If it is leaking out the exhaust then either valve lash is too tight, Valve is burnt, valve is bent, or something is holding it open. Take a quick look at your carb to make sure nothing is missing (squirter, venturi screw etc) then take a straight edge and hold it against your valve retainers ( preferably against the tops of the valve stems and see if #2 is lower than the rest. on a properly machined head these are even when the valves are closed). If they are all even then the valve must be burnt (unusual for it to happen all of a sudden) if they are uneven and #2 is sitting down a bit then either the valve is bent or something is holding it open. Exhaust valves are the ones to get bent because the piston chases them up the cylinder on exhaust stroke, weak spring can do this. I have had a piece of intake gasket hold a valve open before and it lost all compression. Your endoscope up the exhaust port might show this and your endoscope will feel right at home...