I have a 48" exmark turf tracer that wants to bog down when the blades are engaged and encounter a load. It mostly does it going up hill and now it is starting to do it all the time. When it is on pavement the blades run at full throttle with no problem. Sometimes if I put my speed on half it will run ok up hill. I've changed the fuel filter, fuel pump, emptied the gas tank and cleaned it and cleaned the carb. Does anyone have any suggestions? At this point I'm at a loss and will try anything.