MGB Fuel Pump: Symptoms, Testing, and Replacement Guide

If your classic MGB is sputtering, losing power, struggling to start, or simply refusing to run, a failing fuel pump is a prime suspect. The fuel pump is the heart of your MGB's fuel delivery system, responsible for drawing gasoline from the tank and delivering it under consistent pressure to the carburetors. Understanding its function, recognizing signs of trouble, knowing how to test it, and being prepared to replace it are essential skills for any MGB owner. This guide provides a comprehensive look at the MGB fuel pump, covering both the original mechanical type and common electric replacements, empowering you to diagnose and resolve fuel delivery issues effectively.

The Critical Role of the Fuel Pump in Your MGB

Without a functioning fuel pump, your MGB's engine starves. Its sole purpose is to create a steady flow of fuel from the tank, located at the rear of the car, up to the engine bay where the carburetors mix it with air for combustion. The pump must generate sufficient pressure to overcome gravity, fuel line resistance, and the suction created by the carburetors, ensuring the float chambers remain adequately filled under all operating conditions – idle, acceleration, cruising, and climbing hills. A weak or failing pump disrupts this delicate balance, leading directly to engine performance problems. For MGBs, this component is especially critical due to the distance fuel must travel and the design of the carburetion systems.

Understanding MGB Fuel Pump Types: Mechanical and Electric

MGBs left the factory equipped with a mechanical fuel pump. This type is bolted directly to the engine block, typically on the right-hand side (passenger side for LHD cars), driven by an eccentric lobe on the engine's camshaft. As the camshaft rotates, it actuates a lever arm inside the pump, creating a reciprocating motion that draws fuel in from the tank on the intake stroke and pushes it towards the carburetors on the discharge stroke. These pumps are generally reliable but are subject to wear of internal diaphragms, valves, and the lever arm pivot over time and miles. Their output pressure is directly linked to engine speed – slower at idle, faster at higher RPMs.

Many owners opt to replace a failing mechanical pump with an electric fuel pump. Electric pumps offer several potential advantages: they can provide more consistent pressure, are often easier to install in accessible locations (like near the fuel tank), can help with vapor lock issues, and prime the carburetors instantly when you turn the key (before cranking). Common choices include low-pressure (3-5 PSI) rotary vane or diaphragm pumps specifically designed for carbureted engines like the SU or Zenith Stromberg carbs found on MGBs. Crucially, using a pump with excessive pressure can overwhelm the carburetor needle valves, causing flooding and potential engine damage; correct pressure specification is paramount.

Common Symptoms of a Failing MGB Fuel Pump

Recognizing the signs of a failing fuel pump early can prevent roadside breakdowns and more extensive troubleshooting. Here are the most frequent symptoms:

1. Engine Sputtering or Stalling at Speed: A classic sign of fuel starvation. The engine may run fine at idle or low load but starts to miss, sputter, hesitate, or even cut out completely when you demand more power (accelerating, climbing hills, sustained highway speeds). This happens because the pump cannot deliver enough fuel volume to meet the engine's increased demand.
2. Difficulty Starting (Especially When Hot): While hard starting can have many causes, a weak fuel pump is a common culprit, particularly when the engine is hot. Heat can exacerbate vapor lock or reduce the efficiency of a worn pump. You might crank the engine for extended periods before it fires, as the pump slowly refills the carburetor float chambers after the fuel has drained back or evaporated.
3. Loss of Power and Hesitation: A general lack of power, sluggish acceleration, or a noticeable hesitation when pressing the throttle often points towards insufficient fuel delivery. The engine feels like it's being held back.
4. Engine Dies at Idle or Low RPM: If the pump is weak, it might not maintain enough pressure to keep the carburetor floats adequately filled when the engine is idling or running at very low RPMs, causing the engine to stall unexpectedly.
5. Unusual Noise from the Pump Area: Mechanical pumps can develop audible clicks, ticks, or rattles if internal parts are worn or damaged. Electric pumps might whine excessively or buzz erratically if they are failing or not receiving proper voltage.
6. Fuel Leaks: Visible fuel dripping from the pump body (mechanical) or its connections is a clear sign of failure, often due to a ruptured diaphragm in a mechanical pump or a cracked housing/seal. This is a serious fire hazard and requires immediate attention.

