Start Your Engines! Fuel System Revitalizer is designed to ensure easy starting of 2 and 4 cycle motors, smooth rough idling and stalling, and helps remove gum and varnish from throttle plate and carburetor. Start Your Engines! should be used anytime your engine begins to run rough or becomes difficult to start. It is particularly useful for use after long term storage when engines can be reluctant to start. When used as directed, Start Your Engines! will not harm gasoline engines.