MGB Fuel Pump: Complete Guide to Operation, Troubleshooting & Replacement for Your Classic MG

Ensuring reliable fuel delivery is critical for your MGB's performance and driveability, and the heart of that system is the fuel pump. Neglecting it can leave you stranded. Whether your classic MG has its original mechanical pump or a modern electric conversion, understanding how the MGB fuel pump works, recognizing signs of failure, and knowing how to maintain or replace it are essential skills for any owner. Choosing the correct pump and installing it properly directly impacts engine health, fuel efficiency, and overall driving enjoyment. This definitive guide covers everything from fundamental operation to detailed troubleshooting and step-by-step replacement for both mechanical and electric MGB fuel pumps.

Understanding How Your MGB Fuel Pump Works

The MGB fuel pump has one primary, non-negotiable job: delivering gasoline from the tank at the rear of the car to the carburetors or single carburetor at the engine bay, at the correct pressure and consistent volume required for combustion. Without an operating pump, fuel cannot reach the engine, resulting in failure to start or sudden stalling. While the basic function is simple, the execution differs depending on whether the system is mechanical or electrical.

The Two Main Types: Mechanical vs. Electric Fuel Pumps for MGB

  1. Original Mechanical Fuel Pump:

    • Location: Mounted on the engine block, driven by a dedicated eccentric lobe on the camshaft.
    • Operation: As the camshaft rotates, the eccentric lobe pushes a lever arm inside the pump. This lever motion works an internal diaphragm.
      • Suction Stroke: The diaphragm moves downward, creating vacuum. This vacuum opens an inlet valve, drawing fuel from the tank through the main supply line into the pump chamber.
      • Pressure Stroke: The diaphragm moves upward, compressing the fuel in the chamber. This pressure closes the inlet valve and forces open the outlet valve, pushing fuel towards the carburetors.
    • Output: Directly proportional to engine speed – faster RPM equals more pumping action.
    • Pressure: Low-pressure (typically 2.5 - 4.5 PSI for SU carburetors), regulated by pump design and spring tension on the diaphragm.
    • Common Pumps: Original SU pumps (AUF series), AC Delco replacements, and quality aftermarket equivalents like those from Moss Motors or Frontline Classics.
    • Pros: Period correct, self-regulating pressure (within design limits), simple operation (fewer electrical components).
    • Cons: Susceptible to heat soak in the engine bay (vapor lock potential), diaphragm wear/failure common with age/ethanol fuel, lever arm wear against cam lobe, performance limited by engine speed, requires engine removal for access on later MGB models (Rubber Bumper), engine oil can leak into pump chamber if diaphragm fails.

  2. Retrofit Electric Fuel Pump:

    • Location: Typically mounted near the fuel tank (rear boot floor area), sometimes under the car close to the tank (following safety guidelines). Must be mounted low to promote gravity feed to the pump inlet.
    • Operation: Powered by the vehicle's electrical system (usually wired via a relay triggered by ignition or oil pressure). The most common type uses a solenoid that rapidly pulses a diaphragm back and forth. Newer solid-state pumps may use impellers or rollers.
      • Suction: Internal mechanism creates suction on the inlet side, pulling fuel from the tank.
      • Pressure: Internal mechanism pressurizes fuel on the outlet side.
    • Output: Constant volume when powered, independent of engine speed (once running).
    • Pressure: Regulated by a separate fuel pressure regulator (FPR) installed near the carburetors. This is crucial. Without an FPR set to the correct low pressure (3 - 4.5 PSI), an electric pump can overwhelm carburetor float valves, causing flooding. Choose pumps rated for low-pressure carbureted applications.
    • Common Pumps: Facet Red Top or Gold Top (Cube style), SU Electric (EFS series), Huco, Carter P4070, Airtex E8016S. Always select the PSI rating for carburetors.
    • Pros: Less prone to vapor lock (cooler location), immediate fuel pressure at key-on (aids starting), can overcome vapor lock at idle, often easier roadside replacement than mechanical pump, performance not limited by engine RPM.
    • Cons: Requires correct wiring and safety measures (inertia switch, relay, fuse), adds electrical complexity, requires correct mounting orientation and location, must include a pressure regulator, requires robust fuel line connections (pulsation), creates audible ticking noise.

