Those carbs have many small orifices so a soak may not be the best way to clean them. We use an ultrasonic cleaning machine and it works wonders on those carbs. By you closing off the choke you are causing a surge in vacuum and that may be enough to draw the gas up the tube within the carb especially if the tube is partially blocked.The other thing is, have you checked for any vacuum leaks at the base of the carb? If there is a vacuum leak, closing the choke allows the leak to provide enogh air to make it run good. Start the engine, run it with the closed choke and spray carb cleaner around the base of the carb. If the engine increases rpm's that is your vacuum leak since the carb cleaner is adding extra fuel to the mix.