The 1979 MGB Fuel Pump: Your Essential Guide to Reliability & Replacement

Owning and driving a cherished 1979 MGB requires understanding its unique mechanical heart. A critical component demanding specific attention is the mechanical fuel pump. Unlike modern electric pumps, the original '79 MGB pump operates mechanically, driven by an eccentric lobe on the engine camshaft, making its condition paramount for smooth running. Crucially, standard modern ethanol-blended gasoline rapidly degrades the original pump's diaphragm material, making failure highly likely over time. Proactive understanding, identification of symptoms, and choosing the right replacement path are essential for maintaining your MGB's performance and reliability.

Understanding Your 1979 MGB's Mechanical Fuel Pump

The heart of the 1979 MGB's fuel delivery system is a mechanically actuated, diaphragm-type fuel pump. Mounted on the right-hand (passenger side) of the engine block, directly above the crankshaft pulley, it's driven by a lever arm pushed by an eccentric lobe on the front of the engine camshaft. As the camshaft rotates, it moves the lever, which flexes a rubber diaphragm inside the pump body. This diaphragm action creates suction, pulling fuel from the gas tank through the fuel line. On the return stroke, the diaphragm pushes fuel up towards the carburetors via the outlet port. Two check valves inside the pump ensure fuel flows only in the correct direction: into the pump body and out towards the engine. This design is simple and robust under ideal conditions, but vulnerable to modern fuels.

Why Modern Ethanol-Blended Gasoline is the #1 Enemy

The single biggest factor contributing to premature failure of original and early reproduction 1979 MGB fuel pumps is modern gasoline containing ethanol. Ethanol, a common oxygenate added to reduce emissions and extend petroleum supplies, has significant properties that attack vintage fuel system components:

1. Material Incompatibility: Ethanol acts as a powerful solvent. The nitrile rubber (Buna-N) diaphragms and seals used in most original and some lower-cost replacement pumps were not designed to withstand prolonged exposure to ethanol. The ethanol swells, softens, and eventually degrades these materials.
2. Diaphragm Degradation: When the diaphragm degrades, it loses its integrity. Microscopic cracks form. Eventually, it can develop holes or tears large enough to allow raw fuel to leak past the diaphragm.
3. Internal Leakage: The degraded diaphragm allows liquid fuel to pass from the pumping chamber into the interior cavity of the pump housing. This cavity is designed to be dry. From here, fuel has a direct path to leak externally onto the engine block or, more dangerously, into the crankcase via the lever arm opening.
4. Fuel Dilution of Engine Oil: Fuel leaking past the diaphragm and down the lever arm shaft enters the engine's crankcase. This contaminates the engine oil, drastically thinning it and reducing its lubricating properties. Severe engine wear or catastrophic failure can occur if this goes unnoticed.
5. Accelerated Wear: Even before catastrophic failure, a swelling and softening diaphragm increases friction and resistance against the pump lever and camshaft eccentric, accelerating wear on these components.

Essentially, running modern E10 (10% ethanol) or higher ethanol blend gasoline through an original-spec pump is a ticking time bomb for diaphragm failure and potential engine damage.

Recognizing the Symptoms of a Failing 1979 MGB Fuel Pump

A failing fuel pump rarely just stops working completely without warning. Watch for these common symptoms:

• Strong Gasoline Odor Under the Hood: Especially after driving or when the engine is hot. This often indicates raw fuel leaking externally from the pump body, gaskets, or connections. A significant external leak is a major fire hazard.
• Difficulty Starting or Long Cranking Times: Insufficient fuel pressure caused by a weak diaphragm or failing valves prevents the carburetors from getting the fuel they need immediately upon starting. The engine may crank excessively before firing, or may require pumping the accelerator pedal repeatedly.
• Engine Stalling or Hesitation, Particularly Under Load: As engine speed and fuel demand increase, a weak pump cannot deliver adequate fuel volume and pressure. This manifests as hesitation, stumbling, or a complete stall when accelerating or climbing hills. Performance feels flat.
• Engine Sputtering or Misfiring at High RPM: Similar to hesitation, the pump struggles to maintain flow at higher engine speeds, causing the engine to lose power, sputter, or misfire.
• Noticeable Drop in Fuel Economy: While carburetor issues can also cause this, a failing pump delivering inconsistent fuel pressure can lead to incomplete combustion and increased fuel consumption.
• Visible Fuel Leak at the Pump: Wetness, dripping, or pooling fuel around the base of the pump, lever arm opening, or fuel line connections. THIS REQUIRES IMMEDIATE ATTENTION.
• Increased Engine Oil Level or Smell of Gasoline in the Oil: Check the dipstick. If the oil level is inexplicably high above the full mark, and/or the oil has a strong gasoline smell, fuel is leaking into the crankcase – a sure sign of a ruptured diaphragm. Driving with gasoline-contaminated oil WILL destroy your engine.

