The float valve is probably worn out if it's not just dirty. I have seen this before. There is a little rubber tipped needle thing that the floats open and close, if the rubber on the tip gets deformed just a little bit, the valve won't completely seal and gas will seep past the valve and overfill your float bowl. This will cause gas to come out of the overflow tubes on the carb whenever you leave the petcock on. It can also fill the entire motor up with gas.