How to Change the Coolant in a 1991 Yamaha YZ125: A MotoSport.com Maintenance Guide
Shop parts for a 1991 Yamaha YZ125 Dirt Bike.
Keeping your 1991 Yamaha YZ125 running cool is vital for performance, reliability, and engine longevity – especially when pushing your two-stroke hard on trails or tracks. This model uses a liquid cooling system designed to stabilize temperatures during intense rides, but over time, the coolant degrades and loses effectiveness. Regular coolant replacement is crucial, and with the YZ125's accessible layout, you can do it in your own garage with basic tools and supplies.
What You Need
- High-quality ethylene glycol coolant (with anti-corrosion suitable for aluminum engines)
- Distilled or soft water for flushing and mixing (50/50 coolant/water ratio)
- Drain pan or container (minimum 1.5 liters)
- 8mm socket or wrench
- Funnel
- Shop rags
- Torque wrench (recommended)
Step-by-Step Coolant Change Process
- Let Everything Cool
For safety, ensure the engine is cold before opening the cooling system. Hot coolant can cause serious burns – never open the radiator cap when the engine is hot.
- Prepare the Bike
Set your YZ125 upright on level ground, using a stand if available.
- Remove the Radiator Cap
Slowly and carefully remove the radiator cap to release any residual pressure.
- Drain the Old Coolant
Place the drain pan under the cylinder near the water pump. Using an 8mm wrench, remove the coolant drain bolt (typically located on the water pump cover or lower crankcase area). Allow all coolant to drain completely. Take care that coolant does not spill onto painted or plastic surfaces. If it does, rinse with water immediately.
- Flush the Cooling System
Reinstall the drain bolt and copper washer temporarily. Pour clean distilled water into the radiator. Replace the cap, start the engine, and run for a minute to circulate the water. Shut off, allow to cool, then repeat steps 3 & 4 until the drained water runs clear. This ensures any old coolant, sediment, or residue is removed.
- Reinstall the Drain Bolt and Washer
Once finished flushing, securely reinstall the drain bolt and its copper washer. Tighten to 10 Nm (7.2 ft–lb) to avoid stripping the threads or leaks.
- Add New Coolant
Mix coolant and distilled water 50/50, unless using a premixed product. Using a funnel, fill the radiator slowly until the level reaches the bottom of the filler neck. The system capacity is 0.95 L (about 1.00 US qt).
- Bleed Air From the System
Tilt the bike side to side to remove trapped air bubbles. Slowly squeeze radiator hoses (if accessible) to help purge any air pockets.
- Reinstall the Radiator Cap
Ensure the surface is clean, then reinstall the cap securely.
- Warm Up and Recheck Level
Start the engine and let it idle for a few minutes. Shut off, allow the bike to cool, then check coolant level again. Top off if necessary.
Notes Specific to the 1991 YZ125
- This model uses aluminum engine components, making it essential to use coolant with aluminum-safe corrosion inhibitors to prevent internal damage.
- Check hoses, radiator cap, and clamps for wear or leaks during this service; these are common failure points from age or vibration.
- The coolant should be changed at least every two years, or sooner if you race or ride frequently.
- If you detect persistent overheating, inspect for blocked radiators, damaged water pump, or leaking system components.
Coolant Disposal Tip
Never pour used coolant down the drain or into soil. Collect it in a sealed container and take to a local automotive recycling center.
Keep Your YZ125 at Its Best
Changing your coolant is simple preventive maintenance that protects your engine, keeps you on the track, and maximizes the bike's value. For replacement hoses, radiators, or other cooling system parts, Shop parts for a 1991 Yamaha YZ125 Dirt Bike. at MotoSport.com.
Related Shopping Categories
Shop Radiators for a 1991 Yamaha YZ125 Dirt Bike.
Shop Coolant for a 1991 Yamaha YZ125 Dirt Bike.
Shop Temperature Sensors for a 1991 Yamaha YZ125 Dirt Bike.
Shop Tools for a 1991 Yamaha YZ125 Dirt Bike.
Shop Radiator Hoses for a 1991 Yamaha YZ125 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.