When your lawn mower sits for a while, this can happen and fuel will either spill all over the floor or will end up inside the engine crankcase.
As the fuel in the carburetor begins to degrade and breakdown over time, it can cause the seal of the needle and seat to lose its seal. When this happens, fuel from the tank will seep through the carburetor and overflow the float bowl.
The fuel then will flow out the throat of the carburetor and through the air filter and then you will see it dripping from the air box cover.
In cases like this, the carburetor will need to be cleaned, the oil changed and the air filter as well. You may also have to remove all the fuel from the fuel tank and replace with fresh fuel.
If you need help with this problem, our Locations pages has lawn mower repair companies that can clean the carburetor, change the oil and air filter and get your lawn mower working as it should once again.
Leaking fuel can cause a fire and should not be operated until the source of the problem has been corrected.