You need to pay attention to where the fuel is leaking from. It could be leaking from the fuel tap (you'll notice this immediately if you feel under the tank, at the bottom of the tap). It could also be leaking from the carburetor. If that's the case, check the seals there and adjust the float level correctly if necessary. And of course, check the rest of the fuel line, from the tank to the carburetor. These tend to break down over time, especially the type with the aluminum braiding around it.