Find the source of the leak
Because leaking gasoline and even just gasoline fumes can be highly dangerous, take precautions when searching for a fuel leak in your riding mower, and make sure you handle all inspections and repairs in a well-ventilated area. Look for cracked or loose fuel lines that may be the source of the leak. If you spot oil stains, your mower may be in need of more significant repair.
Check the carburetor and head gasket
Either of these riding mower parts can leak oil, leading to a potentially dangerous situation and future damage to your mower's engine. Consider rebuilding or replacing the carburetor if it's the source of the problem. Replacing the head gasket involves removing the cylinder head, then cleaning the surfaces.
Perform preventative maintenance
To keep your riding mower and tractor from leaking fuel, perform regular maintenance on it and keep it in good working order. Check the ignition, clean the engine cooling fins, and make sure the battery, throttle and choke controls are in good shape. Change the oil filter, air filter, spark plug, fuel filter and oil. This type of maintenance is especially important if your mower spends part of the year in storage.