Universal MGB Fuel Pump Components (Shared Responsibilities)

Regardless of type, certain components within the pump mechanism work similarly:

  • Diaphragm (in most mechanical & some electric): A flexible membrane moved mechanically (cam lever) or electrically (solenoid) to create the pumping action. Subject to fatigue, ethanol degradation, and rupture.
  • Check Valves (inlet and outlet): Small, one-way valves that control the direction of fuel flow. Must seal perfectly. Can become clogged, warped, or develop weak springs.
  • Levers & Linkages (Mechanical): Transmit cam motion to the diaphragm. Subject to wear against the cam lobe or at pivot points.
  • Seals and Gaskets: Prevent external leaks and maintain separation between chambers (e.g., between oil and fuel in a mechanical pump). Can harden, crack, or shrink over time.
  • Strainer/Sock (inside tank): While inside the tank, not the pump, this filters fuel before it even enters the pump line. A clogged strainer starves the pump.

Diagnosing MGB Fuel Pump Problems: Common Symptoms and Causes

A failing MGB fuel pump rarely presents a single, clear-cut symptom. Observing driving behavior is key. Here are common indicators:

  • Failure to Start (Engine Cranks but Won't Fire):

    • Check: Can you hear the pump? Mechanical pumps may have a faint click near the engine; electric pumps have a distinct ticking near the rear. Do not crank endlessly. Check for fuel at the carburetor inlet – carefully loosen a fuel line union at a carburetor while cranking (catching fuel in a rag). NO fuel = pump issue or severe blockage.
    • Possible Pump Causes: Diaphragm rupture (mech), failed pump motor (elec), stuck check valves (both), severe internal wear (both), total electrical failure to pump (elec), broken lever arm (mech).

  • Engine Stalling During Operation:

    • Check: Does it stall under load? At idle only? After the engine bay gets hot? Immediate restart possible? Rapid stalling points to sudden fuel loss. Hesitation then stall might be pump struggling.
    • Possible Pump Causes: Vapor lock (especially mechanical pump on hot days), diaphragm fatigue causing intermittent failure under load (mech), failing pump motor struggling to maintain flow (elec), partially clogged filter/valves restricting flow (both), weak pump unable to meet demand (both), leaking fuel line losing pressure.

  • Loss of Power / Hesitation / Stumbling Under Load:

    • Check: Most noticeable accelerating uphill or during hard acceleration? Feels like a fuel starvation issue? Does engine run smoothly at constant low speed?
    • Possible Pump Causes: Pump cannot deliver sufficient volume at higher RPM/demand (worn mechanical pump, under-rated electric pump, clogged inlet strainer/pump filter/check valves), internal pump wear, fuel pressure too low. This symptom often has other causes too (ignition, carb mixture), so pump is one possibility.

  • Rough Idling:

    • Check: Idle is erratic, lumpy, or inconsistent? Combined with other symptoms?
    • Possible Pump Causes: Less common as the sole cause, but possible if the pump produces inconsistent pressure or flow fluctuations (worn mechanical components, failing electric pump). Often points more towards ignition or carb issues, but don't rule out fuel delivery.

  • Engine Running Rich or Flooding:

    • Check: Black smoke? Strong gasoline smell? Fouled spark plugs? Check fuel pressure!
    • Possible Pump Causes: Overwhelmingly points to excessive fuel pressure: Failed pressure regulator (electric pumps only), stuck outlet check valve (both, though rare), damaged regulator diaphragm. This is critical – too much pressure forces excess fuel past the carburetor float valves.

  • Audible Pump Noise Change:

    • Check: Electric pump ticking becomes much louder, intermittent, or silent? Mechanical pump has loud metallic clicking or grinding?
    • Possible Pump Causes: Imminent electric pump failure. Mechanical pump lever arm wear or potential cam lobe wear. Air leak into the suction line causing noisy cavitation (both).

  • Visible Fuel Leaks:

    • Check: Look near pump mounting points, hose connections, pump cover plates/diaphragm cover. Extreme fire hazard!
    • Possible Pump Causes: Failed gasket or seal (both), cracked pump body (rare), loose fittings (both), perforated diaphragm venting fuel externally (mech). Stop immediately and repair.

