Those square ports are for idle pressure relief. They allow water to exit the powerhead, when the motor is in the water in neutral.
The carb floats are a normal replacement item. The original floats were a yellowish hard foam. These tended to dissolve in today's fuel that contains alcohol. Replacement floats were hard white plastic. Check those for cracks and to make sure the small springs are intact and not rotted away.
Set the float height properly as well, and use a fresh 50::1 fuel mix when adjusting the idle mixtures on the carbs. Idle mixtures must be set in the water, motor warmed up and in gear. Idle mixtures are set rich enough to allow proper acceleration, but lean enough to allow decent idle.