Armor, Muriatic acid (used to clean concrete, ect.) will do it for you. Some stores have quantity limits & ID required for purchase. I've bought it at Home Depot. I would find a suitable pan/tray/bucket & pour a couple gallons it to submerge the intake. You have to scrub it with water once your done, so be ready to blow it off, warm it up & paint it before it rusts. Tape up the carb surface, plug the vacuum ports & water ports. fog some WD-40 up the ports & tape the intake surfaces, flip it upside down & lay an oil damped rag or two on the bottom side & cover it with a garbage bag or plastic sheet & get some paint on it quickly. Once the paint is dry, spray some more oil/WD-40 in it through a hole poked in the carb surface tape, then tape the hole shut. You can literly watch bare cast iron rust on a hummid day! Some places will turn Greenish/Black, but doesn't seem to cause a problem. Just be careful with spilling, touching ect...Wear rubber gloves & googles! Hope this helps, Lorne