Berryman Chem-Dip Carburetor and Parts Cleaner (part #0996) is safe on for use on plastic, rubber, and most paints and metals, but it can react with some grades of aluminum and aluminum alloys. For that reason, we recommend on the container that soak times be limited to 15-30 minutes per dip with a total soak time not to exceed 4 hours for aluminum and aluminum-based parts. The darkening you see is caused by the surface formation of aluminum hydroxide and/or aluminum oxide, which are reaction products from extended soaking. Typically, the carburetor simply looks different but will still continue to function normally once it thoroughly dries.