I've used it to soak pistons that had frozen rings. When starting, the rings seemed welded into the pistons. A week or two soak, occasionally attemting to move pistons, will eventually get all the old carbon and gum out. I've salvaged old, non replaceable pistons this way.
My brother has used it in motorcycles straight into the carb while running and it's helped rough running motors.
I found that it removed gum type deposits quiclky, and softened carbon very slowly - over many days. It didn't just desolve it even soaking in pure sea foam.
My experiences would seem to indicate that it would be of marginal help with carbon, but a lot of help with gum deposits.