Even though you replaced the plug, check that you have a good spark before you do anything with the carburetor. Then check you have fuel at the carb before you take off the carb or fiddle with it. It would be a shame to start messing with such a sensitive part as the carb, if the trouble was just a bad HT lead or a blocked filter or fuel line from the tank. Is the air filter clean? Does it now start from cold? It seems to have started ok after you changed plug and cleaned it up, then it stopped. Could be the air cleaner is blocked and making an over rich mixture for a hot engine. That wouldn't show if it was cold, because you want a rich mixture then, hence the choke lever.
Tony
December 2007