Abstract: A tool for modifying openings in carburetor jets includes a base, upwardly extending guide rods, a relatively heavy tool-carrying block slidable on the rods, and a plate for holding the jet or jets to be modified. The plate has locating openings and the base has a locating pin, the locating openings being spaced so that when the proper one is placed on the pin the selected jet is accurately located beneath a tool carried by the block. The block, with the proper tool installed therein, is then lifted and allowed to drop so that the tool reduces the jet orifice size. The tools have rounded ends and diameters to properly modify various jets.