It’s not particularly common for an engine to smoke when running B-12 Chemtool Fuel Injector Cleaner (part #0116) at the recommended concentration of 1 fl oz/gal, However, it is certainly possible if there are some components, namely the carburetor, fuel injectors, or intake valves, that have a high amount of gum or varnish on them as those types of fuel residues do not burn especially well.