How to Test Your MGB's Fuel Pump

Before condemning the pump, it's wise to perform some basic tests to confirm it's the culprit. Always work in a well-ventilated area, away from sparks or open flames, and have a fire extinguisher rated for gasoline fires (Class B) nearby. Avoid skin contact with fuel.

• Visual Inspection:

  • Check for any obvious fuel leaks around the pump body and fuel line connections. Tighten connections if loose, but replace components if leaks persist.
  • Inspect fuel lines (from tank to pump and pump to carbs) for cracks, brittleness, or damage. Replace as necessary.
  • For electric pumps, check wiring connections for corrosion or looseness.

• Flow Test (Mechanical or Electric): This is the most direct test of pump function.

  1. Disconnect the fuel line outlet from the pump (the line going to the carburetors). Place the end of this line into a clean, clear container capable of holding at least a pint of fuel. Ensure the container is placed safely away from hot engine parts or electrical components.
  2. Have an assistant crank the engine (for mechanical pumps) or briefly turn the ignition key to the "ON" position (for electric pumps – only if the pump is wired to run with ignition on). Do not crank excessively.
  3. Observe the fuel flow. A healthy pump should deliver a strong, steady, pulsing stream of fuel into the container. A weak, intermittent, or dribbling flow indicates a failing pump. Compare the volume collected over 10-15 seconds of cranking or pump operation to specifications if available (often around 1 pint per minute or more is considered healthy for these engines).
  4. Important: After testing, reconnect the fuel line securely before attempting to start the engine.

• Pressure Test: While flow is crucial, pressure is also important, especially for electric pumps. You'll need a low-pressure fuel pressure gauge (0-15 PSI range).

  1. Install the gauge in line between the fuel pump outlet and the carburetor inlet line. Some test kits have T-fittings for this purpose. Alternatively, temporarily disconnect the carb fuel line and connect the gauge directly to the pump outlet.
  2. For mechanical pumps: Start the engine and let it idle. Observe the pressure reading. It should be relatively stable and typically within the range of 2.5 - 4.5 PSI for SU carbs, or check your specific carburetor manual. Rev the engine; pressure should increase slightly but remain within spec. A reading significantly below spec or one that drops rapidly when the engine is revved indicates a weak pump.
  3. For electric pumps: Turn the ignition key to the "ON" position (without cranking) and observe the pressure. It should rise quickly to the specified pressure (usually 3-5 PSI for MGB carb applications) and hold steady. If it struggles to reach pressure, fluctuates wildly, or bleeds down quickly, the pump is likely faulty. Never exceed the carburetor manufacturer's maximum pressure recommendation.
  4. Important: Remove the gauge and reconnect the fuel line securely to the carburetor(s) after testing.

• Check Valve Test (Mechanical Pumps - Optional): A common failure point in mechanical pumps is the internal check valves, leading to fuel drain-back and hard hot starting.

  1. Disconnect the fuel line inlet from the pump (the line coming from the tank).
  2. Place a finger firmly over the pump's inlet port.
  3. Have an assistant crank the engine briefly. You should feel strong suction against your finger. If suction is weak or non-existent, the inlet check valve may be faulty. This test is less definitive than flow/pressure tests but can offer clues.

Replacing the MGB Fuel Pump: Step-by-Step Guide

Replacement is generally straightforward. Decide beforehand whether you're replacing with another mechanical pump or switching to an electric one.