Ignoring these symptoms, especially oil dilution, risks severe engine damage far exceeding the cost of a simple pump replacement.

The Critical Choice: Replacement Options for Your 1979 MGB

When replacing the failed or failing original pump, you have several options with significant differences in compatibility and longevity:

1. Standard Ethanol-Sensitive Replacement Pumps:

   • Appearance: Often look identical to the original SU AUF-214 or AUF-300 pumps fitted to the 1979 model.
   • Materials: Typically still use nitrile rubber (Buna-N) diaphragms and seals. While better than a 45-year-old original, they are still not compatible with modern ethanol-blended fuels. Degradation and failure will occur, often within a few thousand miles or even faster. They offer poor value long-term.
   • Best For: Restoration projects where the car will be driven very infrequently, and only with non-ethanol fuel (if reliably available locally), or very short-term emergency fixes. Generally NOT recommended.

2. Ethanol-Resistant Replacement Pumps:

   • Appearance: Visually similar to the original style pump.
   • Materials: The key upgrade is the use of Viton (Fluoroelastomer) or other advanced synthetic rubber diaphragms and seals. These materials possess excellent resistance to ethanol's solvent effects, fuel additives, and engine heat.
   • Performance: Maintains the original mechanical design and function, delivering correct fuel pressure (typically 3-4 psi) for the twin SU HS4 carbs on the '79 MGB. Expect years of reliable service when paired with ethanol fuels, providing the best balance of originality and practical usability.
   • Brands: Pumps marketed by reputable suppliers specifically as "Ethanol Resistant" or featuring "Viton" diaphragms are your best bet (e.g., offerings from Moss Motors, British Parts Northwest, many aftermarket brands). The core casting might be new or remanufactured originals.
   • Best For: The overwhelming majority of drivers seeking reliable performance with modern gasoline. This is the strongly recommended choice.

3. Electric Fuel Pump Conversions:

   • Function: Replaces the mechanical pump with an electric pump, usually mounted near the fuel tank or along the frame rail. Requires wiring to a switched ignition source (often with a safety inertia switch) and sometimes a pressure regulator. Bypasses the mechanical pump location entirely.
   • Potential Benefits: Solves the mechanical diaphragm failure issue completely. Can provide very consistent fuel pressure, sometimes easing hot-start issues associated with vapor lock. Offers flexibility in mounting location.
   • Drawbacks: Significant alteration from original specification. Requires careful installation (wiring, plumbing, potential regulator). Lower pressure electric pumps (specifically designed for carbs, typically 2.5-4.5 psi) are essential; modern EFI pumps will overwhelm the float needles. Initial cost and complexity are higher than a direct mechanical replacement. Proper safety wiring is critical to prevent fire risk if the engine stalls. Purists may object.
   • Best For: Cars with significant modifications (e.g., engine swaps) or persistent vapor lock issues in very hot climates. Generally not necessary for a standard 1979 MGB if a quality ethanol-resistant mechanical pump is used.

For the vast majority of 1979 MGB owners seeking reliability without modification, choosing a high-quality ethanol-resistant mechanical fuel pump featuring a Viton diaphragm is the optimal solution.

Step-by-Step Guide to Replacing Your 1979 MGB Fuel Pump

Replacement is a straightforward mechanical task best performed on a cool engine with a relatively empty fuel tank (below 1/4 tank significantly reduces spillage risk). Always disconnect the negative battery terminal before starting.

Tools & Parts Needed:

• New Ethanol-Resistant Fuel Pump (ensure correct model for 1974-80 MGB 4-cylinder)
• New Fuel Pump Inlet Gasket (often included with pump, but verify)
• New Fuel Pump Block Mounting Gasket (absolutely critical, often included with pump)
• Line Wrench Set (for fuel line fittings)
• Standard Wrench Set (13mm usually for pump mount nuts)
• Medium Screwdriver or small pry bar (to gently leverage pump)
• Socket Set & Extensions
• Rags
• Small Container for Dripping Fuel
• Penetrating Oil (like PB Blaster or Kroil - if bolts/nuts are stubborn)
• Safety Glasses & Gloves

Safety First: Fuel vapors are EXTREMELY FLAMMABLE. Work in a well-ventilated area away from sparks, open flames (pilot lights!), or heat sources. Have a fire extinguisher rated for flammable liquids (Class B) readily available. Handle fuel spills immediately with rags, and properly dispose of them.

