Kohler 19 HP Courage was running rough. Removed, cleaned, and rebuilt Walbro carb (new needle valve and seals). Changed oil, replaced fuel filter, new carb gaskets, new fuel solenoid, new spark plug. Fired it up and it ran like a champ for the 1.5 hours required to cut the yard. Next day, someone else attempted to start the mower with full choke for an extended cranking time. Would not start. Removed air filter and the choke butterfly was stuck closed. Lots of fuel in the carb. Reset the choke and it started up but was running very rough and about 1/2 power on full throttle. I smelled smoke, opened the hood, and the exhaust manifold and muffler were RED HOT! I immediately shut down the engine and let it cool. Removed muffler - it is toast. Removed and re-cleaned carb. Now, when trying to start, engine just cranks and doesn't fire. Tried with starter fluid in direct into cylinder - no luck. Spark plug is dry. I'm out of ideas. Any suggestions? Had anyone seen where a muffler has gotten red hot?