If you have a Briggs & Stratton Quantum lawnmower and notice gas leaking near the air filter, there is a problem with how your engine is processing gas. There are just a few issues that can cause gas to flow out of the air filter, mostly attributable to the carburetor or a safety mechanism that has failed. Check the shutoff valve, the fuel float and the carburetor in general to see if there is a major problem that needs to be fixed.
The shutoff valve on your Briggs & Stratton Quantum prevents gas from escaping into other parts of the lawnmower when the engine is not in use. This valve automatically closes when the engine stops running. Gas that's leaking from the air filter may indicate that the valve is not working correctly. It may be staying open all the time, allowing gas to leak out. You may need to replace this valve to keep it from sticking again since this problem often indicates a worn-out valve that will continue to fail more frequently.
The fuel float is designed to maintain pressures inside the carburetor by closing off the system at a certain fuel level as the carburetor operates. If the fuel float becomes stuck, a problem similar to a stuck shutoff valve will occur. Fuel will be able to escape from the carburetor, and its main exit is the air line, which is blocked by your filter. The result is fuel leaking from the filter and a float that needs to be cleaned or replaced.
Other Carburetor Problems
The fuel float is not the only problem the carburetor can develop, which may cause the lawnmower to leak gas out of the air line. Dirty carburetors, cracked components and fuel quality can cause issues of their own, which may result in leaking fuel. Replacing the carburetor may be required but be careful. Sometimes taking apart the carburetor or installing a new one can cause these problems instead of correct them.
Fuel Line Issues
If your fuel filter has become blocked by debris or the fuel line connections are failing, gas may start leaking from the mower. Sometimes fuel lines simply become too old to properly carry fuel without cracking and leaking. While this rarely causes fuel to leak directly from the air filter, it can cause leaks in nearby engine components, which may be fooling you, so run through a line check as well as a carburetor check.