I bought a used plane and engine from a fellow flyer. The engine is a Thunder Tiger 91. Last night I was ready to start the engine to see how it would run. After a few needle valve adjustments and throttle adjustments the engine started. It sounded real good at idle and up to half throttle. But when I advanced the throttle beyond half throttle the engine sounded like it was going to come apart. A large rattling sound. Any ideas? If I wanted to get the engine looked at for diagnostics, what and where would be the best place for that. I am not confident enough to work on the engine myself. Any help would be appreciated.