the old coolant drains.
- Drain the Coolant:
- Place your drain pan under the bike.
- Remove the drain bolt & let all coolant flow out completely. If equipped, remove the bleed bolt to help drain air-locked coolant.
- Flush the System (Optional but recommended for neglected bikes):
- With the drain plug re-installed, pour distilled water into the radiator, start the engine gently, and circulate the water for a few minutes.
- Turn off the engine, drain the distilled water, and repeat until clear.
- Replace Sealing Washers & Reinstall Bolts:
- Fit a new sealing washer to the drain bolt. Tighten securely, but do not overtighten.
- Add New Coolant:
- Using a funnel, fill the radiator up to the base of the filler neck with your motorcycle-specific coolant (pre-mixed or properly diluted).
- Rock the bike gently and squeeze the coolant hoses to free trapped air.
- Bleed Air from the System:
- With the radiator cap still off, start the engine and let it idle for several minutes. Occasionally blip the throttle and rock the bike side to side.
- Watch for bubbles escaping. Once the bubbling stops, top off the coolant if necessary.
- Reinstall the Radiator Cap:
- Fit the radiator cap securely.
- Check the Reservoir, if equipped:
- Fill to the correct level line on the tank.
- Heat Cycle & Recheck Levels:
- Run the engine up to normal temperature. Let the bike cool and recheck both radiator & reservoir levels; top up if needed.
Critical Notes for the 1981 YZ100 Owner
- The original 1981 YZ100 is an air-cooled, two-stroke motocross bike; it does not use liquid coolant unless an aftermarket system has been installed.
- A properly functioning cooling system – air or liquid – is crucial for high-revving engines prone to overheating.
- Always use distilled water in any coolant mixture to avoid mineral deposits clogging small passages.
- If unsure about your system, consult with a MotoSport specialist or a qualified technician before proceeding.
Maintenance Tips
- Inspect coolant lines, radiator, and cap for leaks whenever you ride hard or after a crash.
- Flush & renew coolant every two years or as often as your kit’s manufacturer recommends.
- Safely dispose of old coolant at a certified recycling center – never pour it on the ground.
- Mark your service date and keep records in your maintenance log.
Related Shopping Categories
Shop Radiators for a 1981 Yamaha YZ100 Dirt Bike.
Shop Coolant for a 1981 Yamaha YZ100 Dirt Bike.
Shop Temperature Sensors for a 1981 Yamaha YZ100 Dirt Bike.
Shop Tools for a 1981 Yamaha YZ100 Dirt Bike.
Shop Radiator Hoses for a 1981 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.