I ran my enigne up again today to balance the carbs. Got half way through and noticed that coolant was squirting out of the overflow in my header tank. switched off and let cool down. removed the header tank cap and it was FULL.

eventually it went down. started the engine and after a minute or two it happened again.

I ran the engine with the cap off to see what is happening. The top hose on the tank is connected to the breather outlet on the thermostat. there is water running out of this the whole time the engine is on. It is hot water (obviously) and it isn't going to the rad which concerns me a bit. Leave the engine running and after a couple of minutes, the water level rises fairly quickly. switch off and all returns to normal.

The top half of the header tank is higher than the thermostat, which sits on top of the R1 engine.

I have bled the radiator several times by running the enigne, stopping it, taking the bleed screw out of the top of the rad and waiting (for easily 30 seconds) until water started to come out. Put the screw in and repeat.

The coolant is still getting hot and expanding (is it boiling?) what might be wrong and how do I fix it? My SVA is on Thursday and I'd like to get through it without droping boiling water on the inspector's feet.

Sounds like the system is airlocked somewhere.
I used to have this problem with a shitroen Xantia.
Absolute b'stard to fill.

I spent some more time bleeding the radiator. Run the engine, stop bleed, run the engine...

Refitted the filler cap and ran the engine until the fan came on, ran for a bit and switched off three times. No more water coming out. Probably OK now. Thanks for the advice.

I had a similar problem. What radiator are you using? Do you have an expansion tank or a header tank?

--Chris

Did you have a rad fan running at the time? I think you will find without a fan any bike engine will overheat eventually if just idling for as long time.

If you have a fan, then its most likely an airlock in the system as others have pointed out.

Cheers

Paul

I am using the radiator supplied by MNR. It has one inlet and one outlet with a small bleed screw at the top. I believe that I am using a header tank as it is open to the rest of the cooling system but sealed with a pressure cap. The escaping coolant must have been under enough pressure to force it's way out.

Once I have bled the system more, the coolant in the radiator got hot enough to switch on the fan but without spilling over. I think I'll still be careful for a while.

use your fan override swith to keep the fan going...

The rad is a polo rad and should be okayish, but with no airflow you might have issues..

If in doubt you can look at the vw golf mk2 rad which I believe (have been told) is same size only thicker core, or for more money a pace westfield rad...