The Complete Guide to MGB Fuel Pumps: Symptoms, Diagnosis, Replacement & Maintenance

Your MGB's fuel pump is critical to its operation, and failure, while common in these classics, is often straightforward to diagnose and replace. The mechanical fuel pump on MGB models (excluding later US V8 models with electric pumps) is a relatively simple device mounted on the engine block. Its sole job is to reliably draw fuel from the tank and deliver it at low pressure to the twin SU carburetors. However, age, internal diaphragm wear, leaking valves, contaminated fuel, or physical damage can lead to frustrating symptoms or complete failure. Understanding how your MGB fuel pump works, recognizing the signs of trouble, knowing how to test it, and mastering replacement and maintenance procedures are essential skills for any MGB owner to ensure smooth running and prevent being stranded. This comprehensive guide covers everything you need to know about your MG’s vital fuel delivery component.

Understanding the MGB Mechanical Fuel Pump System

The vast majority of MGB roadsters and GTs, from the earliest 1962 models through the rubber-bumper era ending in 1980, utilized a mechanical fuel pump driven directly by the engine. Here's the breakdown:

  1. The Pump Itself: Mounted low on the engine block, typically near the oil filter housing. It's powered by an eccentric cam on the engine's camshaft. As the camshaft rotates, a lever arm inside the pump is pushed, creating the reciprocating motion needed to pump fuel.
  2. How It Works: This lever arm action operates a flexible diaphragm inside the pump housing.
    • Intake Stroke: As the diaphragm is pulled back, it creates suction. This suction opens an inlet valve, drawing fuel from the fuel tank through the inlet pipe and filter sock (often located in the tank or inline).
    • Delivery Stroke: The diaphragm is then pushed forward by the cam action. This closes the inlet valve, opens an outlet valve, and pushes fuel out towards the carburetors. A small chamber inside the pump and the flexible diaphragm help smooth the pulsing flow into a more consistent stream.
  3. Low Pressure: Unlike modern high-pressure fuel injection systems, the SU carburetors on the MGB require only a low, steady fuel pressure – typically in the range of 2.5 to 4 psi. The mechanical pump is perfectly designed to deliver this.
  4. Self-Regulating: The mechanical design inherently limits the maximum pressure generated. The pump only pushes what the carburetors are using. There is no external pressure regulator needed.
  5. Simple Design: Key components include the pump body, diaphragm, inlet and outlet valves (usually small discs or flaps of material), the lever arm, return spring, top cover, and associated gaskets. Its simplicity is its strength... until wear sets in.

Common Symptoms of a Failing or Faulty MGB Fuel Pump

When the fuel pump starts to deteriorate or fails, it manifests in ways that can often mimic other fuel system issues or ignition problems. Here are the most frequent warning signs:

  1. Engine Cranks But Won't Start (No Fuel Delivery): The most obvious sign. If the pump diaphragm ruptures significantly, valves stick shut, or the lever arm breaks, the pump cannot deliver fuel to the carbs. Check for fuel in the carburetor float chambers (carefully remove the domes to inspect).
  2. Engine Starts But Stalls Under Load or at Speed: A partially failing pump might deliver enough fuel for idle but cannot keep up with the engine's demand during acceleration or higher RPM driving. The engine feels hesitant, lacks power, sputters, and may cut out. This symptom is often gradual.
  3. Hard Starting After Sitting: If the pump cannot build or maintain adequate pressure when the engine is off (like overnight), fuel can drain back to the tank. This results in extended cranking before the pump manages to refill the carburetor float chambers.
  4. Engine Surging at Steady Speeds: Insufficient fuel delivery can cause the engine to momentarily "starve," causing a surge or stumble sensation while cruising.
  5. Visible Fuel Leaks: While less common than internal failures, external leaks can occur. Check for wetness or fuel stains around the pump body, especially near the seam between the top and bottom halves, at the pipe connections, or around the gasket where it mounts to the engine block. WARNING: Fuel leaks are a significant fire hazard. Address immediately!
  6. Fuel in the Engine Oil: A critical failure mode! If the diaphragm develops a pinhole leak or tears, fuel can be pumped into the crankcase past the diaphragm shaft, contaminating the engine oil. This dilutes the oil, drastically reducing lubrication and risking severe engine damage. Check your dipstick – if the oil level is inexplicably high and smells strongly of gasoline, suspect a ruptured pump diaphragm. STOP ENGINE. DO NOT DRIVE.
  7. Excessive Engine Vapor Lock (Hot Starting): While vapor lock is often caused by heat boiling fuel in the lines, a weak pump struggling against low pressure in the line makes vapor lock more likely to occur, especially in hot weather or after shutting off a hot engine.

