BBD Accelerator Pump Circuit

Understanding the Carter BBD Accelerator Pump Circuit: Function, Troubleshooting, and Maintenance

The Carter BBD 2-barrel carburetor is renowned for its reliability in classic vehicles. A crucial component of its performance is the accelerator pump circuit, which ensures smooth acceleration by delivering an extra fuel shot when the throttle is engaged.

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How the Carter BBD Accelerator Pump Works

When the throttle is opened, the accelerator pump plunger descends, pushing fuel through the discharge check valve into the carburetor's venturi. This immediate fuel delivery prevents hesitation or stalling during acceleration. As the throttle closes, the plunger ascends, drawing fuel into the pump chamber through the intake check valve, preparing for the next acceleration.

Key components include:

• Pump Plunger: Drives fuel into the venturi upon throttle engagement.

• Intake Check Valve: Allows fuel to enter the pump chamber during plunger ascent.

• Discharge Check Valve: Prevents backflow and ensures fuel is directed into the venturi.

• Air Bleed: Prevents fuel dribble when the pump is inactive by relieving pressure.

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Common Accelerator Pump Issues and Solutions

1. Hesitation or Stumble During Acceleration

Symptoms: Engine hesitates or stumbles when the throttle is quickly engaged.

Potential Causes:

• Dried or Damaged Pump Cup: Over time, the pump cup can harden or crack, reducing its sealing ability.

• Clogged Passages: Debris or varnish can obstruct fuel flow within the accelerator pump circuit.

• Worn Check Valves: Faulty intake or discharge check valves can impede proper fuel delivery.

Solutions:

• Inspect and replace the pump cup if it's hardened or damaged.

• Clean all passages with carburetor cleaner and compressed air to remove obstructions.

• Examine check valves for wear or debris and replace if necessary.

2. Fuel Dribble at Idle

Symptoms: Fuel leaks into the venturi when the engine is idling, leading to a rich mixture.

Potential Causes:

• Faulty Discharge Check Valve: Allows fuel to leak into the venturi when not under pressure.

• Missing or Damaged Air Bleed: Fails to relieve pressure, causing fuel to dribble.

Solutions:

• Replace the discharge check valve to ensure it seals properly.

• Ensure the air bleed is present and unobstructed to prevent pressure buildup.carburetor-parts.com

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Maintenance Tips for Optimal Performance

• Regular Inspection: Periodically check the accelerator pump components for wear or damage.

• Use Quality Fuel: Ethanol-blended fuels can degrade rubber components; consider using ethanol-free fuel when possible.

• Proper Storage: If the vehicle is stored for extended periods, run the engine periodically to keep components lubricated and prevent drying out.

The lag can be caused by the distributor advance not working correctly.

A lean condition could cause a lag including popping through the carburetor.

A rich condition could cause a lag.

With those conditions out of the way lets see about the accelerator pump circuit.

Has the vehicle been sitting for a long time? This might cause the check balls, inlet and outlet to get varnished up causing them to stick. The passage itself could be varnished and plugged.