Carter WCD Technical

The accelerator pump circuit in the Carter WCD is a mechanically operated system designed to provide an immediate, metered squirt of fuel into the venturis. This compensates for the sudden rush of air when the throttle is opened quickly, preventing a lean stumble before the main metering system has time to catch up.

Here is how the circuit operates, broken down by its key phases:

The Intake Phase (Throttle Closing)

When you let off the gas and the throttle plates close, the mechanical linkage lifts the pump plunger upward inside its cylinder, which is submerged in the float bowl.

• Creating a Vacuum: The upward movement of the plunger creates a low-pressure area (suction) inside the pump cylinder.

• Intake Check Valve: This suction lifts the intake check valve (typically a small steel ball) off its seat at the bottom of the cylinder, pulling liquid fuel from the float bowl into the pump well.

• Discharge Check Valve: Simultaneously, the discharge check valve (a ball usually held down by a small brass weight) seats firmly. This prevents air from being sucked backward through the pump discharge jets into the cylinder.

The Discharge Phase (Throttle Opening)

When you step on the gas, the throttle linkage pushes the plunger assembly downward into the fuel-filled cylinder.

• Sealing the Intake: The downward pressure of the fuel immediately forces the intake check ball back onto its seat, trapping the fuel in the cylinder so it can't escape back into the float bowl.

• The Plunger Assembly: The linkage doesn't force the plunger down rigidly. Instead, it compresses a duration spring on the plunger shaft. This spring pushes the plunger down smoothly, providing a prolonged, steady stream of fuel rather than a single, violent squirt.

• The Leather Cup: The WCD uses a leather pump cup to seal against the walls of the pump cylinder. Note that this leather pump cup does not use an expander spring behind its lip to maintain a seal; the leather is shaped and conditioned to maintain the seal dynamically against the cylinder wall under pressure.

• Fuel Delivery: The pressurized fuel lifts the discharge check valve off its seat and travels up through the internal passages. It exits through the pump discharge jets (nozzles) located in the air horn, spraying a targeted stream of fuel directly into the airflow of each barrel.

Common Troubleshooting Areas

If the WCD is experiencing a hesitation off idle, the pump circuit is usually the culprit. Key areas to check during a rebuild include:

• Dried Leather Cup: If the leather cup dries out or shrinks, fuel will bypass it during the downward stroke instead of being forced out of the jets.

• Stuck Check Balls: Old fuel or debris can cause the intake or discharge check balls to stick to their seats. Tapping them lightly with a brass drift during a rebuild helps reshape the seats and ensure a tight seal.

• Clogged Passages: The cross-drilled passages leading to the pump jets are very narrow and easily clogged by varnished fuel.

• Back out throttle lever set screw to allow throttle valves to seat in the bores of the carburetor and loosen metering rod arm clamp screw (F).
• With metering rods in place, press down on vacumeter link (G) until metering rods bottom in carburetor body casting (H).
• Holding rods in downward position and throttle valves seated revolve metering rod arm until finger on arm contacts lip of vacumeter link (J).
• Hold in place and carefully tighten clamp screw (F)