I have a MTD lawn tractor with a 14.5 hp briggs&stratton motor.my gas is draining into my oil. WHY?

the small shutoff valve that is attached to the float in the carburetor is not shutting off the gas, letting it drain your gas into the cylinder after you shut the engine off. it will seep around the piston rings and into your crank case into the oil. you can rebuild the carburetor or put a shut off valve between the tank and carburetor that you have to shut off every time you stop using the mower