Diagnosing MGB Fuel Pump Problems: Step-by-Step Testing

Before condemning the fuel pump, it's wise to perform some basic diagnostics to confirm it's the culprit.

  1. Visual Inspection:
    • Look for external fuel leaks around the pump body, fittings, and mounting flange.
    • Check for damage to the pump body or inlet/outlet pipes.
    • Verify all pipe connections are tight and the banjo bolts securing fuel lines to the pump have their copper washers in good condition.
  2. Check Operation - "Sight Glass" Test (Early Pumps): Many original SU fuel pumps incorporated a glass "sight dome" on top. With the engine idling (or cranking if it won't start), observe the dome.
    • Good Pump: You should see a regular, strong pulsing movement of fuel inside the dome.
    • Weak/Failing Pump: The pulsing might be very weak, erratic, or non-existent.
    • Important: Later pump revisions (common replacements) may not have this glass dome.
  3. Check Operation - Flow Test: If no sight dome or to verify, disconnect the fuel line from the pump outlet to the carburetors. Place the end of this line into a suitable container. Have an assistant crank the engine (ignition coil disconnected to prevent starting – disconnect coil wire or use a dedicated starter switch lead). Observe fuel flow:
    • Good Pump: Fuel should pulse out in strong, distinct squirts corresponding to cranking speed. A steady stream is less common and can indicate an issue on SU pumps designed to pulse.
    • Weak Pump: Fuel pulses out weakly, or flow is inconsistent/stops and starts.
    • No Flow: No fuel emerges during cranking. Confirm the tank has fuel! Also, check for a blocked inlet filter or fuel line.
  4. Check Operation - Pressure Test: The most definitive test. Obtain a low-pressure fuel pump tester (0-15 psi range).
    • Disconnect the fuel line from the pump outlet to the carbs.
    • Connect the fuel pump tester inline between the pump outlet and a spare piece of fuel hose leading to your container.
    • Start the engine and let it idle. Observe the gauge.
    • Good Pump: Should produce a pulsing pressure reading between approximately 2.5 psi and 4.0 psi for MGB SU carburetors. The reading will oscillate slightly as the pump pulses.
    • Weak Pump: Pressure consistently below 2 psi, especially under simulated load (increase engine RPM to ~2000 rpm – pressure should stay relatively constant or only drop slightly).
    • Excessive Pressure: Highly unusual on a stock mechanical pump but could indicate debris jamming a valve open or a failed internal component. Pressure significantly over 4 psi could overwhelm carburetor needle valves, causing flooding.
    • Zero Pressure: No pressure reading confirms pump failure. Test again at cranking speed if engine won't start.
  5. Check Fuel Volume: While connected to the pressure tester or with the outlet pipe into a container, run the engine at idle and measure how much fuel is pumped in 30 seconds.
    • Good Pump: Should deliver well over 1 pint (approximately 0.47 liters) in 30 seconds at idle – consult a workshop manual for precise spec, but flow should be strong and obvious. Reduced volume indicates wear.
  6. Check for Diaphragm Leak (Oil Contamination): As mentioned, a critical test. Remove the engine oil dipstick and wipe it clean. Reinsert fully and remove again.
    • Good Pump: Oil level should be normal, and oil should not smell strongly of gasoline.
    • Failed Diaphragm: Oil level may be abnormally high. The oil will have a very strong gasoline odor. This requires IMMEDIATE pump replacement and an oil/filter change.

When to Replace Your MGB Fuel Pump: Making the Decision

Based on the symptoms and diagnostic tests above, replacement is warranted in these scenarios:

  1. Confirmed Failure: No fuel pressure or flow during cranking or running.
  2. Weak Output: Pressure significantly and consistently below specification (under 2 psi) or inadequate flow volume, especially under load conditions (observed as hesitation/stalling).
  3. Internal Leak: Fuel contamination found in the engine oil (a non-negotiable reason!).
  4. External Leak: Visible fuel weeping from the pump body or gasket surfaces (a safety hazard).
  5. Routine Preventive Replacement: If the pump is very old (decades), known to be original, or of unknown history, replacing it before failure during a major service or engine work can provide peace of mind, especially if driving long distances.
  6. Conversion: Mechanical to Electric: While not strictly replacing the mechanical pump for another mechanical pump, this is when many choose to switch systems (see next section). If converting, the mechanical pump must be bypassed or removed.

