Anytime a used RC especially nitro is purchased a good tear down is a really good idea. The engine should be taken out, cleaned externally then tear down, inspected, cleaned internally with rubbing alcohol and ad a few drops of after run to the engine bearings then reassemble and make sure all the seals are good. There are several reasons for this as there could be a bad gasket seal or a bad one way bearing in the pull start causing a lean running engine, ( a bad front engine bearing can cause this as well). Also there could be a bad clutch spring or something broken in w/ the clutch causing it to engage prematurely. It’s also a good idea to tear the whole truck down for the same reasons and that way you can familiarize yourself with the truck and how everything works.