He is not trying to slow it down, it won't run at low throttle if I read the post correctly. This sounds like and engine/motor issue. Is this a AC Pulmore motor? I would look at brushes and motor maintenance, followed by engine running gear. Your running at 80% throttle to get it to go, indicates an issue with the engine.
A PW engine (similar to yours if pulmore) would not need 80% throttle to move with a 1033. G