Corvec britony 11T how can I fix it?

Pilot light went out removed burner (brushed it clean) checked for spark and clean thermocouple.replaced burner including small gasket lit pilot but the flames when fired up are to far forward of the heat exchanger and the casing is red hot in seconds.ihave gone through the service guide and everything seems to be in the right position now I'm stuck ! Anyone any idea what's wrong?