Keeping on top of the MGB Fuel System - Part 1

This is part one in a four-part series covering many of the common problems and some of the solutions often found within the MGBs fuel system, many of which can also apply to most other MGs running SU carbs.

The MGB has a really quite simple fuel system, certainly when comparing it with any later car with fuel injection, which is a bonus for owners when keeping it working efficiently.  However, it can still suffer problems and being that it is from a bygone era there is not the widespread professional knowledge to keep on top of it that there used to be when the car was in production. Here I will run through the fuel system from the petrol station pump into the tank through to when it is used by the engine.

Firstly it is pertinent to take a quick look at today’s UK petrol as it is in its overall make up quite different to petrol that was available when the MGB was in production (1962 to 1980).  Petrol is a mix of various compounds and it is the detail change in this mix over time that creates a few specific and well documented problems for classic cars with cast iron carburettor fed engines; most recently of course it is the volume introduction of ethanol.  Ethanol content in mainstream UK 95RON pump petrol reached a ceiling of 5% in 2014, then rising to 10% in 2020.  However, Super unleaded that in the UK means 97RON or higher octane rating could until recently be found with no ethanol, but now it is only safe to assume that it will have up to 5% ethanol and is officially described as the ‘Protection Grade’ intended for older vehicles not suited to E10 levels.

Petrol with ethanol has today been noted as a major cause of a concurrent rise in problems with classic cars and the MGB is no exception, but it is interesting to note that Cleveland Discol petrol was available until 1968 in the UK so it’s certain that some MGBs used it.  The pertinence of this is that this petrol contained unleaded petrol and also ethanol, yet there is no known history of the much wider problems seen in recent years.

For more information on the effects of ethanol-blended fuels, check out our blog - Using Ethanol-Blended Petrol in your Classic Car

Fuel Tank and Filler

When you remove the filler cap to put fuel into the tank you will be inadvertently looking at the first potential problem; the seal in the cap which on older caps can see the seal become softened and swollen by ethanol and that can lead to the air vent path becoming restricted or blocked.  This won’t allow the tank to breathe and take in air as petrol is used, so after twenty to thirty minutes driving the pressure inside the tank lowers and this can be to a point where the fuel pump is no longer able to overcome the outside air pressure that is trying to force its way into the tank via the fuel line and reducing or stopping fuel flow. 

The filler cap is a simple item, but be aware that ethanol in petrol can cause rubber seals to swell and block the breather path.

This won’t be a sudden problem and will develop over time and every time the tank is filled then when the cap is removed there is a rush of air into the tank to even air pressure, which is your warning.  Eventually if you engine stops when out driving the first check is to remove the cap and listen for that air rush.  If this is the problem then once the pressure has equalised switching on the ignition will be greeted with a rapidly working pump filling the float chambers and the engine will now run for another period of time before the problem reoccurs. Obviously a new cap or unblocking the vent is the obvious cure.

As we follow petrol down the filler pipe towards the tank it’s worth a quick mention that there haven’t been any major issues with the hose connecting the filler pipe to the neck of the tank, but age will eventually demand a new hose.  Inside the tank is where several nasties can occur and as most tanks are made from untreated mild steel and internal corrosion can be accelerated due to the ethanol’s hygroscopic trait of drawing water from the atmosphere and air is always being exchanged through the filler caps vent. 

Phase separation does seem to occur quicker with modern fuel than occurred with fuel from previous decades, although there is no definitive time scale.  I have evidence of phase separation occurring in fuel added to the tank of an MGB in September 2021 with the car not run until a non-start in December.  Detailed examination didn’t occur until February when it still wouldn’t start.  Being an earlier car with HS4 carbs it was easy to remove the float chamber lids dry out the remaining fuel and simply add fresh pump petrol.  An instant start with smooth running followed initially but then less so as the fresh fuel was being replaced by older fuel from the tank.  The engine continued running and once warmed it would restart again, but it would not after an overnight cool down until fresh fuel replace what was in the float chambers again.  Whilst other owners have reported similar issues the age of their fuel when the problem appeared was quite a bit longer. 

The author's experience with his MGBs has been quite different and both have had summer 2021 fuel that lasted well over 18 months before a refuel.  No additives were used and there were no starting or obvious running issues.  He puts this down to both cars having fuel injected engines that sees much greater fuel agitation both in the tank from the high pressure/volume fuel pump and its associated high volume spill return providing significantly greater disturbance of fuel in the tank even when the car is stationary.  The other influence is that the injectors deliver a high pressure fine fuel spray into the engine that burns easier than the droplets delivered from a carburettor.  This seems to be corroborated by the same aged fuel in his TF also not giving problems; however he considers this to just provide a delay to when fuel degradation causes engine running problems.