Distinguishing Pump Issues from Other Problems: The Vapor Lock Dilemma

A signature problem, especially for mechanical pumps on MGBs is vapor lock. It mimics pump failure but is fundamentally different:

  • Cause: Fuel in the lines or pump chamber overheats (engine bay heat), boils, and turns into vapor bubbles. Vapor is compressible, so the pump cannot push it effectively, blocking fuel flow. Ethanol-blended fuels vaporize at lower temperatures than classic petrol, worsening the problem. More common on hot days and during heat soak after stopping a hot engine.
  • Symptoms: Sudden loss of power and stall, often after running fine initially or after a hot restart. Difficulty restarting while hot. May restart after cooling period.
  • Mechanical Pump Risk: High. Pump is heat-soaked directly from the engine block.
  • Electric Pump (Rear-Mounted) Solution: This is one of the primary benefits of converting to electric. Locating the pump cooler near the tank significantly reduces vapor lock potential. Combined with modern fuel lines less prone to heat transfer, it nearly eliminates this issue if installed correctly.

Testing Your MGB Fuel Pump

Don't guess – gather evidence before replacing the pump or condemning it:

  1. Visual Inspection & Sound:

    • Check for leaks at pump body, hose connections, and fuel lines.
    • Inspect mounting bolts and electrical connections (elec) for tightness/corrosion.
    • Listen: Electric pumps should tick steadily when ignition is turned on (or during cranking/running, depending on wiring). Sudden silence is bad. A weak or irregular tick might indicate wear or blockage. Mechanical pump clicks are harder to isolate but a change or metallic grinding is bad. Air bubbles in fuel lines are audible in many systems.

  2. Pressure Test (CRITICAL):

    • Equipment Needed: Low-pressure fuel pressure gauge (0-15 PSI scale is ideal). Tee-piece adapters or appropriate fittings for your carburetor inlet line.
    • Procedure: Disconnect fuel line to a carburetor. Connect the gauge either directly inline between the pump and carburetor inlet, or use a tee to connect the gauge while still feeding the carburetor. Ensure all connections are secure.
    • Mechanical Pump: Run engine at idle, observe pressure reading (should be stable ~3-4.5 PSI). Rev engine - pressure should increase slightly but remain stable. Pressure should not drop significantly at high RPM. Turn off engine; pressure should hold for several minutes without leaking down rapidly.
    • Electric Pump: Turn ignition to 'On' (pump should run) - observe pressure without engine running. Engine running at idle, observe pressure. Pressure reading should match the pressure regulator's setting (confirmed separately on the regulator if adjustable) and be stable. Pressure should not significantly fluctuate with RPM changes on an electric pump. Turn off ignition; pressure should hold for several minutes without leaking down rapidly.
    • Interpretation: Low pressure = pump wear, blockage (inlet strainer, filter), air leak on suction side. Erratic pressure = failing pump, severe blockage. Pressure significantly drops at high RPM = pump cannot meet flow demand (worn, blocked inlet). Pressure higher than specification = Electric Systems: Pressure regulator failure. Mechanical Systems: Possible blockage downstream or faulty gauge (less likely). Holds Pressure Well: Pump check valves and downstream system seals are likely good. Rapid Pressure Drop: Leaky outlet check valve in pump, faulty carb float valve, leaking fitting downstream of pump/gauge. A dead pump produces zero pressure.

  3. Flow Rate Test (Supplemental):

    • Equipment: Catch container (metal or approved fuel container), measuring jug, stopwatch.
    • Safety First: Ensure perfect ventilation, no sparks, fire extinguisher handy. Point hose into container securely. Disconnect the fuel line after the pump. Route the outlet into the catch container. Place the container in a larger spill containment tray.
    • Mechanical Pump: Disable ignition (coil wire) and crank the engine (do not run!) for a known period (e.g., 30 seconds). Measure the fuel collected. Calculate flow per minute. Consult service manual or pump specs for minimum expected volume during cranking (flow under cranking RPM is less than running). Expect at least 150-300ml per 30 seconds on crank as a bare minimum guideline.
    • Electric Pump: With ignition 'On' (engine NOT running), run the pump into the container for a known period (e.g., 30 seconds). Measure fuel volume and calculate flow per minute. Compare against the pump manufacturer's specified free-flow rate (often listed in Gallons Per Hour or Litres Per Hour). Most carb applications need around 15-35 GPH. Good low flow under cranking is more important for starting than peak GPH. Flow should be smooth without spurts or air bubbles.
    • Interpretation: Low volume = pump wear, blockage, leak on suction side. Air bubbles in the stream = air leak on suction side (tank outlet, cracked line, loose hose clamp, failing lift pipe seal inside tank). Erratic flow = failing pump.

