^I'd try some top-end foam cleaners and idle flush additives for this app; for what is unseen.
I've had 'consistent' carbon deposit removal with those two products on my '92 so much so that now(after 3-4 oil change time treatments, a couple of top-end cleanings and some idle time flushes like amsoil and Lubro Moly Pro Line engine flush), I no longer see carbon deposits with the used oil I drain out. Before I'd get some deposits with the drain.
That, fuel system cleaners, replacing the fuel filter, etc; so no entry of contaminants via the fuel system making things 'worse'. Piston soaks are good for carbon removal as well. Chevy knows all about that, just hard to do on a chevy truck b/c of spark plug access...
In fact, now that I think about it. This could get done via manual cleaning, rinse with oil, idle flush and oil change. Valve-train could be clean in a day if the other side looks like this too.