Think his fan is fine so he says, tested it on a 12v source.

How old's the bike? How many miles?

I'm thinking about the radiator cap itself, whether thats faulty.
They're designed to be able to handle a certain pressure, thats how the coolant is allowed to go above 100c without boiling because its kept under pressure.

If the cap is faulty or doesn't produce a seal well, it may leak and boil over.

Might be worth having a look at the radiator cap, checking the condition of the seal and sealing faces.

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09 XT660X:

CCC Stainless Rounds, Stage 1 DNA, Stage 2 DNA, PCV With Custom Autotune Map, PCV AutoTune, Home made O2 eliminator, Self machined Fork Spacers, DIY Autotune Switch.