I have a Lawnboy 10730, with a Kohler XT650 engine. Five years old. Seems to have developed hydro-lock, whereby I can't pull the starting cord. I remove the spark plug, and I can pull the cord fine fine (though nothing comes out of the plug hole - I would have expected some oil to shoot out). I replace the spark plug, and I can't pull the cord again. What's the fix???