MGB Fuel System: A Deep Dive

Understanding the MGB's Fuel System: Part 1

The MGB, a beloved classic from the 1960s to the 1980s, boasts a fuel system that, by modern standards, is remarkably straightforward. This simplicity is a significant advantage for owners looking to maintain their vehicles efficiently. However, as with any classic car, time and evolving fuel compositions can introduce unique challenges. This article, the first in a four-part series, delves into the MGB's fuel system, tracing its path from the petrol station to the engine, and highlighting common issues that can affect its performance, particularly in relation to modern fuels.

The Impact of Modern Petrol on Classic Cars

One of the most significant factors impacting the MGB's fuel system today is the composition of modern UK petrol. Unlike the fuel available during the MGB's production years (1962-1980), today's petrol is a blend of various compounds, with the introduction of ethanol being a primary concern. Ethanol, an alcohol derived from plants, is added to petrol to improve its octane rating and reduce emissions. However, it's also highly hygroscopic, meaning it attracts and absorbs water from the atmosphere. This can accelerate corrosion, particularly in older, untreated mild steel fuel tanks.

The UK's 95RON unleaded petrol now typically contains up to 10% ethanol (E10), a significant increase from the 5% (E5) seen until 2020. Even 'Super Unleaded' (97RON and higher), once a safe haven for classic cars due to its potential lack of ethanol, now often contains up to 5% ethanol. This shift has led to a documented rise in problems for classic cars, and the MGB is no exception.

Fuel Tank and Filler Cap: The First Line of Defence

The journey of fuel begins at the filler cap. A seemingly simple component, the filler cap's seal can be susceptible to swelling and softening due to ethanol in modern fuels. This swelling can restrict or even block the crucial air vent path within the cap. When the engine consumes fuel, the tank needs to 'breathe' – allowing air to enter and replace the consumed fuel. A blocked vent prevents this, creating a vacuum within the tank. After a period of driving, this vacuum can become so significant that the fuel pump struggles to overcome the external air pressure, leading to a reduction or complete stoppage of fuel flow. You might notice a rush of air into the tank when you remove the cap after a drive – this is a tell-tale sign of a blocked vent. The solution is straightforward: replace the filler cap or ensure the vent path is clear.

Following the fuel down the filler pipe, the hose connecting it to the tank is generally robust, but age can eventually necessitate replacement. The real challenges, however, lie within the fuel tank itself. As most MGB fuel tanks are constructed from untreated mild steel, internal corrosion is a significant concern. Ethanol's hygroscopic nature exacerbates this, as it draws moisture from the air, which is constantly exchanged through the filler cap's vent. Using a fuel additive like 'Classic Etha-Guard Plus' can help mitigate these effects by countering the hygroscopic properties and stabilising the fuel, preventing 'phase separation'.

Phase Separation: A Modern Fuel Menace

Phase separation occurs when ethanol-blended fuel breaks down, separating into its original components. This can happen more quickly with modern fuels than with those from previous decades. An example of this issue involved an MGB with HS4 carburettors. Fuel added in September 2021 caused starting issues by December. After draining and refilling with fresh petrol, the engine ran smoothly initially, but problems recurred as older fuel from the tank was drawn into the carburettors. This highlights how degraded fuel can affect engine performance, even after a seemingly successful refill.

Interestingly, some MGB owners have reported their cars running on older fuel for extended periods without issues. This is often attributed to fuel-injected engines, which experience greater fuel agitation from high-pressure pumps and spill returns. This agitation, along with the finer fuel spray from injectors, may help delay fuel degradation. However, it's likely only a temporary reprieve.

Fuel Tank Drain Plugs: An Essential Feature

A common issue with MGB fuel tanks is the lack of a drain plug. Draining a full 12-gallon tank without a drain plug is a considerable task. While original factory tanks often had drain plugs, they could become so tight over time that removing them would damage the tank itself. Thankfully, MGOC Spares now offers improved tanks for various MGB models (1965-1976 and 1976-1980) that feature drain plugs. These plugs are fitted to a stronger metal base, reducing the risk of damage. The availability for earlier strap-type tanks (1962-1965) is under review.

