When removing a Mikuni carb, you have to drain off some coolant to avoid it back spilling into the manifold (don't worry, EVERYONE does it ) Either siphon some from the radiator or have an ice cream bucket etc under one of the manifold hoses (heater bypass hose to manifold base is the best place) and let it drain until you have no coolant pouring out. That way the coolant won't try to back log up through the coolant port under the carb