 Such systems were originated by Carter Carburetor in the 1950s for the primary two venturis of their four barrel carburetors, and step-up rods were widely used on most 1-, 2-, and 4-barrel Carter carburetors through the end of production in the 1980s.The step-up rods are tapered at the bottom end, which extends into the main metering jets.The tops of the rods are connected to a vacuum piston or a mechanical linkage which lifts the rods out of the main jets when the throttle is opened (mechanical linkage) or when manifold vacuum drops (vacuum piston).