Procedure:

1. Preparation: Disconnect the negative battery cable. Place rags underneath the pump area to catch drips. Identify the inlet (from tank) and outlet (to carbs) fuel lines. Use a line wrench on the fuel line fitting nuts (where they connect to the pump inlet and outlet ports) to prevent rounding the soft brass.
2. Relieve Fuel Pressure (Limited): Unlike EFI, carbureted systems don't hold high pressure, but fuel in the lines will leak. Loosen the inlet line fitting at the pump slightly to allow pressure to bleed off slowly into your rag or container. Retighten slightly once pressure is relieved.
3. Disconnect Fuel Lines: Using the line wrenches, completely disconnect both the inlet fuel line from the pump and the outlet fuel line going to the carbs. Have rags ready to catch drips. Plug or cap the open ends of the fuel lines temporarily to minimize leakage and prevent debris entry. Avoid bending the hard lines excessively.
4. Remove Mounting Nuts: Locate the two (sometimes four, but usually two) mounting bolts/nuts holding the pump body to the engine block casting. They are typically 13mm. Remove these nuts and any associated washers. Hint: These nuts can be tight or corroded. Penetrating oil applied earlier helps. Use a 6-point socket to avoid rounding.
5. Remove the Old Pump: The pump may be stuck due to the gasket. Gently pry or lever the pump body away from the engine block using a screwdriver or small pry bar. Be careful not to damage the mounting surface on the block. Wiggle it slightly as you pull outward. Once free, lift the pump away, noting the position of the lever arm. You will see the operating rod projecting from the block.
6. Clean the Mounting Surfaces: Remove all remnants of the old block mounting gasket from the engine block surface and the pump mounting surface (on the new pump too, if needed). A scraper and solvent (carb cleaner works well) are suitable. Ensure both surfaces are clean, flat, and dry. This is critical for a leak-free seal.
7. Lubricate the Lever Arm: Apply a small amount of clean engine oil or assembly grease to the operating rod projecting from the block. This helps the new pump lever slide on easily.
8. Prepare the New Pump: Check the new pump's lever arm position. It should be oriented correctly to engage the operating rod on the block. Apply a light film of oil to the lever arm pad that contacts the rod. Fit the new pump block mounting gasket onto the new pump.
9. Install the New Pump: Carefully position the new pump assembly (with its new mounting gasket) over the operating rod. Engage the pump's lever arm with the rod correctly. Slide the pump straight onto the mounting studs until the mounting surface is flush against the engine block. Do not force it. It should slide on smoothly if the lever is properly engaged. Wiggle slightly if needed.
10. Install Mounting Nuts: Hand-start the mounting nuts onto the studs. Ensure the pump body is seated flat and straight. Tighten the nuts evenly and firmly in a criss-cross pattern using a wrench. Do not overtighten, but ensure they are snug to compress the new gasket. Refer to the pump manual for a torque spec if provided (often around 15-20 ft-lbs).
11. Install Fuel Line Gasket: Ensure the sealing washer/gasket is in place on the pump's inlet port. These are often fiber or copper crush washers. Use a new one.
12. Reconnect Fuel Lines: Carefully reconnect the fuel inlet line to the pump inlet port. Use the line wrench to tighten the fitting nut. Avoid cross-threading. Reconnect the fuel outlet line to the pump outlet port, tightening securely with the line wrench. Double-check these connections are tight to prevent leaks. Remove any plugs/caps from the fuel lines.
13. Final Checks: Visually inspect all connections and the pump body for leaks. Carefully reconnect the battery negative terminal.

Priming and Starting:
Since the fuel lines were opened, the pump needs to refill itself and the lines. With the fuel cap on, crank the engine in 10-15 second bursts with short rests in between. Do this several times to allow the pump to draw fuel from the tank. You should hear fuel moving through the lines after a few cranks. Be patient; it may take numerous attempts. Once the pump is primed and fuel reaches the carbs, the engine should start. Run the engine and immediately re-check very carefully for any signs of fuel leaks at all connections and the pump body. Shut off the engine immediately if any leak is detected and correct the issue before proceeding.