Options for Replacement: Original, Rebuilt, or Electric Conversion?

MGB owners have several choices when replacing the fuel pump:

  1. New Mechanical SU-Type Pump: Several reputable suppliers manufacture brand-new mechanical pumps designed as direct replacements for the MGB:
    • Burlen Fuel Systems Ltd: The successor to SU Carburetters, widely considered the gold standard for genuine SU parts. Pumps are manufactured to original specifications.
    • Moss Motors, Victoria British, British Parts Northwest, etc.: Major MGB parts suppliers stock brand-new SU pumps, often sourcing from Burlen or other quality manufacturers like Accles & Pollock.
    • Smiths Industries: Also produced original SU pumps under license. New equivalents are available.
    • Pros: Maintains originality, bolt-on replacement, known quality from established brands, self-regulating pressure for SU carbs.
    • Cons: Subject to the same potential wear mechanisms as the original, though diaphragm materials are often improved. Typically the highest cost new option.
  2. Rebuilt/Reconditioned Mechanical Pump:
    • Burlen Rebuilt Service: Burlen offers a rebuild service for original pumps, replacing the diaphragm, valves, and seals.
    • Specialist Rebuilders: Some independent classic fuel system specialists offer rebuild services.
    • Pros: Can restore an original pump to like-new condition, good for concours where originality is paramount.
    • Cons: Can be costly, turnaround time involved sending the pump away. Quality depends entirely on the rebuilder.
  3. Pattern Mechanical Pump (Alternative Brand New): Lower-cost new pumps are available from various sources.
    • Pros: Lower cost.
    • Cons: Quality can be highly variable and sometimes questionable. Risk of leaks, incorrect pressure, or early failure. Research reviews and forums carefully before choosing this route. Avoid bargain-basement options for such a critical component.
  4. Electric Fuel Pump Conversion: Some owners choose to replace the mechanical pump with an electric one.
    • Reasons: Perceived increased reliability (debatable, a good mechanical is very reliable), ability to prime the system before starting, can be mounted lower/in-line for potentially better fuel supply from the tank, sometimes necessary for high-performance upgrades.
    • Types: Facet-style solid-state pumps (common, relatively quiet) or rotary vane pumps (typically quieter). Vital: Must be a low-pressure pump (3-4.5 psi max) suitable for carburetors. DO NOT USE an EFI pump!
    • Key Components Needed:
      • Suitable electric fuel pump (e.g., Facet Gold-Flo/Purolator, Huco, SU electric).
      • Inertia safety cut-off switch (essential safety device to cut fuel in an accident).
      • Relay and fused power supply.
      • Wiring loom/kit specific to MGBs.
      • Appropriate fuel hoses and fittings.
      • Often includes a pre-pump fuel filter.
      • Sometimes a fuel pressure regulator (if pump pressure exceeds carb needs).
    • Bypassing the Mechanical Pump: If keeping the engine stock, the mechanical pump cannot simply be disconnected; its internal pumping action blocks fuel flow. You have two options:
      1. Remove Mechanical Pump: Block the hole in the engine block using a blanking plate kit available from suppliers. This is the cleanest solution.
      2. Use a Bypass Pipe: Some kits provide a pipe that connects the inlet to the outlet port on the mechanical pump body, effectively turning the mechanical pump into a passive fuel pipe manifold. This is less common and requires careful installation.
    • Installation Location: Usually mounted near the fuel tank, below fuel tank level (check pump specification - some are pushers, some pullers). Must be securely mounted away from heat and exhaust with anti-chafe protection. Mount the inertia switch vertically on a solid part of the chassis/body.
    • Pros: Eliminates camshaft drive wear, easier pre-start fuel priming (hear it run), potential location benefits (cooler area).
    • Cons: Added complexity (wiring, relay, safety switch), introduces potential electrical failure points and noise (some pumps can be buzzy), potential need for pressure regulator, not original to the car. A well-maintained mechanical pump is arguably simpler overall.
    • Recommendation: Only consider an electric conversion if you have a specific reason beyond just replacing a mechanical pump (e.g., performance mods, recurring mechanical pump issues due to block wear). For a stock road car, a quality new mechanical pump is often the best, simplest, and most reliable choice.

Step-by-Step Guide: Replacing Your MGB Mechanical Fuel Pump

Replacing a mechanical pump is generally straightforward. Gather your tools and new pump beforehand.

