Walk Behind Lawn Mowers

My lawnmower has trouble the number is Hu725Awd Briggs and Stratton 725 ex series and it has been having trouble with low power if I would if I would bump in to something the engine power would get higer but then get lower again I have put new spark plug in and new gas cleaned Carberator and that stuff no water in gas. When I lean it backwards it gets more power sometimes it works normal then sometimes u start and It shuts off.