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Unfortunately it is a bit of a faff to check the coolant level, since you have to lift the tank and apart from the 2+4 bolts that hold the tank there are 9 pieces of fairing that need to be undressed.
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Minor correction in my previous post, now that I've raised the tank yet another time to check the I/C level.
If you don't have massive hands you can get away with only raising the tank an inch. For this it doesn't even have to be empty (mine was full). Only seven pieces of fairing need to be removed, since the rear ones can stay in place, as you don't need to remove the rear 4 bolts holding the tank.
Managed to unscrew the I/C tank cap and it was (very) slightly pressurised (24 hours after the last ride).
It was also full to the brim. Last time I had not set it full to the brim, and it had burped 50-100cc since, so it must have pulled the missing coolant from the OEM coolant reservoir. Which is nice.
So the 'burping' issue has been solved apparently. The new overflow hose arrangement seems to be working.
