When I went through Mechanic night classes in the late 1960 and 1970s, several of the instructors suggested putting a very small amount ( a couple of table spoons) of transmission fluid in the gas tank. It is supposed to clean off the valves by loosening up built up carbon. Don't over do it though. It might not be a good idea to put in a car with fuel injection. It could clog the injectors.I have tried it in my 72 Z , but can't really say if it helped or not. It hasn't stopped my car from passing smog inspection. The best way of cleaning valves is using a good grade oil and gas with clean filters and running at sustained 60 plus speeds for about an hour.