a fuel system for use with an internal combustion engine includes primary and secondary carburetors and vaporizing apparatus . when starting the engine , liquid fuel is supplied by a fuel pump to the primary carburetor via a first switchable valve means . after the engine has warmed sufficiently , liquid fuel is prevented from reaching the primary carburetor , and instead is supplied to the vaporizing apparatus via second switchable valve means . vaporized fuel is supplied to the primary carburetor from the vaporizing apparatus via the secondary carburetor .