How to Tune a 750 Double Pumper Holley Carburetorby Don Bowman
A 750 double pumper Holley carburetor is a high performance carburetor that incorporates two accelerator pumps that are manually operated. Used for high performance or larger displacement engines, they are infinitely tunable. The mechanically operated, double pumper carburetors seldom come with an electric choke. The choke is cable-operated from within the vehicle.
Remove the air cleaner. Start the engine and allow it to warm up to operating temperature. Shut the engine off. Remove the passenger side fuel sight plugs on the float bowls using the large common screwdriver.
Adjust the fuel level by setting the float first. The ideal fuel level is when the fuel can be seen at the bottom of the sight hole. Start the engine for a minute and shut it off again. To set the floats on a Holley, use the 5/8-inch wrench to hold the nut on the top of each float bowl, and loosen the lock screw on top of the bolt. As the nut is turned clockwise, it is turning the needle and seat down, raising the fuel level. Turning the nut counter-clockwise lowers the fuel level. Once correct, tighten the top lock screw. Pull the plug, check the fuel level again and adjust the floats until correct.
Adjust the accelerator pumps. Notice that at the bottom of each float bowl there is an accelerator pump and arm. The throttle linkage uses a spring loaded bolt with the head of the bolt resting on the arm of the pump. Use the 3/8-inch wrench to adjust the bolt head where it meets the pump rod so that the accelerator pump begins to move when the throttle linkage travels 1/4-inch.
Start the vehicle to set the mixture. Attach the tachometer to the negative terminal of the coil and a good ground. Turn the driver's side front mixture adjusting screw in until the engine RPM drops. Turn it out until it reaches maximum RPM and does not rise any farther. Turn the screw back in until the RPM just starts to drop and stop there. Repeat the process for all four of the mixture screws.
Adjust the idle with the idle screw located on the throttle linkage. Turning the idle screw clockwise raises the idle. Turning the idle screw counter-clockwise lowers the idle.
Items you will need
- Small screwdriver
- 5/8-inch wrench
- 3/8-inch wrench
- 7/16-inch wrench
- Large screwdriver
- Remote tachometer
- Don Bowman, A.S.E. Certified Mechanic