My wife and I use a JD 925A zero turn to maintain our 2.2 acre lawn. It has the 27HP Kawasaki engine and is running a 54" deck set up to mulch. Lately, the unit feels (and sounds) like it is operating with about18-20 of those 27 horses. I have recently changed fuel filter, primary air filter, and spark plugs with not much improvement. What could I be missing? We bought the mower used several years ago with about 800 hours from a dealer. It now has 1100 hours on it. Thanks for your help.