I have a new Toro Z-Master 20 hp Kohler with 52-inch deck. The machine has approximately 20 hours on it and I have noticed a problem with it while going up steep hills (more than 25 degrees). The motor bogs down like it is about to stall (real close). The hill is in my back yard and I have gone up it many times with no problems. Today the machine was doing it every time I mowed up the hill. I tried the hill without the blades on and no problem, turned the blades on and tried the hill again, no problem, what gives? Runs great on the flat and once at the top of the hill where it levels out. The hill is about 60 feet to the top. Could it really be the plugs already? I did notice black smoke at one point when I just happen to look back.