Hi all. Engine gets hot, temp reading good, over about half hour or so temp gradually drops and drops. Pull over, let engine cool, sucks all the coolant out of the header tank until its bone dry. Add more coolant - same reoccurs. Any suggestions? zzr1100 c1. ps stat works.

If no obvious water leaks? - gotta be a headgasket?

Different engine (pinto with Polo rad) but mine did exactly the same. It was just plumbed in all wrong.
Has it been doing this from the first run or just started after a bit of use?

Kind of always done it, the coolant bubbles in the header to the piont it forces some out the overflow pipe. No gunk on oil cap and last time changed oil it was ok.

It's the plumbing then. Get it looked t mate or post some pics etc

ok cheers mate.

I have re-bled, hopefully properly this time, got to bleed at pump, rad cap and rad cap nipple. As the engine or rad get no direct feed of cold air, I have now fitted 3rd rad. 1x on oil cooler. 1x on rad - both controlled by fan switch and a second one on rad with perminant feed to immitate air flow. The overflow pipe from stat housing to the header tank is now much longer and coiled up. hopefully this will allow coolant to cool bit more on way to h/tank and not boil over out the overflow pipe. I will test tomorrow and give update.

sorted!

Excellent. Well done