  • Tools Needed: Combination wrenches (spanners - SAE sizes, typically 9/16" and 5/8" or 1/2"), socket set & ratchet (often 1/2" drive needed), screwdrivers, drain pan, container for fuel, new copper washers for banjo bolts, safety glasses, gloves, rags.
  • Parts Needed: New fuel pump, new pump mounting gasket, new inlet/outlet pipe seals/washers, optional but recommended new inlet pipe banjo bolt copper washers.
  • Safety First: Ensure ignition is OFF. Disconnect the battery negative (-) terminal. Work in a well-ventilated area away from sparks or flames. Have a fire extinguisher rated for fuel fires nearby. Relieve any residual pressure - carefully crack a pipe fitting while holding rags to catch drips.
  • Location: The pump is mounted low on the engine block, usually near the oil filter housing. Access is usually reasonable on the B-Series engine. You might get slightly better access from underneath the car.

Procedure:

  1. Drain Carburetors (Optional but Recommended): Minimizes fuel spillage when pipes are disconnected. Place a container under each carb float chamber drain screw (small slotted screw on the bottom of each chamber). Carefully open each drain screw fully and let fuel drain out. Tighten drain screws when done.
  2. Disconnect Fuel Lines: Identify the inlet pipe (coming from the fuel tank via the filter and sometimes a sediment bulb) and the outlet pipe(s) going to the carburetors (one pipe to front carb, one to rear carb, usually via a T-piece or banjo).
    • Use the correct spanner to hold the fitting on the pump body while unscrewing the pipe union nut. Banjo bolts require holding the bolt head while unscrewing the nut.
    • Caution: Banjo bolts have TWO copper washers per bolt – one under the head and one under the nut on the pump body side. Note their position. Have replacement washers ready.
    • Be prepared for fuel spillage – have rags and a container ready. Cap the pipes temporarily if possible. Plug the pump ports to prevent debris entry.
  3. Remove Mounting Bolts: There are typically two bolts securing the pump body to the engine block. One is often longer than the other (pay attention for reassembly). Be ready for oil seepage around the pump mounting flange.
  4. Remove Old Pump: Carefully withdraw the pump, ensuring the operating lever clears the block. Fuel and oil will drip – catch it with rags/pans. Remove the old gasket from the engine block face. Clean the mounting surface thoroughly of old gasket material and oil.
  5. Prepare New Pump:
    • Check the new pump operating lever is correctly positioned and rotates freely (don't force it).
    • Ensure the new mounting gasket is correctly fitted to the pump (if not supplied separately).
    • Optional: Lightly coat the gasket surfaces with a smear of gasket sealant (like Hylomar Blue) or engine oil. Check pump instructions – sometimes dry is best. Avoid excessive sealant.
    • Put new copper washers on the banjo bolts/pipes if replacing them.
  6. Install New Pump:
    • Crucial Step: Rotate the engine until the fuel pump cam lobe is on its base circle (low point). This allows the pump lever to sit correctly on the cam without binding. Failure to do this can result in immediate damage to the new pump or lever arm when tightening the bolts.
      • Simplest Method: Engage a high gear, release handbrake, and carefully push/pull the car until the cam rotates. Alternatively, briefly crank the starter with plugs/coil disconnected (can be messy).
      • Feel the cam through the pump hole: Lubricate your finger, insert it into the pump mounting hole, rotate the engine slowly (by hand via the fan belt or crank pulley bolt), feel the cam come around – stop when you feel the lowest point (the "dwell" where it's flat).
    • Carefully insert the new pump, guiding the operating lever onto the cam face correctly. It should sit flat against the block without pressure. Ensure the gasket is correctly aligned.
  7. Hand-Tighten Mounting Bolts: Start both mounting bolts by hand to ensure they aren't cross-threaded. Use the longer bolt in the correct hole if applicable.
  8. Torque Mounting Bolts: Follow the correct torque setting if available (consult workshop manual, typically firm but not excessive - often around 12-15 ft-lbs). Tighten the bolts evenly and gradually in a criss-cross pattern to avoid distorting the pump housing.
  9. Reconnect Fuel Lines:
    • Reattach the fuel outlet pipe(s) to the pump outlet. Use new copper washers where applicable. Tighten securely but do not overtighten, especially banjo fittings.
    • Reattach the fuel inlet pipe to the pump inlet. Use new washers/seals. Tighten securely.
  10. Double-Check Connections: Ensure all fittings are tight and secure.
  11. Reconnect Battery: Reattach the battery negative terminal.
  12. Prime the System & Check for Leaks:
    • Turn the ignition ON (do not start).
    • If carburetors were drained: Open the throttle fully 4-5 times (or use the starter in short bursts) to activate the mechanical pump and refill the float chambers. You may hear gurgling from the carbs as fuel flows in.
    • If using an electric pump conversion: Turn the ignition on to run the pump briefly until fuel flows steadily to the carbs.
    • Crucially: Visually inspect ALL pipe connections, banjo bolts, and the pump body/mounting flange for any sign of fuel leakage. Use a mirror underneath if needed. Repair ANY leak found BEFORE starting the engine. Tighten fittings slightly more if necessary, but avoid overtightening.
  13. Start Engine & Verify Operation: Once confident of no leaks, start the engine. It may take a few extra cranks initially to fully prime the system.
    • Listen for unusual noises from the pump (should be a quiet clicking).
    • Recheck all connections again for leaks while the engine is running and the pump is under pressure.
    • Observe engine performance – should idle smoothly and accelerate cleanly without hesitation.

