I'm running an exmark TT with the 17 hp kaw. Last week it started acting a bit funny, sputtering just a bit every once and awhile. Today I mowed 8 hours and it got progressively worse. I changed the fuel filter, no change. It only is sputtering though with the blades engaged. Engine ran without blades engaged, no problems. What might be the most likely cause? Spark plugs maybe? Ive not changed them in the 2 years I've had the mower. Air filter appears fairly clean. Any suggestions?