Go-kart Engine Help!!!!?

I have a 3 hp briggs and stratton engine that is probably about 20 years old. It was working perfectly until yesterday and then it kinda stopped working. It starts and idles perfectly but when i pull the throttle at all the engine lags down and dies. I was running 7 month old gas so I thought that it was the carborater was clogged and i sprayed some carb cleaner down it but it didnt seem to help. Im thinking that i may need to take the carb apart and clean it (hopefully not). Another thing that i think it may be is the spark plug. Since it idols good i think that the spark plug may be missing at high rpms. it is a very old spark plug as well (15 years). Anyone with any knowledge about engines please help me out. Thanks