An intake manifold assembly includes an intake manifold having an inlet configured to receive an air/fuel mixture from a carburetor, an outlet configured to discharge the air/fuel mixture into a combustion chamber, an interior wall at least partially defining an intake passageway fluidly communicating the inlet and the outlet, and an aperture in the interior wall. The aperture is positioned between the inlet and the outlet. The intake manifold assembly also includes a first regulator received in the aperture and at least partially positioned in the intake passageway to effectively decrease the cross-sectional open area of the intake passageway. The first regulator is selectable from a plurality of regulators configured to each be at least partially positionable in the intake passageway. The first regulator is configured to be removed and replaced by a second regulator from the plurality of regulators without disassembly of the intake manifold from the carburetor.