Bet you have a briggs engine with a diaphragm carb fitted (the type where standing behine the machine the carb is to your right on the side of the engine and is connected directly to the top of the fuel tank.
If so, then your problem is simply the carb diaphragm that needs replacing. Obtain part number 795083 from briggs and stratton for around £2 or £3 pounds and go to you tube to see a video of how to fit it. Very common problmem and easy to cure.
pleasant
November 2011