Some fuel additives claim to clean the valves, but it's usually done by breaking down the engine and using an industrial solvent to break up the carbon deposits. I've also seen wire wheels used.
You'll need to crawl under neath the cab of your car and locate the poppet valve, using a ratchet set, remove the pipe and clean it with a pipe cleaner-and-grease.
Clean valve through a brake cleaner or heavy duty degreaser. If got it off of vehicle soak it or spray it. Crawl underneath cab of car to locate poppet valve. Use a ratchet set and remove pipe tp clean with agents I metioned above.