I have an Evinrude sportwin 9.5, not sure of age probably 60's. It runs
so sweetly I cant believe it, (miles better than the Honda 2006 5hp I
have) but siezes after 15 mins running.

I changed the impeller, which was pumping well anyway, changed the
plugs for NGK with a cooler rating and tried again but it still
happened.

Water gushes out, the plugs are light tan, maybe a little lean, but not
much.

I guess it should be 32:1 or 50:1 at least, I am using unleaded fuel -
is that a problem?
not sure if it has a thermostat or where to find it- does anyone know?
Any other sugestions?

Will post the model number later as I am at work now - would be
interested to hear what year it is if anyone knows

thanks
Nick