Finally fixed,yam 80
At last all problems solved!
It was overheating and loosing pee at 4000 rpm.
Advice on forum was the powerhead gasket had not sealed ( that was the case ) I was advised to use STAG WELLSEAL jointing compound ( top stuff ) with a new gasket, at the same time replaced all the impella parts and housing, new stat and poppet valve.
Great must be good now! No still overheat at 3500rpm.
Only one place left to look exhaust cover, Sure enough the previous owner had demolished an impella and debris had got trapped in the cooling system.
Only problem was out of 18 stainless bolts 13 snapped, shit! the whole powerhead had to come off again, every one had to be drilled out and heli coiled.
But im happy to say its now running great and pees for england.
A very big thanks to everybody for the advice, And the stag wellseal is the dogs!