How to Change the Coolant in a 1990 KTM 300EXC: Step-by-Step Guide

Shop parts for a 1990 KTM 300EXC Dirt Bike.

Changing the coolant on your 1990 KTM 300EXC is vital for keeping your two-stroke performing at its best and preventing overheating on the trail or track. This guide walks you through the process, notes unique aspects for the 1990 model, and provides care tips to maximize the life of your engine and cooling system.

Tools & Supplies Needed

  • Fresh motorcycle-specific coolant or 50/50 pre-mix
  • Drain pan
  • Sockets, ratchet, or spanners for drain bolt and radiator cap
  • Funnel
  • Distilled water (for flushing, if needed)
  • Shop towels or rags
  • Gloves and eye protection

Before You Start: 1990 KTM 300EXC Notes

  • The 1990 300EXC uses two radiators connected in parallel–always ensure both drain completely.
  • This vintage KTM typically uses a simple cooling system without a reservoir. Fill and check the radiators directly via the cap.
  • Inspect hoses and drain bolt washers for age-related wear–replace as needed to avoid leaks.

Step 1: Let Your Bike Cool Completely

Never service a hot cooling system. Ensure your bike is on level ground and fully cooled off for safety.

Step 2: Remove the Radiator Cap

Locate the radiator cap on either side. Loosen it slowly to release any pressure before removing it fully.

Step 3: Drain the Old Coolant

  • Find the coolant drain bolt, typically at the base of the water pump or lowest radiator point.
  • Place a drain pan beneath and carefully remove the bolt.
  • Allow coolant to drain from both radiators completely. Opening the radiator cap at this stage helps coolant flow out faster by letting air in.
  • If your bike has a petcock or hose at the bottom of the radiator, open or disconnect it as well.

Step 4: Flush the System (Recommended)

If the old coolant looks dirty or has changed colors, flush the system for maximum protection:

  • Replace the drain bolt temporarily and fill the radiators with distilled water.
  • Start the engine and let it run for a few minutes with the radiator cap off.
  • Shut off, let cool, and drain again. Repeat until water drains clear.

Step 5: Inspect, Replace, and Reinstall

  • Check the condition of the drain bolt washer–if in doubt, replace it.
  • Ensure the radiators and all fittings are free of debris or corrosion.
  • Reinstall the drain bolt securely.

Step 6: Add Fresh Coolant

  • Using a funnel, slowly fill both radiators to the base of the filler neck.
  • Gently tilt the bike side to side to help release trapped air pockets.
  • Leave the cap off for now.

Step 7: Bleed and Heat Cycle

  • Start the engine and let it idle. Watch the coolant–top off as needed as air bubbles escape.
  • Once the level stays steady, replace the radiator cap.
  • Allow the engine to reach operating temperature, then shut it off and let it cool completely.
  • Check the coolant level again when fully cool. Top up if needed.

Step 8: Final Checks

  • Inspect for leaks around the drain bolt, hoses, and radiator seams.
  • Test ride the bike, watching for any sign of overheating or warning lights (if equipped).

Pro Tips for 1990 KTM 300EXC Owners

  • Vintage radiators and hoses can become brittle–inspect for cracks or soft spots regularly.
  • Use only compatible coolant. Mixing types (red, green, etc.) can cause precipitate and blockages.
  • Consider flushing the system at least once a year, or more often if riding in muddy or harsh environments.
  • This model does not typically have a coolant reservoir–always fill and check at the radiator cap.

Changing the coolant in your 1990 KTM 300EXC is a straightforward job that protects your investment and ensures legendary two-stroke longevity and performance. Keep these steps handy for the next time your KTM needs a refresh.

Related Shopping Categories

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Shop Radiator Hoses for a 1990 KTM 300EXC 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.