I will share THE most effective carbon remover that I've EVER used. I do NOT generally tell about it because it CAN and WILL damage or destroy an engine if not used properly. WATER! Yes, good old water! We used to install secondary misting water injection systems to keep the internals clean on our hot rods way back in the 70's. When I was a heavy line engine mechanic for Cadillac (remember the 425, 472, and 500 Cubic inch mastadons of engines in those?), I would routinely decarbon an engine I was going to tear down for internal repairs using good old water! The water turns into steam in the combustion chamber, and, as steam, it's volume drastically increases. Problem is, you can hydrostatically lock up an engine in a heart beat and cause catastrophic damage!
When I took the engine apart, it was as clean as new! ALL carbon was gone!
Using a carbon cleaner MUST be done with a non combustible chemical. Otherwise, it is consumed as fuel by the engine BEFORE it can do any good.
The oil additive from ATS claims to clean carbon from the UNDERSIDE of the engine, i.e. the underneath of the pistons, the crankcase, rods, etc.
The ATS fuel additive REALLY cleaned dirty fuel injectors. I know because I used it on my own cars! THEY (ATS) claim that the fuel treatment decarbons too. So does API SP rated motor oil. That and API SN-plus employ new chemistry to address this issue to alleviate carbon related LSPI (Low Speed Pre Ignition), which damages pistons and upper rings.
Again, if one is EXTREMELY diligent, water can and will do a very good job of cleaning carbon. Then, once used, the oil and filter must be changed! With our old hot rods, we changed the oil all the time, so it seemed!
PS: My old hotrod back then was a 1966 Galaxie 500XL running a 428CJ, C-6 auto trans, 3:71 posi diff gear with 31 splined axles