Are you using an Edelbrock Performer or Thunder series AVS carburetor? Then you need these Edelbrock metering jets! The jet and rod is changed to get the cruise air/ fuel ratio leaned out. The step in the rod changes the effective jet area between high and low manifold vacuum conditions. The brass metering jets are designed for use with Edelbrock Performer series and Thunder series AVS carburetors only. Do not use them with any other brands.