Is it possible to remove carbon buildup on valves without removing the cylinder head?
Yes, but not with the same results as physically removing the cylinder heads and cleaning them. Using a fuel additive that says fuel system cleaner, valve cleaner, etc. will have some success in cleaning the valves of an engine. The best policy is to use a lighter fuel additive (STP website will explain what to use for what) every few tanks to clean the normal buildup from the fuel system. It’s much easier to clean small deposits than wait a few years and try to do one big cleaning later. I recommend reading the labels of a few fuel additives and making sure you get the right one for the job.