Maintaining Your MGB's Fuel Pump for Long Life

While mechanical fuel pumps are not high-maintenance items, proactive care promotes longevity:

  1. Use Clean, Good Quality Fuel: Contaminants and water accelerate wear on the pump's valves and diaphragm. Avoid running the tank excessively low where sediment can be drawn in.
  2. Replace Fuel Filters Regularly: The pump inlet usually has a built-in filter sock or strainer inside the pipe connection. Ensure this is clean upon pump replacement. Additionally, service any in-line fuel filter (often found near the fuel tank) per the manufacturer's schedule. A clogged filter makes the pump work harder and can starve the engine.
  3. Inspect for Leaks Periodically: During routine maintenance under the bonnet, visually check the pump body and fuel line connections for dampness or stains. Monitor your oil level and smell for gasoline.
  4. Know Its Age: If you replace the pump, note the date on your maintenance log. If the pump is decades old, consider preemptive replacement during a major service to avoid roadside issues.
  5. Avoid Starting Fluid Abuse: Excessive use of starting fluid (ether) is hard on engine components and can potentially affect pump diaphragms. Focus on solving the underlying problem instead.
  6. Electric Pump Considerations: If you have an electric pump, ensure the wiring connections remain clean and tight. Test the inertia safety switch periodically per manufacturer instructions.

Key Considerations for MGB Fuel Pumps: Points to Remember

  • Oil Contamination is Critical: A ruptured diaphragm leaking fuel into the crankcase is catastrophic. Recognize the smell of gas in the oil and act instantly. Always change the oil and filter immediately after replacing a pump that has leaked fuel internally.
  • Quality Matters: Invest in a quality pump (Burlen/SU/Smiths) for reliability. Cheap pattern parts frequently fail prematurely or cause other issues. The relatively small extra cost is worth the peace of mind.
  • The Cam Position Matters: Installing a mechanical pump without checking the cam lobe position risks instant damage. Always rotate the engine to the base circle before tightening the pump bolts.
  • Correct Pressure is Crucial: SU carburetors require low pressure (2.5-4 psi). Using a pump designed for higher pressure, especially an EFI pump by mistake, will flood the carbs. Ensure replacement mechanical pumps or any electric pump are specifically rated for carburetor use with low pressure.
  • Safety First: Fuel leaks are extremely dangerous. Double and triple-check connections before and after starting the engine. Have fire suppression equipment close at hand.
  • Electrical Conversion Requires Care: Converting to electric introduces complexity. If undertaken, use a comprehensive kit designed for MGBs, include an inertia switch, and follow instructions meticulously. Don't cheap out on components or wiring.
  • Keep it Stock for Simplicity: For the majority of stock MGBs on SU carburetors, a correctly diagnosed and replaced mechanical pump with a quality unit remains the simplest, most reliable, and most authentic solution. Focus on diagnosing correctly before replacing parts.

Conclusion: Keeping the Fuel Flowing in Your MGB

The humble mechanical fuel pump is a robust and generally reliable workhorse in your MGB. While failure can be frustrating, diagnosing issues is usually straightforward through symptoms like hard starting, stalling under load, or lack of fuel pressure. Visual checks, pressure testing, and the critical inspection for fuel contamination in the oil lead you to a clear decision. Replacement with a quality SU-type mechanical pump, paying close attention to cam lobe positioning and using new gaskets and seals, restores reliable performance. Regular checks for leaks and maintenance of the fuel filters prevent future problems. Whether you stick with the authentic mechanical pump or opt for an electric conversion for specific reasons, understanding the MGB fuel pump system empowers you to keep your classic MG running smoothly and reliably mile after mile. Enjoy the drive!

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