Is your RS150R running hotter than usual? Or maybe it’s just time for that 24,000 km maintenance interval?

Changing the coolant on a Honda RS150R (or the K56 engine platform) seems simple: drain, fill, and go. But here is the secret most mechanics won’t tell you: this engine layout is famous for trapping air pockets in the cylinder head.

If you use the standard “fill and wait” method, you risk “invisible overheating”—where your dashboard shows normal temps, but your engine is frying internally.

This guide will teach you the “Active Bleeding” technique—using blipping, tilting, and squeezing—to ensure 100% of the air is out and your machine stays cool.

🛠 Tools & Consumables

• Coolant: 1 Liter (Honda Pre-mix or a safe aluminum-friendly brand like Shell).
• Tools: 8mm socket (for drain bolt), Phillips screwdriver.
• Time: 20–30 minutes.

Step 1: Drain the Old Fluid

1. Cool Down: Never open the radiator cap while the engine is hot.
2. Open Up: Remove the right-side fairing to access the radiator and water pump.
3. Drain: Place a tray under the water pump. Remove the drain bolt (the lowest bolt with the copper/aluminum washer) and then open the radiator cap to speed up the flow.
4. Don’t Forget the Reserve: Drain the white plastic reserve tank too! Old, dirty coolant loves to hide there.

Pro Tip: Check the crush washer on the drain bolt. If it’s flat or damaged, replace it now to prevent leaks later.

Step 2: The “Active” Fill (Crucial Step)

This is where most people fail. Don’t just pour the coolant in and wait. You must physically move the air.

1. Tighten the Drain Bolt: Secure the bolt (do not overtighten).
2. Fill Slowly: Pour coolant into the radiator until it hits the neck.
3. Squeeze the Hoses: Gently squeeze the large rubber radiator hoses with your hand. You will see bubbles “burp” out. This acts like a manual pump to dislodge air from the bends.
4. The “Tilt” Trick:
   • Take the bike off the center stand.
   • Carefully lean the bike to the left, then to the right.
   • Watch the filler neck—you will often see a big “glug” of air escape that was trapped in the radiator corners.
   • Top up the coolant as the level drops.

Step 3: The “Blip” Bleed (Engine Running)

Now, we use the engine’s power to flush the final bubbles.

1. Start the Engine: Keep the radiator cap OFF.
2. Idle: Let it run for 2–3 minutes.
3. The Blip:
   • Give the throttle 3 or 4 sharp “blips” (revs) to around 4,000–5,000 RPM.
   • Why? At idle, the water pump spins slowly. Blipping creates a sudden surge of pressure that flushes sticky air bubbles out of the cylinder head and up to the radiator neck.
4. Watch the Drop: As the thermostat opens, the coolant level will suddenly drop. Immediately top it up.

Step 4: The Fan Test (Final Verification)

You aren’t done until the bike passes this test.

1. Let the bike idle until the Radiator Fan spins.
2. Wait until the fan turns OFF automatically.
   • Success: If the fan turns off, your cooling system is efficient.
   • Fail: If the fan never turns off, you still have air trapped. Repeat the “Blip” step.
3. Turn off the engine, top up one last time, and close the radiator cap.
4. Fill the Reserve: Fill the plastic reserve tank to the “UPPER” line.

⚠️ Important Cleanup

Coolant is slippery and can peel paint. If you splashed any on your fairings or tires during the “blip” step, wash it off immediately with plenty of water.

That’s it! You’ve successfully bled your RS150R. No more airlocks, no more overheating risks. Ride safe!