Choosing the Right Replacement MGB Fuel Pump

Selecting the correct pump is paramount:

  1. Confirm Your Type & Specs: Are you replacing a mechanical pump? Converting to electric? Replacing an existing electric pump? Know what you have and what PSI rating it should deliver (usually 3-4.5 PSI max for SUs or HIFs).
  2. Mechanical Pump Replacement:
    • Period Correctness: For stock appearance and function, replace with a good quality replica of the original mechanical pump (SU AUF type, AC Delco equivalent). Ensure mounting gasket material is ethanol-resistant.
    • Considerations: While the mechanical pump is original, understand its inherent vapor lock limitations compared to a relocated electric pump. Premium mechanical pumps offer longer life.
  3. Electric Pump Upgrade - Essential Choices:
    • PSI Rating: *DO NOT USE A FUEL INJECTION PUMP.* Choose a pump specifically rated for carbureted applications - typically 3 to 4.5 PSI maximum output pressure. Higher pressure will flood the carbs.
    • Flow Rate: Aim for 15-35 GPH (Gallons Per Hour) free flow capacity. Higher flow rates within the correct PSI are generally acceptable as the regulator limits pressure, but excessive flow creates noise, heat, and unnecessary power consumption. Sufficient flow for engine demand is key.
    • Pump Type: Facet "Cube" style pumps (Red Top/Posi-Flow, Gold Top) are incredibly popular and reliable for MGBs. SU's EFS series are excellent but higher cost. Huco and Carter also offer suitable low-pressure options. Solid-state pumps are quieter but ensure they have low-pressure models.
    • Mounting: Mount near the fuel tank, as low as possible vertically. The pump inlet should be below the bottom of the fuel tank. Mount it securely using rubber bushings to dampen vibration and noise. Ensure the mounting orientation matches the manufacturer's diagram (inlet/outlet positions critical). Mounting above tank level greatly increases vapor lock risk and primes the pump with air.
    • Critical Safety Components: Do NOT wire directly to the ignition switch!
      • Inertia Safety Switch (Cut-off switch): Mandatory. Mounted securely to the chassis, usually inside the rear boot. Cuts power to the pump instantly if the vehicle experiences an impact/shunt. Prevents a live pump feeding fire from a wreck. Install per the switch instructions.
      • Relay: Essential to handle the pump's electrical load. Wire power from battery (via appropriate fuse) to relay. Trigger the relay from a fused ignition switched source OR an oil pressure switch (see below). Connect the pump power through the relay output.
      • Fuse: Size appropriately for the pump's current draw (consult pump specs, often 5-10 amp). Protect the main power feed near the battery.
      • Wiring Trigger Options: Two main strategies:
        • Ignition Switched Trigger: Pump runs immediately when ignition is turned on. Advantage: Instant prime for easier starting. Risk: If engine stalls in an accident, pump keeps running unless inertia switch activates. Common and generally acceptable with a robust inertia switch.
        • Oil Pressure Switch Trigger: Pump only runs when oil pressure is present (usually above 5 PSI). Requires wiring from the oil pressure sender/signal. Advantage: Pump stops instantly if engine stalls, regardless of ignition position. Disadvantage: Starting requires cranking the engine to build oil pressure before fuel is pumped. Can cause slightly longer cranking times, especially on cold engines. Consider using a momentary bypass switch solely for initial priming if using this method.
  4. The Fuel Pressure Regulator (FPR) - Electric Pump Systems Only:
    • Absolute Necessity: The pump outlet pressure will be too high without a regulator. You must install an FPR.
    • Selection: Choose a low-pressure regulator specifically designed for carburetors. Ensure PSI adjustment range includes the needed 3-4.5 PSI. Holley, Mallory, Summit, and many others offer suitable FPRs.
    • Location: Install the FPR as close to the carburetors as practical, usually near the fuel inlet lines on the firewall.
    • Plumbing: Inlet from pump, Outlet to carb(s), RETURN LINE fitting MUST be piped back to the main fuel tank. A return line is essential for constant pressure regulation. Blocking the return port turns the FPR into a simple restriction, not a true regulator. Secure all fittings.
    • Adjustment: Follow FPR instructions. With the vacuum hose disconnected and plugged (if applicable - some models use manifold vacuum), start the engine. Use the pressure gauge to measure pressure at the carb inlet (after the FPR). Adjust the FPR to achieve the specified pressure for your carburetor(s). Tighten locknut.

