This engine needs a carburettor clean by the sound of it - I guess the slow running jet may be blocked so the engine compensates for this by its internal govenor which is trying to maintain a constant speed. Dismantle the carb completely but clean the outside first with some aerosol carb cleaner. Keep all of the bits in order, and ideally blow out each jet and drilling with compressed air if you can. You shouldn't need new gaskets for this.
