How to Change the Oil in a 1990 Kawasaki KX125: A Complete Guide
Shop parts for a 1990 Kawasaki KX125 Dirt Bike.
Keeping your 1990 Kawasaki KX125 in top running condition requires regular maintenance—one of the most important jobs is a proper transmission oil change. This guide, proprietary to MotoSport.com, walks you through the tools, steps, and model-specific tips to ensure a smooth oil change every time.
Tools & Supplies Needed
- Socket wrench set (8mm or 10mm for drain bolt–confirm by inspection)
- Oil catch pan
- Funnel
- Measuring cup (metric–milliliters)
- New transmission oil (See specs below–AMSOIL, Motul, or comparable two-stroke transmission oil)
- Clean rags
- New crush washer for the drain bolt (Recommended)
- Latex or nitrile gloves (Optional for cleanliness)
- Service stand (to keep bike upright and level)
Oil Specifications for 1990 Kawasaki KX125
- Type: Use a high-quality two-stroke transmission oil. Do not use standard automotive engine oil, as it lacks proper clutch and gear protection for two-stroke bikes.
- Oil Quantity: The recommended capacity is approximately 650-700 milliliters. Check your owner's manual or the fill window if uncertain.
- Brands: Amsoil, Motul, Bel-Ray Gear Saver, or manufacturer-recommended fluids are all excellent options for the KX125's transmission.
Steps: Changing the Oil in a 1990 Kawasaki KX125
-
Warm Up the Engine
Briefly run the engine for 2-3 minutes (outdoors or in a well-ventilated area). Warm oil drains more thoroughly and removes more contaminants. -
Stabilize the Bike
Position the bike upright and level on a service stand. This helps the old oil drain completely and prevents the bike from tipping over. -
Locate the Drain Plug
The drain bolt is at the bottom of the right-side engine case. Place your oil catch pan directly underneath. -
Remove the Drain Plug
Use the appropriate socket to carefully loosen the drain bolt. Avoid stripping threads—these casings are soft aluminum.
Remove the crush washer if it comes off with the bolt; inspect for damage and replace if needed. -
Drain the Old Oil
Allow all oil to fully drain into your pan. Tip: Hold the bike upright during draining–it ensures all used oil escapes the crankcase.
Model note: Like other KX models, the 1990 KX125 drain plug threads are delicate. Do not overtighten the plug during reassembly or you risk stripping the case. -
Inspect and Clean
Wipe away any debris from the drain plug and case. Also check the condition of the crush washer and replace if it's deformed or hard. -
Reinstall the Drain Plug
Reinstall the drain bolt (with a new crush washer). Hand-tighten at first; then tighten snugly with the wrench. Do not overtighten. -
Add New Oil
Remove the transmission oil filler cap (large cap on clutch cover). Using your measuring cup and funnel, slowly pour in 650-700 ml of the correct two-stroke transmission oil.
Model call-out: Because the KX125's clutch pack can absorb oil, after filling, allow a minute for the level to settle and recheck the level window or oil port; if required, carefully add a small additional amount. -
Check the Oil Level
Set the bike upright (not on the stand). On the right side of the engine, find the small inspection window or the check bolt.
The oil should be even with the check port or window; avoid overfilling as excess can be forced out the breather or clutch pack. -
Replace the Filler Cap and Clean Up
Refit the filler cap. Wipe any spilled oil, properly dispose of your used oil, and clean your work area.
Specific Notes for the 1990 KX125
- 2-Stroke Design: The KX125 uses a separate transmission/clutch oil, as engine lubrication is handled by the pre-mix in your fuel. Only change the transmission oil, not engine oil.
- Oil Mixture Reminder: While not part of the oil change, always mix your gas and pre-mix oil at the recommended ratio (typically 32:1), as improper fuel oil mixtures can increase engine and clutch wear.
- Avoid Overtightening: Both the drain plug and fill cap must be snug—never force tight. The magnesium and aluminum transmission cases strip easily.
- Thread Integrity: Inspect the threads and use a torque wrench if you have one; typical spec is about 10 ft-lbs (14 Nm).
- Oil Type: Use only oil labeled for “two-stroke transmission” or “gearbox” use; automotive or four-stroke oils can cause clutch slippage or poor shifting.
Why Change Your KX125's Oil?
- Reduces clutch wear and extends the life of your transmission
- Improves shifting and overall ride quality
- Prevents build-up of dirt and debris inside the gearcase
Recommended Oil Change Frequency
- Racing or intense riding: every 5 hours of use
- Weekend/recreational: every 2-3 rides or at least once a month
By following these steps, you’ll keep your 1990 Kawasaki KX125 shifting smoothly and running strong. For all your oil, tools, and maintenance supplies, head to MotoSport.com—your dirt bike maintenance headquarters.
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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.