.......and cracking a ring. This is one of those things that many people do and never get into any trouble but.......one out of every 1000 or so crack a ring doing this, pouring any liquid down the carb is an invitation to disaster. Yea I know people have been doing it for years but every engine I've torn down needed bead blasting to remove the carbon from the piston domes, I doubt the water gets very much out. Mostly the black you see coming out of the exhaust pipe is the soot that has accumulated in the muffler, not carbon in the engine. I hate to be a party pooper but......This is like the "don't ever put a battery on concrete" wifes tale that never seems to die. I wouldn't do it - ever. A good hard run on the highway will unfoul the plugs just as efficiently.
In case your wondering where the battery fairy tale came from, they used to make battery cases out of glass and the were much heavier than they are today due to the heavy plates. Ever try to put a 150 pd chunk of thin hollow glass down gently on concrete? It would be kind of like trying to put down a full fish tank.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo