Just bought this tractor. When I got the tractor I could run it at low engine RPM fine, both idling and driving. Ran it for about 5 minutes to get it where it needed to be to load it on the truck to take home. Half way to the loading point going up a hill it started slowing down (engine RPM's fell) and I pulled the throttle arm all the way but it ended up stalling. Got it going and used a lower gear and it still had a hard time pulling any kind of RPM's like it should, till it got on flat ground. Once on flat ground it started to race and the RPM's went higher than they should. I shut it off. Loaded it on the trailer got it home. Now anytime I start it, it immediately goes to the engine racing. What might have happened here?