Chances are there is fuel leaking past the float needle valve. When the engine is running, it uses the fuel faster that the leak replenishes it, but when the engine is stopped, float chamber over-fills, and fuel comes up through the carb jet and runs into the engine.
Solution - remove float bowl and float, clean/replace needle and/or seat, and re-assemble.
phil_saunders@bigfoot.com
Phil Saunders
May 2008