A fuel metering apparatus, for supplying metered fuel to an associated combustion engine, has an idle fuel metering and supply system comprising adjustable valving means which, in turn, comprises a discharge orifice means and cooperating valving member which is adjustably selectively positioned with respect to such orifice means in order to thereby establish a desired metered rate of fuel flow through the thusly determined effective flow area of the orifice means for a particular pressure differential thereacross; the adjustable valving member is generally contained within a cooperating body member or portion and access to the valving member, subsequent to it being selectively positioned, is prevented by closure members placed in position after the said selective positioning of said valving member.