its hard to tell, it could be a extremely lean bottom, but scince you set ti to factory settings i doubt this is the case, you may want to check the clutch to see if it is hanging it up, if it was a rich bog you should have smoke......how used was it? did you know who had it.......my next step would be pull off the engine and check the compression........and easy way to do this without taking the engine apart is pull the engine off the truck then hold the engine up by the flywheel the compression should hold up the engine for at least about 10 seconds.........hope this helps....