Black spark plugs on all new units put into field, they all foul between 25 and 75 hrs, no matter the customer and or brand of mower. I was at kaw service school this year in grand rapids and they had 4 tech's there that said " Gosh i dont know". I hate to rejet cause they run so smooth at top end and DONT surge. Ideas??????? bpr5es are the plugs used and are correct in heat range. air filters and top no load speed are mint. alcohol free fuel, what do you all think???