1990 Cadillac Deville Carbon build-up

The valves are not adjustable, they are fixed n a rocker support shaft. Do you think carbon is causing your miss? 90 was a bad year for fuel injectors, have those been checked. A resistance test of warm injectors should read 13-15 ohms. A lot less and the injector is shorted. To answer your question, top engine cleaner through a vacuum port, on a warm engine until it stalls, and let it soak for several hours. That is about the best you can do aside from engine disassembly, to remove carbon. Just be careful when introducing the cleaner, not to induce it at such high rate to potentially cause hydrolock.