Why does a 2 year old 4.5 HP Briggs and stratton lawn mower engine runs fast then slow then fast then slow never gaining a smooth rpm?

its probaly the carburater or the spark plug. ok, take off the carb. and take the two halfs apart. then take some sort of container and soak it in gasoline. after it has soaked for a little wile, scrub with a old tooth brush. if it is the spark plug(wich i would try first since it is easiar) just remove it and clean with a wire brush. it could also be an unbalanced blade.