Holley 0-4776C Model 4150 Double Pumper 600 CFM Square Bore 4-Barrel Mechanical Secondary Manual Choke New Carburetor Mechanical secondary dual accelerator pump for additional fuel under initial acceleration Dual feed fuel inlet with mechanical secondary type Dichromate finish with center hung floats. Primary Power Valve-6.5. Primary Pump Nozzle Size-28. Secondary Pump Nozzle Size-31 Manual choke for easy control of engine warm up 100-percent wet-flow tested and calibrated for street drivability