originally he thought it was a leak in the bushing surrounding the back plate. He has since decided that the leak is from the base of the head and not at the back plate. That is why I suggested doing an air test after sealing all the existing openings in the engine (exhaust, fuel inlet) and pressurising through the air inlet on the carb with low pressure air. By doing that it would force air out any places that leak on the engine. then you can fix all of the leaks with out guessing.