I have an Exmark Turf Tracer 48" deck. Today as I was mowing, the engine (Kohler Motor 14hp) revved up VERY high then shut off. Now it wont start at all! Any ideas on what could be causing this? Oh and when I pulled the spark plug, it was black and the part that you gap (don't know the name of that part) was all the way closed! Plug is only about 1 1/2 months old. I thought that was odd also.