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  5. WCFB Accelerator Pump Circuit

Quick test to see if the accelerator pump is working.

WCFB Pump Circuit The accelerator pump circuit resides on the primary side of this carburetor.

This circuit provides a measured amount of fuel when needed on acceleration until the vacuum and main circuit can take over. This eliminates any bog or hesitation when acceleration.

When closing the throttle, the accelerator moves up in the pump well. The suction along with the fuel weight pulls fuel through the intake check via the passage to the fuel bowl. The discharge check is seated at this time to prevent air being drawn into the pump well.

As the throttle is opened the pump moves down the pump well forcing the intake check closed and forcing fuel out the main discharge.

Once engine speed gets high enough, the carburetor vacuum is strong enough to pull fuel through the main jet and the accelerator pump circuit is no longer used.

During high speed operation a vacuum exists at the pump jets. To prevent fuel from being drawn through the pump circuit, the passage to the pump jets is vented by a cross passage to the carburetor bowl above the fuel level. This allows air instead of fuel to be drawn off the pump jets.

Some models have a pump relief check in the bottom of the fuel bowl. This check prevents excessive pressure in the discharge passage during acceleration.

  • Seat the discharge check by tapping lightly in the hole to form a good seal.
  • Seat the intake check by tapping lightly in the hole to form a good seal.
  • Blow through the intake check passage to be sure the passage is open.
  • Be careful removing the intake check retainer. They are staked in the hole. Sometimes scraping around the edges with an awl will remove the staking.
  • Blow air through the main discharge to make sure the passage is clear all the way to the pump well.
  • Make sure the pump relief check is clean. It should not be opened up.

Carter WCFB Accelerator Pump Adjustment


WCFB Pump AdjustmentThe accelerator pump is used to provide an extra squirt of fuel when you step on the gas. The adjustment of the pump is important in order to get the required fuel.

  • Install the pump connector rod (D) in the outer hole, long stroke.
  • The ends of the connector rod should point to the throttle arm side.
  • Back off the idle screw so that the throttle plates close completely.
  • Put a straight edge across the dust cover as illustrated.
  • The flat on top of the pump arm should be parallel to the straight edge.
  • Adjust by bending the connector rod at the lower bend (E).
  • You can also check the adjustment by measuring (F) which should be 18/64"

Watch a video about how the accelerator pump works:


WCFB Carburetor


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