Essential Supporting Components for Any MGB Fuel System

  • Filters: Protect your pump and carbs!
    • In-Tank Strainer/Sock: Crucial first line of defense for the pump suction. Prevents large debris from entering the pump or lines. Replace if the car has sat long-term or fuel contamination is suspected.
    • Main Fuel Filter: Install a quality paper-element filter between the tank/pump and the carbs. For mechanical systems, usually near the carbs. For electric systems, ideally near the pump outlet. Replace periodically. Ensure correct flow direction. Choose a filter rated for modern fuels.
  • Fuel Lines:
    • Replace Old Rubber: If your rubber hoses are more than 10 years old, or hard/cracked, replace them immediately using SAE J30R9 fuel injection rated hose (high-pressure rating needed for modern ethanol blends, even at low pressure). Standard carb hose is inadequate and will deteriorate rapidly. Use hose rated to 50 PSI minimum.
    • Securing: Use proper fuel line clamps (fuel injection type band clamps preferred) where hoses connect, especially at pump inlet/outlet. Ensure tightness without damaging the hose.
  • Check Valve in Feed Line (Electric Systems): While not always mandatory, installing a one-way check valve near the carb inlets can help maintain a pressurized column of fuel after shut-off, improving hot starts slightly (optional).
  • Ethanol Considerations: Use modern rubber parts throughout the system (pump diaphragm/gaskets, hoses, seals) designed for E10 compatibility. Avoid cheap, outdated materials.

MGB Fuel Pump Replacement Procedure: Step-by-Step

  • SAFETY FIRST: Work outdoors or in well-ventilated space. Disconnect the negative terminal of the battery. Extinguish all open flames, no sparks. Have a large fire extinguisher rated for flammable liquids (ABC or BC) within arm's reach. Wear safety glasses. Place absorbent pads underneath work areas.

A) Replacing a Mechanical MGB Fuel Pump:

  1. Relieve Fuel System Pressure: Loosen the fuel filler cap slightly. For added safety, clamp the main fuel feed hose (carefully) with vice-grips wrapped in a rag near the tank outlet. Avoid damaging the hose. Placing the clamp near the tank minimizes fuel spillage. Have rags ready.
  2. Drain Oil (Optional but Recommended): Place a drain pan under the pump. The cam lever arm extends into the crankcase. Removing the pump will create an opening to the engine oil. Draining the oil first prevents a large spill.
  3. Access: Remove engine compartment components blocking access if necessary.
  4. Disconnect Fuel Lines: Carefully disconnect the fuel inlet and outlet lines from the pump. Plug or cap the lines temporarily. Use wrenches on the fittings, not just the hose. Be ready for fuel spillage.
  5. Remove Mounting Bolts: Remove the two or three bolts securing the pump to the engine block. Note any spacers or washers.
  6. Remove Pump: Carefully pry the pump away from the block (may need gentle rocking) and lift it out, tilting it slightly to clear the lever arm off the cam lobe. Keep track of gasket pieces.
  7. Clean Mounting Surface: Use a plastic scraper or razor blade carefully to remove old gasket material from the block. Ensure surface is smooth and clean. Wipe with solvent rag, let dry. Clean bolts.
  8. Prepare New Pump: Position the new, correct gasket onto the block or the new pump. Some advocate a thin coat of suitable gasket sealant on one side only. Verify orientation.
  9. Install New Pump: Carefully lower the new pump into place, ensuring the lever arm engages the cam lobe correctly. This requires careful angling. Rotating the engine manually (via crank pulley bolt, spark plugs removed) to position the cam lobe at its base circle (lowest lift) can make pump engagement easier. Guide bolts in finger-tight first.
  10. Torque Bolts: Tighten mounting bolts evenly to the proper specification (consult manual, typically 15-20 ft-lbs). Avoid overtightening.
  11. Reconnect Fuel Lines: Securely connect inlet and outlet fuel lines using new clamps if old ones were weak. Ensure connections are tight.
  12. Prime & Check: Reconnect battery. Slowly release any line clamp used. Turn ignition on briefly (if electric pump primes) or crank engine (mechanical pump) to build pressure. Immediately check for leaks at pump body, gasket, and hose connections. If oil was drained, top up to correct level. Run engine to operating temperature and recheck for leaks. Verify fuel pressure and engine operation.

