Coolant Boiling

Slap some sun screen on your bottle

Just a suggestion...
The main time of year that thermostats start to bugger up are at the change from summer and to autumn.
You might want to take out your thermostat and dunk it in some boiling water, just to make sure its opening all the way.

That will also cause your symptoms.
Cheers
Tnate

Put a new bottle on, seems to fine for now. Will see how it goes

The crack and leak is your problem.

When the system is pressurised, it raises the boiling point of the water (from 100deg to about 120). The liquid expands as it boils, and under pressure there is no room to expand. As you have a crack it allows the expansion at the normal temp (100 deg) and so the liquid boils. Fix the crack, fix the boiling problem, as most engines are designed to run around the 100+ mark.

Mike the coolant reservoir doesn't form part of the pressurised cooling system. The Rad cap is where the pressure is held. The catch tank is only to catch the fluid as it heats up and expands, then as it cools it sucks the coolant back into the rad. Changing the bottle shouldn't fix the prob. My guess is either the rad cap is leaking or not holding pressure, or a head gasket is on it's way out.

id go for rad cap, thered be a strong chance of milkyness or air bubbles in the radiator when running if it was the head gasket

The crack and leak is your problem.

When the system is pressurised, it raises the boiling point of the water (from 100deg to about 120). The liquid expands as it boils, and under pressure there is no room to expand. As you have a crack it allows the expansion at the normal temp (100 deg) and so the liquid boils. Fix the crack, fix the boiling problem, as most engines are designed to run around the 100+ mark.

Mike the coolant reservoir doesn't form part of the pressurised cooling system. The Rad cap is where the pressure is held. The catch tank is only to catch the fluid as it heats up and expands, then as it cools it sucks the coolant back into the rad. Changing the bottle shouldn't fix the prob. My guess is either the rad cap is leaking or not holding pressure, or a head gasket is on it's way out.

Maybe it shouldn't fix the problem Marty, but it did exactly that when I had exactly the same problem, as I said before the reason the bike over heats is, as the coolant get hot and expands in to the overflow bottle, it leaks out of the crack, thus when the bike cools down it cant retrieve the coolant from the overflow bottle, like mine this eventually leads to the bike having little or no coolant, sooo the bike overheats.
Once I put the new bottle in the bike hasn't had one drama even sitting at lights for ages on a 33deg+ day

I think you're right mate. When I changed the bottles over and did the whole flush/refill thing, there was not a drop of liquid in either the radiator or the overflow bottle. Very lucky not to do a head gasket I think

the coolant level on the MFP12 has dropped a lot since the 24k service, which was just before the WSB trip. While staying at PI, I had to add about 300ml to the overflow tank....

It had dropped to the low mark once the return trip was over, and seems to have settled there, but it still shits me that it seems to have lost coolant, without any obvious signs of leaks, splits, holes in the radiator, loose hoses etc......

My vote, along with strika, is the rad cap. If it doesnt seal properly it allows the coolant to escape before pressurising thus lowering the boiling point as PM touched on. If the system does not presurise then the boiling point is normal ( 100 deg ) and the thermos dont come on until 102 (?) Also in agreement with the cracked bottle not holding enough fluid for the system to draw upon when cooling.

Gos if the twelve has settled at a happy level now is it possible that when it was last drained and refilled ( pre PI ? ) there could have been an air lock?

I think that must be it - I have looked everywhere for any signs of leakage, and have found *jack shit*.....

Brown coolant can indicate a reaction with engine gasses, which points to a warped head/head gasket issue. Next time you ride it, get it up to temp, then get off with it still running and open the overflow bottle cap. any bubbles?