OK - here's the story..... I had a problem with overheating, power, missing, etc. with a 20 HP Kohler on my Walker mower. Certified Kohler dealer works on it and finds blown head gasket on one cylinder and blown intake gasket. He replaces those, rebuilds carb, and mower runs fine. After about 100 hours symptoms re-appear......same mechanic replaces gummed up spark plug on cylinder that originally had blown gasket.....can't find anything else wrong. Now about 100 hours later that same cylinder has plug gummed up with oily black carbon to the point it closes the gap on the plug. What is causing this? Any ideas???