Are you seeing gas pouring out of the air filter area on your lawn mowers engine? When you remove the air filter is it soaking wet with gas?
This is a common problem when a lawn mower sits for a long time such as over the winter. Some lawn mowers have a shutoff valve that allows you to turn off the gas and not let the weight of the gas overcome the needle and seat in the carburetor. A needle is the size of a pencil lead and it has to force itself down on the seat to seal off the gas from coming through the carburetor and flooding the engine.
If the needle and seat becomes dirty or gummed up with fuel residue, gasoline will begin to seep past it and come out the throat of the carburetor and thus into the air filter. It will continue until the gas tank is empty and could cause a fire in your garage or shed.
This problem is a safety concern and should be inspected and repaired by a small engine repair shop. You can find one near you by visiting our Locations page.