Sometimes the fuelcock diaphragm will wear out and allow fuel to flow to the carburetor when the unit is shut down, which will cause excess fuel to dump out the overflow tube. Replacing it could solve your issue.

After your fuel reaches a certain level, it elevates the float and closes the needle valve though which the fuel flows. If something interferes with this, (sand or dirt in your fuel) the gas level will continue to rise and will overflow out of the overflow tube. It could also be an open float drain valve, wrong float height or your float needle is damaged. You could clean out or rebuild the carb. You could also just upgrade it.

Hope this is helpful