Having a drain plug offers significant advantages, particularly for winter storage. It allows owners to drain the remaining fuel before laying up the car, preventing fuel degradation and moisture absorption. It's important to note that if the drained fuel has been treated with a lead replacement additive, it must not be used in cars with catalytic converters.

External Corrosion: A Persistent Problem

External corrosion on the MGB fuel tank, particularly on the top surface of post-March 1965 flat-top tanks, has been a prevalent issue since the 1960s. The design, with a gap between the ridged boot floor and the ridged tank top, allowed water and dirt to accumulate, quickly corroding the thin paint layer and the tank itself. This often became apparent within five years, leading to fuel smells and visible streaks of dried fuel down the tank sides.

A common solution has been to paint new tanks with a high-quality paint like Hammerite, with extra coats applied to the top surface. This extra protection is vital for mild steel tanks with only a basic paint coating. MGOC Spares' 'Q Parts' tanks, however, are powder-coated for enhanced corrosion resistance, negating the need for additional surface treatment.

Fuel Pick-up Pipes and Filters

In pre-1976 MGB tanks, a 1/4" bore steel fuel pick-up pipe is integral to the tank. Internal corrosion can cause this pipe to rust through, leading to the fuel pump drawing air when the fuel level drops below the hole. This manifests as a hammering noise from the pump and engine stalling. A simple diagnostic test is to add a few gallons of fresh fuel; if the car then runs normally, a perforated pick-up pipe is the likely culprit. Unfortunately, the only permanent solution is a new tank. When fitting fuel pipes, it's crucial to support the fitting with a tool to avoid stressing the pick-up pipe and damaging the tank seal.

From 1976 onwards, the fuel pick-up and sender unit were combined. This design made the pick-up more accessible and easier to replace. Both types of pick-ups typically had a plastic filter screen at the end, designed to draw as much fuel as possible. However, over time, these filter screens can become blocked, perforated, or dislodged, and they are not replaceable. Blocked filters can mimic a weakening fuel pump, especially when climbing hills. A useful diagnostic tip is to bounce the rear of the car to dislodge any debris from the filter screen; if the car then performs better, a tank clean is likely needed before replacing the fuel pump.

The MG RV8, with its fuel injection system, used a variation of the 1976-1980 tank pick-up without a filter screen, as it required higher fuel flow. While this design hasn't shown widespread blockage issues, debris can still enter the tank, as evidenced by an RV8 sender unit that had an object sucked into the pick-up pipe, stopping the engine. The RV8 also introduced a pre-pump filter, a concept that could potentially be retrofitted to MGBs to protect the fuel pump.

Common Issues Summary

Frequently Asked Questions

Q1: Can I use E10 fuel in my MGB?
While some MGBs may tolerate E10, it's generally recommended to use E5 or higher-octane fuels, which typically contain less or no ethanol, to minimise the risk of damage to fuel system components, particularly rubber seals and tank linings.

Q2: How often should I treat my fuel with additives?
It's advisable to use fuel additives, especially if your car is stored for extended periods or if you are concerned about the ethanol content in your fuel. Follow the product manufacturer's recommendations for usage.

Q3: Is it worth replacing my MGB fuel tank?
If your current tank shows signs of significant corrosion or a perforated pick-up pipe, replacing it with a new or reconditioned unit, preferably one with a drain plug and improved corrosion resistance, is a worthwhile investment for the longevity and reliability of your MGB.

Q4: What are the signs of a blocked fuel filter in the tank?
A common sign is a noticeable loss of engine power when accelerating or climbing hills, as the engine demands more fuel than the blocked filter can supply. The engine may run fine at lower demand but falter under load.

This concludes the first part of our series on the MGB's fuel system. In the next instalment, we will delve into the intricacies of the fuel pump, pipes, and hoses.

Disclaimer: The information provided is intended as a general guide. Always consult your vehicle's official workshop manual and seek professional advice if you are unsure about any procedures.

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