Last summer in July I bought a new Exmark lazer Z hp with the 21 hp w/ 48" deck. The mower ran right for about 2 hrs. then started misfiring and I removed the plugs only to find dark carbon black plugs. I took the mower to Mutton Power Equipment in Fort Wayne where I bought it. It was an 01' model with the new type of exhaust system. The exhaust smells like it cannot burn off the fuel that it is being given. The exmark tech at Muttons said it sounded like A Kaw engine to him. Then why were the plugs so fouled up? It backfires when i shut it off and the new set of plugs I put in it were fouled after about 2 hrs. of use again. The mower is great when it comes to cutting but I know the engine isnt running right. Also oil shoots out of the exhaust. please help me.