Quoted:
I have used Seafoam to clean the carbon buildup off internal engine components such as intake manifolds and the valves (high mileage vehicles). Just very slowly suck some in through an intake vacuum line while the engine is running. The amount at the moment I can't recall. Turn off the motor and let it sit for 1/2 hour and allow the Seafoam to soak in to all the deposits. Then start the motor and watch all the crap get burned off. Word of warning, do this away from anything you don't want to turn black. So don't do it next to the house or another vehicle. Also, it's going to put out a lot of white smoke from the exhaust.
The main method of cleaning when sucking seafoam through a vacuum line isn't solvency, but rather thermal shock. The cold liquid hits the hot carbon and causes it to quickly contract and break free from the surface.
Water was used for this for years! I've talked to all kinds of old people that would pour water into their carbs for this. I've also heard of brake fluid, but why I never understood.
Seafoam is just a light (pale) oil, lighter fluid, and rubbing alcohol. No scientifically advanced additives, super solvents, or cleaners in it.
I have thought about trying LC20 by www.lubecontrol.com before. I have seen it just melt away carbon when I tested its solvency a many years ago. Also it has a viscosity of about a 5w20 motor oil and a flash point above 325F (Seafoam's flash point is 55F). LC20 is only $38 a gallon too.