The popular Holley 650 Double Pumper carburetor has a variety of adjustments. In addition to setting the idle speed and the idle mixture, the carb's accelerator pump can be adjusted. Proper adjustment of the float level is also critical. Proper adjustments on the Holley 650 Double Pumper are vital to maintaining drivability.
Start the engine and move the idle speed screw on the carburetor linkage in or out until the correct idle speed is achieved.
Attach a tachometer to the engine and turn the idle speed screw on the carburetor baseplate on the driver's side (U.S.) of the car. Adjust until the idle rpm is within the car manufacturer's recommendations.
Connect a vacuum gauge to the vacuum port at the base of the carburetor. With the engine running, adjust the idle mixture screws on either side of the metering block until the highest vacuum is achieved. After adjusting the idle mixture, you'll have to reset the idle speed.
Park the vehicle on a level surface. Remove the sight plug, located near the fuel inlet on the side of the fuel bowl. Loosen the lock nut surrounding the adjusting screw on the top of the fuel bowl. With the engine running, adjust the float level by turning the adjusting screw on the top of the fuel bowl. When properly adjusted, fuel will just trickle out of the sight plug. Tighten the lock nut and replace the sight plug. Each fuel bowl has a float that must be adjusted.
Adjust the accelerator pump override spring. This is on the end of the accelerator pump operating lever. Tighten or loosen the screw to compress or relax the spring, until even the slightest movement of the throttle activates the accelerator pump.
