If it isn't leaking from a hole in the tank, my first place to check would be the petcock... if not there, its probably leaking from the floatbowl of the carb, especially if it has been sitting a real long time.. you'll probably need a new float needle and float if it has a foam one like in the bing carb on my Maxi...
After sitting in crud and fuel for a while, they tend to "sink" in the gas and need repalced... I've 3 bing carbs with the same foam float in them....