Fair enough. If this developed slowly then chances are the piston is worn out (that usually takes a LOT of running). If it happened all of a sudden then something's loose, either not sealing on the head gasket or the centre post of the plug has lost its seal. To check for a worn out piston, lay the engine on its side (without the muffler), close the exhaust port with the piston then fill the port with fuel. Turn the piston to the top of the stroke and look for a lot of bubbles through the fuel. There goes the compression. For leaking plugs or head gasket I use a little bottle of leak detector like they use when checking for gas leaks but a solution of detergent and water should work just as well. Run it around the head joint or a drop on top of the plug terminal and turn the piston to TDC again and look for bubbles. Oh yeah, and check the plug gasket at the same time.