Maintenance Tips to Extend the Life of Your New Pump

Investing in an ethanol-resistant pump is wise, but care extends its life:

1. Use Fuel Stabilizer Regularly: If the car sits unused for more than a month, add a quality fuel stabilizer (like STA-BIL 360 Marine Ethanol Treatment or PRI-G) to a full tank of fresh gas. This combats fuel degradation and the formation of varnish and gum, protecting the entire fuel system, including the pump valves.
2. Avoid Storing with Old Fuel: Before storing the car for the winter or extended periods, try to run the tank nearly dry or add stabilizer and drive the car enough to ensure treated fuel circulates fully through the system. Old fuel degrades.
3. Minimize Ethanol Exposure (If Possible): If non-ethanol premium gasoline (E0) is reliably available in your area, using it is the best option for your entire vintage fuel system. Use apps or websites (like PureGas.org) to locate stations.
4. Address Ethanol Issues If Necessary: If only E10 (or higher) is available, consider adding an ethanol-specific fuel treatment with phase-separation inhibitors and corrosion inhibitors at every fill-up (products like Lucas Oil Ethanol Fuel Conditioner, Star Tron Enzyme Fuel Treatment). Ensure the treatment is compatible with Viton.
5. Inspect Regularly: Make it part of your routine under-hood checks. Look for any signs of wetness or fuel stains around the pump body, mounting surface, or fuel line connections. Catch minor weepy leaks early before they become major or cause a crankcase leak.
6. Listen for Changes: While quieter than an electric pump, a normally functioning mechanical pump should have a consistent, quiet rhythmic ticking sound at idle (the lever arm clicking over the cam lobe). Any change to a louder clatter or other unusual noise warrants investigation.

Troubleshooting Persistent Fuel Delivery Problems

If you've replaced the pump with an ethanol-resistant unit and still experience issues, investigate further:

• Clogged Fuel Filter: The MGB typically has a small in-line fuel filter near the carburetors, sometimes within the fuel pump outlet fitting itself, and sometimes a mesh screen in the fuel tank sender unit. A clogged filter anywhere in the line will starve the pump and carbs. Check and replace filters/screens as needed. Don't overlook the tank pickup screen!
• Blocked Tank Vent: The gas tank needs to vent air in as fuel is drawn out. A blocked or clogged vent (either in the gas cap or a separate tank vent line) creates a vacuum that prevents fuel flow. Symptoms include poor idle, stalling as the tank empties, and potentially surging after opening the gas cap.
• Pinched, Kinked, or Collapsing Fuel Lines: Inspect the entire fuel line from the tank to the pump and from the pump to the carbs. Steel lines can rust internally or get dented/kinked. Rubber hoses (especially old ones) can collapse internally when under suction or degrade externally.
• Perished or Leaking Hose Connections: Check the condition of rubber hoses connecting steel lines to the tank sender, pump inlet/outlet, and carburetor inlets. Brittle, cracked hoses leak air into the system (causing fuel pump suction loss) or leak fuel out.
• Damaged Fuel Tank Sender Pickup: The brass pickup tube in the fuel tank can corrode or break off, especially in cars that sat with old fuel/water. A broken pickup tube means the pump can't draw fuel effectively once the level drops.
• Severe Rust Debris in Tank: Corrosion inside the steel fuel tank can shed flakes and rust particles. This constantly clogs filters, screens, and can even jam fuel pump valves. Persistent clogs after filter replacements strongly point to tank contamination.
• Faulty Carburetor Components: While the pump supplies fuel, issues within the carburetors themselves (stuck floats, clogged jets, debris in needle valves) can prevent fuel from entering the float bowls effectively, mimicking a pump failure. Ensure fuel is reaching the carb inlet filters/fittings.
• Weak Engine Compression: The mechanical pump relies on camshaft rotation. Extremely worn camshaft lobes (uncommon, but possible) or timing chain issues can reduce lift on the fuel pump eccentric rod, resulting in low pump output.

Table: Key Differences in Replacement Fuel Pumps

Ensuring Long-Term Reliability

The mechanical fuel pump is a vital component on your 1979 MGB. While its failure mode due to ethanol can be catastrophic, understanding its function, the risks of modern fuel, and proactively installing a high-quality ethanol-resistant replacement pump with a Viton diaphragm provides long-term peace of mind. Regular visual inspections, proper fuel system maintenance, and troubleshooting any subsequent issues effectively will ensure your MGB continues to deliver the classic driving experience you cherish. Keeping this simple yet critical component in top condition safeguards your engine and ensures countless more miles of enjoyable motoring.