I had the same problem with my boat.
the float is sticking in the carb, which allows the gas flow to increase
drematicaly. YOU DON'T HAVE TO REBUILD THE CARB!!! Just open it up
carefully. clean out the the area where your float is located. try not to
damage the gasket.
Make sure the float will move up and down very easly.
However, if it does not, then you will need to install a carburater kit.