Back before the high-precision manufacturing we currently enjoy, when people used to get valve jobs at 50,000 miles, a trick for decarboning engines was to slowly introduce water into the carb bellmouth. When it hit the combustion chamber & turned to steam, it would pressure-clean the combustion chamber. Water expands as steam to 1700 times its liquid volume. The engine would make quite a racket, and the exhaust would emit what looked like a disgusting and scary cloud, but it would eventually dissipate.
Now, I have never done this to my engines, but I have read of it, so don't try this at home unless you know what you are doing, or some such caveat.
If you search on here, I have posted about Marvel Mystery Oil, Seafoam, Start Your Engines, and Sta-Bil. Search by any of them and my screen name or try MSDS. It's all there from before, including links to the MSDS info.