These carburetors have all of the performance features of Edelbrock`s performer carburetors, with an added twist: the Qwick Tune secondary air valve. This air valve can be easily adjusted to kick in the secondaries early or late, no extra springs are needed. These CNC-profiled, ball-burnished aluminum carbs feature black accented hardware and GM/AMC throttle linkage. Also included with every Edelbrock carburetor is an owner`s manual. These manuals consist of two sections: Section 1, theory of operation and tuning Procedures, will help you to develop your own individualized calibration. Section 2, tuning procedures, will take you through a step-by-step procedure that will enable you to achieve a desirable calibration. For added ease of tuning, a calibration reference chart for your model of carburetor will be included. For 4x4 owners, these thunder series AVS off-road carburetors include spring-loaded needles and seats for outstanding performance in rough, uneven terrain.