Water works like a steam cleaner...I usually use a vacuum hose off the carb and restrict the end that goes into the water container down to a small jet size to prevent stalling the engine. Get the engine fully warmed up so things are nice and hot, set the engine to about 1500 rpm, and suck water in thru the hose for about five minutes. It will crack some of the carbon off and clean stuff up in the ports and combustion chamber...but, it will not work miracles on something that has a half inch clod of hard carbon/gunk on it. Choke the hose down so that the ingested water does not kill the engine.
Better is something like top engine cleaner that you dribble into the carb at fast idle, and then pour the last part of the can or bottle so that the engine dies and let it set for a bit. The downside is the tremendous smoke cloud that comes out the tail pipes. SeaFoam, GM Top Engine Cleaner, and others are designed for this. I always put new plugs in afterward