Loss of power when blades engage

I have an old Craftsman Zero Turn lawn mower. It was running good until it was parked on an incline. Now its blowing out white smoke and when the blade is engaged it runs maybe 1 mile an hour. We have changed the spark plugs, cleaned the carburetor, changed the oil and fuel filter. It will run fast and then bog down slow, then pick up some power and then bog down again slow. It will eventually die out and have to be restarted. It runs with speed until the blades are engaged.