Hey Guys: I was mowing my last account last night with my ~15 years old belt driven 48inch scag walk behind with a 14 Kohler Command when I noticed my mower seemed to be operating at a lower than usual RPM and was having difficulty getting through the grass. The motor runs great. It starts on the first pull, does not use oil, and doesn't smoke. There are also newer belts on the mower. When the mowing deck is off the engine seems to be operating at a correct RPM, but when the deck is enaged, the mower does not have enough RPM and power to power the deck. Fourth gear also seems slower than usual. Any help with this would be greatly appreciated, as I have to mow with this tommorrow. Air filter is clean, newer spark plug and fuel filter.