You can't dissolve carbon...the trick is to think of carbon as grapes. Carbon molecules is held together by resin which you can think as the stems. The trick is to dissolve the stem.... I know a little about this process as my father and I where part of a company called CarbonClean and later called MotorVac and then we created Engine Clean Technologies. The process of removing carbon has been around for ages and the product is basically gun soap, which was the original formula. It was used to clean the gun barrels of destroyers, battleships, etc. It was by far the best product for removing carbon, gum and varnish until silicone windings came along and it dissolved them on the injectors, then we had to reformulate and it was still good, but never like the original.
You can buy fuel injector cleaner that you pour into your tank, but the dilution ratio is too high IMO to be effective at any level. I still have the original formula here in bottles at the house and I just spray it down the carb intake with a hand held sprayer. If your system is FI you can pull off a vacuum line off your intake and spray it through there..cleans the IAC, throttle plate, etc. Unfortunately its a bit harder to get the volume into the intakes to do the valves. Best way to see if it worked is a compression check prior and after...you will see an increase in compression as the valves seal better. O2 sensors are cleaned also with the process.
Basically the machine was nothing more than a pump with a return line regulated. For carb you hooked up after the fuel pump and you became the fuel pump and ran it at 4psi. For FI you had to use the return line as you did not want product going to the tank. Also because the line was regulated you could crank up the pump. squeeze down the return and do a high pressure cleaning at the end that garnered even greater results. You could also clean too fast and hang a valve with carbon debris.
High mileage motors you should refrain from as it the carbon (talking engines never cleaned) is what is keeping it together already

4 gas before and after and watch your emissions zero out...my pickup is a 1994 and has literally no emissions as I do the crankcase and FI prior to smog..275k miles on it all original.
Water works well and was part of the reformulation to generate more heat. Never heard of ATF for gas, but used it many times in diesel.
BMW pulls the plugs and bead blasts it with walnut shells, that is the factory method then evacuates it and checks with a boroscope.
Regardless the system works...we used it in China to reduce emissions in the mines, the railroads, the Chinese Military and was a tool in the Bejing Clean Air report to get the Olympics. In the USA we manufactured for WYNNS, BG, OTC, US Military, John Deere, and so on. Even GM tested and approved it use but refused to cover any warranty if it occurred. We could handle up to 10,000HP with the equipment we made.
Average cleaning time is 20 mins.
GM Top End is a great one to use... you can drizzle it in, run at approx 1200 RPM and start lightly pouring...you will start to get a ton of exhaust water vapor and that is the best sign.