Replacing a Mechanical Fuel Pump:

1. Preparation: Disconnect the negative terminal of the battery. Relieve any residual fuel pressure by carefully loosening the fuel line connections at the carburetors and catching any drips with a rag. Place rags under the pump area.
2. Disconnect Fuel Lines: Carefully disconnect both the inlet (from tank) and outlet (to carbs) fuel lines from the pump. Plug or cap the lines temporarily to minimize fuel spillage and prevent dirt ingress. Note their orientation.
3. Remove Mounting Bolts: Remove the two bolts securing the mechanical pump to the engine block.
4. Remove Old Pump: Gently pry or wiggle the old pump away from the block. Be mindful of the actuating lever arm inside the pump that rests against the camshaft eccentric. Remove the old gasket material from the engine block mounting surface.
5. Prepare New Pump: Compare the new pump to the old one. Ensure the actuating lever arm moves freely. Some pumps may require priming – follow the manufacturer's instructions. A light coating of oil on the lever arm tip can aid initial lubrication. Fit a new gasket (dry or with a very thin smear of gasket sealant if recommended).
6. Install New Pump: Carefully position the new pump against the block, ensuring the actuating lever arm correctly engages with the camshaft eccentric. This might require gently rotating the engine slightly (via the crankshaft pulley bolt) if the eccentric lobe is at its peak. Hand-start the mounting bolts.
7. Tighten Bolts: Tighten the mounting bolts evenly and securely to the manufacturer's specified torque (if available), usually around 15-20 ft-lbs. Avoid overtightening.
8. Reconnect Fuel Lines: Reconnect the inlet and outlet fuel lines securely to the correct ports on the new pump. Use new copper washers if applicable.
9. Prime and Check for Leaks: Reconnect the battery. Before starting, crank the engine for a few seconds (without starting) to allow the pump to fill the carburetor float chambers. Visually inspect all connections for leaks. If leaks are found, shut off immediately and rectify.
10. Start Engine: Start the engine and check for smooth operation and the absence of leaks. Verify that the engine idles and accelerates correctly.

Installing an Electric Fuel Pump (Replacing Mechanical):

1. Choose Location: Select a suitable mounting location. Common spots are near the fuel tank (under the rear of the car, protected within the chassis rails if possible) or low down in the engine bay away from excessive heat. The pump should be mounted lower than the bottom of the fuel tank and as close to the tank as practical. Mount it securely using the provided bracket or clamps, ensuring it's vibration-resistant. Crucially, the pump must be mounted lower than the tank outlet to ensure gravity can feed fuel to the pump inlet.
2. Remove Mechanical Pump (Optional but Recommended): While you can leave the mechanical pump in place (effectively acting as a passive junction block), it's generally better practice to remove it to eliminate a potential failure point and reduce complexity. Follow steps 1-4 above for mechanical pump removal. You will need a blanking plate kit specifically designed for your MGB engine block to cover the mounting hole where the mechanical pump was. Install this plate with a new gasket.
3. Install Fuel Filter: It's highly recommended to install a new, high-quality inline fuel filter before the electric pump (between the tank and pump) to protect the pump from debris in the tank. Install another filter after the pump (before the carbs) for maximum protection. Ensure filters are installed in the correct flow direction.
4. Plumb the Fuel Lines: Run new fuel lines or adapt existing ones:
   • Tank Outlet -> Pre-Pump Filter -> Electric Pump -> Post-Pump Filter -> Carburetor Inlet.
   • Use appropriate fuel-rated hose (SAE 30R7 or 30R9) and secure all connections with proper fuel hose clamps (screw type preferred over spring clamps).
   • Ensure lines are routed safely, away from hot exhaust components, sharp edges, or moving parts. Secure with P-clips or cable ties.
5. Wiring the Pump:
   • Safety is Paramount: The pump must be wired to shut off automatically if the engine stops running. The safest and most common method is to wire it through an inertia safety switch (also known as a fuel pump cutoff switch or impact switch). This switch cuts power to the pump in the event of a collision. Mount this switch securely, typically on a vertical surface inside the cabin (e.g., front bulkhead) or in the trunk.
   • Power Source: Connect the pump's positive (+) wire: Battery Positive -> Fuse Holder (install an appropriate fuse, e.g., 10-15A, close to the battery) -> Inertia Switch -> Electric Fuel Pump Positive Terminal.
   • Ground: Connect the pump's negative (-) wire to a clean, bare metal point on the chassis near the pump. Ensure a solid ground connection.
   • Ignition Control (Optional but Recommended): For convenience, you can wire the circuit so the pump only runs when the ignition is on. This usually involves connecting the wire after the inertia switch to a switched +12V source in the fuse box (e.g., the ignition coil + terminal, only if it's switched with ignition). Alternatively, use an oil pressure safety switch wired in series, though this is less common for carbureted retrofits than inertia switches. Consult a wiring diagram specific to your chosen pump and safety setup.
6. Prime and Check for Leaks: Before reconnecting the battery, double-check all wiring and plumbing. Reconnect the battery negative terminal. Turn the ignition key to the "ON" position (do not crank). You should hear the electric pump run for a few seconds (if wired through ignition) or continuously. Listen for its normal operating sound and visually inspect every fuel connection for leaks. Fix any leaks immediately.
7. Start Engine: Start the engine. The pump should run continuously while the engine is running. Verify smooth operation, correct idle, and acceleration. Listen for any abnormal pump noise.