B) Installing or Replacing an Electric MGB Fuel Pump (Full Installation Guide):

  1. Plan Mounting Location: Ideal: Inside rear boot on the transmission tunnel hump, or on the boot floor itself, near the tank outlet area. Alternatively, securely mounted underneath on a chassis crossmember near the tank, protected by the frame rail. *Key: Pump inlet MUST be at a lower level than the bottom of the fuel tank to promote gravity feed.* Ensure location is protected from road debris (use a shield plate if necessary). Ensure pump orientation matches manufacturer specs (inlet/outlet ports UP/DOWN etc.).
  2. Relieve Fuel Pressure: Loosen fuel filler cap. Clamp the rubber line feeding the pump inlet near the tank, or drain tank significantly. Have rags ready.
  3. Access Fuel Tank Outlet: Locate the hard line exiting the top of the fuel tank (inside the boot). Disconnect the existing rubber line or steel line connected to this outlet.
  4. Run New Suction Hose: Using fresh SAE J30R9 fuel injection rated hose (short length ~6-12 inches), connect from the tank outlet fitting to the inlet port of the new pump. Install new, proper fuel injection clamps and tighten securely. Route hose neatly avoiding kinks or sharp bends. Ensure gravity feed works.
  5. Run New High-Pressure Delivery Line:
    • From the pump outlet, run fresh SAE J30R9 hose towards the front of the car. Secure firmly with P-clips or Adel clamps every 12-18 inches. Avoid contact with sharp edges, exhaust, moving parts, or excessive heat sources. Bring this line to the area near the carburetor fuel inlet(s).
  6. Install Fuel Pressure Regulator (FPR): Mount the FPR securely near the carb(s) (firewall common). Connect the inlet port of the FPR to the high-pressure hose coming from the electric pump.
    • Connect the outlet port of the FPR to the carburetor(s) – either via a tee-block for dual carbs or directly to the single carb inlet.
    • Connect the return port of the FPR back to a RETURN PORT on the fuel tank. Many MGB tanks have a plugged return fitting near the main outlet. If yours doesn't, a competent garage can solder in a bung fitting. Connecting the return directly into the filler neck is unsafe and creates fumes. A proper tank return port is necessary. Install a small return line filter near the tank. Use proper hose/clamps for return line (lower pressure, can be standard J30R7, but J30R9 is fine too).
  7. Wire the Pump Electrically: This is critical for safety and reliability. Follow the diagram below:
    • Fuse: Run a new, appropriately sized wire (e.g., 14 AWG) from the battery POSITIVE (+) terminal via an inline fuse holder (mounted near battery) to the mounting location of the relay. Size fuse per pump specs (e.g., 5A, 10A).
    • Relay: Mount the relay securely in the boot area.
      • Connect the fused battery wire to relay terminal 30.
      • Run a new wire (e.g., 14 AWG) from relay terminal 87 to the positive terminal of the electric fuel pump.
      • Connect the negative (-) terminal of the fuel pump to a clean, bare metal chassis point near the pump using a ring terminal and bolt. Ensure excellent contact.
      • Relay Trigger: Connect relay terminal 86 to a clean chassis ground point.
      • Relay Trigger Power Source: Connect relay terminal 85 to one of the following: Option 1 (Switched Ignition): A fused ignition switched source that has 12V+ only when the key is ON/START. Use a multimeter to verify. Tapping into the main harness in the boot is common. Option 2 (Oil Pressure Switch): Wire terminal 85 to the "S" terminal (signal) of a simple oil pressure switch (not the sender/gauge unit) plumbed into an oil gallery near the filter. This switch closes at 5-7 PSI oil pressure. The 'signal' wire from this switch goes to ground when the switch closes (providing oil pressure). Check switch operation with a meter before wiring. This trigger grounds terminal 85 when oil pressure is present. Consider a momentary priming switch for Option 2.
