It is a 3.5 horsepower briggs motor. ( model #9D902) When I start the mower up it runs weird. It runs at an up and down speed. Runs full idle then almost dies repeatedly. I took the carburator off and cleaned it. I cleaned the fly wheel and the armature. I changed the oil. I changed the spark plug. I also changed the air filter. The only thing I am noticing that is odd, is the spark plug keeps turning black. I can clean it and put it back in and it turns black again. I can't think of anything else to do. HELP!!!!!! Thank-you ahead of time for your help.