Preventative Maintenance for Your MGB Fuel Pump

Extend the life of your fuel pump and avoid problems with these practices:

1. Use Clean, Fresh Fuel: Stale fuel or fuel contaminated with water or debris accelerates wear and can cause internal corrosion or valve sticking. Avoid letting the car sit for extended periods with old fuel. Consider a fuel stabilizer for storage.
2. Replace Fuel Filters Regularly: A clogged filter forces the pump to work harder, potentially leading to premature failure. Replace the pre-pump and post-pump filters according to the manufacturer's schedule or annually. Inspect filters whenever fuel issues arise.
3. Keep the Fuel Tank Clean: Sediment and rust in the tank are the enemies of fuel pumps and carburetors. If your tank is old or shows signs of internal corrosion, consider professional cleaning or replacement. Installing an in-tank strainer (if accessible) provides an extra layer of protection.
4. Avoid Running Extremely Low on Fuel: Consistently running the tank very low increases the risk of sucking debris from the bottom of the tank into the pump and filters. It can also cause the pump to overheat (especially electric pumps, which rely on fuel for cooling).
5. Inspect Periodically: During routine maintenance, visually inspect the pump and surrounding fuel lines for any signs of leaks, damage, or corrosion. Listen for any unusual noises from the pump while the engine is running.

Choosing the Right Replacement Fuel Pump for Your MGB

Selecting the correct replacement is vital:

• Mechanical Replacement: Ensure you get a pump specifically listed for your MGB model year and engine type (e.g., 18V, 18GG). Reputable brands like SU (now owned by Burlen Fuel Systems), AC Delco, or quality aftermarket suppliers are recommended. Avoid the cheapest options, as quality varies significantly.
• Electric Replacement: Pressure is Key! Choose a pump rated for carbureted applications, delivering no more than 3-5 PSI (confirm the exact requirement for your specific carburetors – SU carbs typically need 2.5-4.5 PSI). Popular reliable options include the SU AUF300/AUF302, Facet Posi-Flow (e.g., 40104), or Huco low-pressure pumps. Ensure the pump's flow rate is adequate (generally 15-30 gallons per hour is sufficient for MGBs). Consider noise levels if mounting inside the cabin is planned (though rear mounting is preferred). Purchase a complete kit designed for MGBs if available, as it often includes the pump, filters, wiring, and sometimes an inertia switch.

Conclusion: Keeping the Fuel Flowing

A healthy fuel pump is non-negotiable for reliable MGB operation. By understanding the different types (mechanical vs. electric), recognizing the tell-tale symptoms of failure (sputtering, power loss, hard starting), knowing how to perform basic flow and pressure tests, and being prepared to tackle replacement using the correct parts and procedures, you can confidently address this common issue. Prioritize safety throughout any fuel system work, use quality components, and adhere to correct pressure specifications, especially when fitting an electric pump. Regular preventative maintenance, particularly fuel filter changes, will significantly extend the life of your MGB's fuel pump, ensuring many more miles of enjoyable classic motoring. When problems arise, methodical diagnosis starting with the fuel pump can often lead you quickly to the solution.