I cleaned the mower deck and lubed the spindles, checked the gear oil, and made sure the blades were spinning freely. None of that fixed the problem of the engine bogging down under load. Did a little more trouble shooting and found out that if you open the fuel valve 5 turns, the tractor runs fine without bogging down. So, the problem seems to be in the fuel valve. The manual says it should be open 2 turns to use the main tank fuel and all the way open to use the reserve. I’m thinking that the main tank line is clogged. In order to remove the fuel valve for checking and cleaning, the tank will have to be drained of fuel.