Repair 7

How to Adjust the Mixture on a Double Pumper

Adjusting the air fuel mixture on a Holly double pumper is critical for smooth idling and for a smooth transition from idle to acceleration. If the mixture is too lean, the vehicle will hesitate during acceleration until the carburetors accelerator pumps can deliver fuel. In contrast, the engine will slightly flood from excessive fuel on acceleration if the idle mixture is set too rich. While the settings are critical, the task is easy.

Instructions

1

Connect a tachometer to the engine, following the tachometer and engine manufacturers instructions.

2

Screw the idle mixture screws clockwise with a small flat blade screwdriver, until they gently seat. Turn each needle valve 1 1/4-turn counterclockwise. The idle mixture screws are located in the front of the metering block on either side of the metering block. The metering block is located between the venturi body and the float bowl.

3

Slip the clear plastic line of a vacuum gauge onto the vacuum port on the carburetors throttle body. The throttle body is the plate at the bottom of the carburetor where the throttle linkage attaches.

4

Start the engine. Apply the parking brake. If the engine is equipped with an automatic transmission, put the transmission in drive.

5

Turn the idle mixture screws clockwise with the screwdriver until attaining the highest possible vacuum reading. Turn both screws equally.

6

Set the idle speed to the manufacturer specifications by moving the title speed screw counterclockwise or clockwise, depending on the need. Do this while viewing the reading on the tachometer until you attain the proper idle speed.

7

Put the vehicle in park and turn off the engine.