Blown my radiator cap?

The cooling system on a bike is relatively low pressure. If you're getting that much pressure in there I'd be checking the head gasket.

The radiator cap is where the pressure relief is. It is possible that the radiator cap is bad, and with a 13 year old bike that wouldn't be surprising.

I suspect, however that something else is wrong.
Like Julien said, a leaking head gasket is a very common source of this problem. What happens is that the very high pressure from the cylinder leaks past the gasket into the cooling system. This keeps raising the pressure in the cooling system until the radiator cap pops open to relieve it. Unfortunately, when it relieves the pressure it also dumps coolant and it doesn't take long to run out.

Another possibility is that the water pump has failed. If water isn't circulating through the system then what is in the engine will boil, which increases the pressure.

The first thing I would do is take the radiator cap off, start the engine and look down in the radiator. If the water pump is working you should be able to see water moving in the radiator. If it is sitting there very calm and placid then I would suspect the water pump. If if you see a lot of bubbles (it might look like it is boiling, even though it is still cold) then I would suspect the head gasket.

How long has it been since your last top end rebuild? If it has been a while then I would start there.


Rod