I've sprayed water into the carb of a running engine before to knock out the carbon. We'd have it running at high idle and squirt water into the carb as fast as we could and try to keep the engine running. It would puke out all kinds of carbon out of the exhauast. Make sure you do it outside though.
A possible problem it could create is chunks of carbon getting stuck in the valves, so I wouldn't try it on a whim, but if you definitely know you have a haevy carbon problem...you don't have much to loose.
I would check out the indexing of the plug first.