    • Inertia Switch: Install the inertia safety switch per manufacturer instructions in a secure location on flat chassis metal in the boot area.
      • Disconnect the wire running from relay terminal 85 (the trigger power source).
      • Connect the trigger power source (ignition or oil switch feed) to one terminal of the inertia switch.
      • Connect the other terminal of the inertia switch back to relay terminal 85. The inertia switch is now interrupting the relay trigger circuit. If triggered, it breaks the ground path (oil switch method) or power path (ignition method), instantly stopping the relay.
  8. Double Check Wiring: Verify all connections are tight, properly insulated, and protected from chafing. Ensure pump polarity is correct (+ and - terminals). Verify inertia switch is mounted correctly and direction arrow (if any) points towards the front of the car.
  9. Final Fuel Line Connections: Ensure all hose clamps are properly tightened, especially at pump inlet/outlet and carb inlets. Check entire fuel line route for kinks or interference points.
  10. Prime, Pressurize, Purge & Check:
    • Reconnect battery negative terminal.
    • If using an Inertia Switch, ensure it's RESET (button pressed in).
    • If wired to Ignition: Turn key to 'On' (do not crank). Pump should run and stop after a few seconds if the system pressurizes properly. Listen for pump ticking.
    • If wired to Oil Pressure: Briefly activate a priming switch or expect pump to run only during cranking/once oil pressure builds.
    • Carefully check every single fuel connection from tank to carbs for leaks (especially at fittings and clamps). Look underneath too. Fix ANY leak immediately.
    • Crank engine until it starts (may take slightly longer if converting from mech pump and system needs purging). Air bubbles should clear quickly.
  11. Set Fuel Pressure:
    • With engine running at idle, connect pressure gauge at carb inlet (downstream of FPR).
    • Adjust the FPR according to manufacturer instructions (usually an adjusting screw) to achieve the specified fuel pressure for your carbs (typically 3-4.5 PSI). Tighten locknut after adjustment.
    • Recheck pressure stability at idle and during revving. Adjust FPR if needed.
    • Ensure pressure stays within specs and doesn't creep up. Watch closely when shutting off engine.
  12. Final Inspection & Test Drive: Once pressure is set and stable, let the engine reach operating temperature. Check for hot leaks. Go for a short test drive on quiet roads, paying attention to idling, acceleration from stop, hill climbing power. Park, turn off, inspect for leaks again, especially warm/hot spots.

MGB Fuel Pump Maintenance & Troubleshooting Tips for Longevity

  • Filter Management: Replace the inline fuel filter annually or per manufacturer guidance. Inspect/replace the tank strainer sock every few years or if contamination is suspected. Clean filters prevent pump damage.
  • Regular Pressure Checks: Periodically check fuel pressure with a gauge. It costs little time and can detect failing pumps or regulators before symptoms become severe.
  • Ethanol Resistance: Use ethanol-compatible replacement parts (gaskets, diaphragms, hoses). Consider ethanol fuel treatments if pump issues arise with modern fuels. Never use old rubber fuel hose with E10.
  • Preventing Suction Leaks: Ensure all clamps pre-pump are tight. Air leaks cause poor running, vapor lock, and pump cavitation (damaging).
  • Winter Preparation (Non-Use): For storage, consider adding fuel stabilizer and running it through the system. Use a full tank to minimize condensation.
  • If Trouble Strikes On The Road:
    • Sudden Stall: Check inertia switch (if fitted) - RESET IT. Check fuel pump power (fuse, relay trigger). Check for leaks. Verify mechanical pump: Tap pump body lightly with a wrench handle while cranking (can free stuck valve). Check for vapor lock symptoms (hot engine bay - wait to cool).
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