From experience the water trick may work for getting carbon, etc off the valves and top of the pistion. If I thought there was a problem with stuck rings and excessive carbon on the valves and top of the pistions, like said earlier get the engine warm and then put in GM Top Engine Cleaner by pouring down the carb. Pour in until the engine stalls or the can is done. Then let the engine sit for several hours or overnight before restarting. Besides working on the carbon on the valves, it will also soften up anything that might be causing the rings to be tight. Here is a link with a picture and your local dealer may have it. Gm top engine cleaner for cap tires - HobbyTalk