Here is a new one to me. The instructions to my OS FS A .56 state, "Do not close the needle valve and mixture control valve too far as this will cause a lean setting and overheating of the engine. This can, in turn, create nitromethane oxide leading to internal rusting of the engine." So, that being said, does it mean if ya run your four cycle engine on the rich side, no rusting will occur? interesting.