The Edelbrock 1801 9 Remanufactured Thunder Series AVS 500 CFM electric choke carburetor has been designed and calibrated to deliver optimum street performance. Several characteristics make the 18019 Remanufactured Thunder Series AVS 500 CFM electric choke carburetor an outstanding street performer. The first one is that metering rods are utilized to transition between circuits. The carburetor is unaffected by engine backfires, which means that there are no power valves to blow out, and the rods can be changed in seconds without carburetor removal or fuel draining. Secondly, the carburetor has the unique ability to hold a tune, so once it is tuned, the carburetor stays tuned. What this means to you is that compared to other carburetors, the performance remains consistent and the calibration stays unchanged. The third characteristic is the primary and secondary booster clusters that are utilized offer the most accurate and up-to-date calibration available for todays fuels and octane levels. Fourth the unique Thunder Series Qwik-Tune Secondary Air Valve allows for limitless calibration of the secondary circuit with simple hand tools while on your vehicle. (CALIBRATED FOR SINGLE CARB APPLICATIONS) Designed and calibrated for small cubic-inch engines such as 305 c.i.d and smaller Chevrolet, 302 c.i.d. and smaller Ford and dual-quad applications such as our new RPM Dual-Quad Air-Gap, Edelbrock C-26, F-28 and Street Tunnel Ram. Match with an Edelbrock Performer or Performer EPS manifold or other brands of similar design. No.1801/No.1802 are calibrated for single-quad applications and factory equipped with: Metering Jets - Primary .086, Secondary .095, Primary Rod .065 x .052, orange spring (5 Hg). No.1803/No.1804 is calibrated for dual-quad applications and factory equipped with: Metering Jets - Primary .086, Secondary .077, Metering Rods - .065 x .057, No.1804 includes Primary .086, Secondary .077, Metering Rods - .065 x .052, Step-Up Spring - orange (5 Hg).