The MGB owner with a non-starting engine had a full tank of fuel, (he was trying to reduce the exposed inner tank surfaces to air and reduce the fuel surface area to reduce water absorption), which unfortunately meant there was no room to dilute the affected fuel with sufficient fresh fuel to overcome the problem. Consequently this required the tank be drained that was a considerable task as it had no drain plug in the bottom of the tank and decanting and dealing with 12 gallons of fuel isn’t easy!

Interestingly, whilst no drain plug has been the norm on replacement MGB fuel tanks for many years MGOC Spares have commissioned improved tanks, now with drain plugs, for the 1965 to ’76, and the different 1976 to ’80 bolt up tanks.  (Availability for the early MGB 1962 to 1965 strap type tanks is under review). Original MG factory tanks did come with a drain plug but over time the plug would become so tight that when it was removed often it was a case that part of the tank would rip out as well! MGOC Spares tanks with drain plugs benefit from a stronger metal base where the plug fits to reduce the possibility of this problem, aided by the use of PTFE tape on the plug threads – which incidentally should be renewed whenever refitting the plug after it has been removed.

NRP1174Q - MGB (76on) fuel tank with removable drain plug, developed exclusively for MGOC Spares.

Additionally having the drain facility also now provides a practical and ideal over wintering storage option for classic MGBs as now the fuel level can be run down prior to lay-up and the remaining fuel drained and used in another vehicle where possible or responsibly disposed of.  This completely eliminates any issues of fuel degradation or ethanol drawing in water to cause corrosion and poor running.  Note that if that removed fuel has been treated with a lead replacement additive such as Classic Valvemaster Plus it MUST NOT be used in a car with an exhaust catalyst!

We now move outside the tank and discuss the traditional MGB fuel tank issue of external corrosion that has been around since the 1960’s.  On the post March 1965 flat top tanks that are bolted to the underside of the boot floor it was almost endemic.  There is a gap between the boot floor and top of the tank, (both boot floor and tank top metal are ridged to provide rigidity), so even when the tank is fully secured water and dirt was able to enter the gap and remain on the top of the tank that soon ate through the standard thin paint layer and then through the top of the tank. 

A traditional MGB problem is rusting and perforation on the top of the tank…

…that can be easily seen in this close up of just one of several areas affected.

This would on average take about five years to occur and be announced to the owner with fuel smells when around the back of the car that wasn’t coming from the filler cap area.  It would be even more obvious if the tank were filled higher than half way as very quickly streaks of dried fuel stains could be seen down the visible sides of the tank or even dripping of the tank.  What has become standard preparation for most owners when fitting a new tank to significantly extend its lifespan was simply painting the new tank with a proprietary paint such as Hammerite, with the top benefitting from extra coats (see image).  This still remains a very sensible thing to do with any original mild steel new tank that only has a weak thin paint cover that isn’t good protection.  Note specifically here that MGOC Spares range of Q Parts tanks are powder coated for improved corrosion resistance, negating the need for the extra surface protection.

For mild steel tanks with just a thin coat of paint additional protection of the top is vital to delay the damaging onset of corrosion.

Returning inside the tank sees pre 1976 tanks having a ¼” bore steel fuel pick up pipe as an integral part of the tank that emerges on the right side of the tank through a metal screw-on connection point. Occasionally, issues of internal corrosion can see this pick up pipe rust to a point causing a hole, so when fuel levels drop below this hole the pump then starts to draw just air and sound like a demented jack hammer with the engine stopping. A tell-tale to help identify this is to just add a couple of gallons of fresh fuel that takes the internal fuel level above the hole and if the car then drives normally this becomes the primary suspect. Unfortunately, aside from keeping the fuel level higher the solution is a new tank. 

A technical tip here has to be to mention that it’s essential when fitting an existing or new fuel pipe to the tanks fuel outlet connection to mechanically support the fitting when tightening the brass nut to avoid flexing of the fixed pick up pipe otherwise there is a risk of breaking the seal between the fuel outlet and the tank, which can render even a new tank U/S.  You will see on the image that these tank unions have two flats to allow a tool to be used. MGOC Spares stocks Petro Patch Putty in the event a hasty repair is needed, but can only resolve the external side of the fuel pick-up.

The 1965 to 1976 format MGB tank has a screw-on union on the right side of the tank (arrowed) It is important to take note of the main text in respect to bracing this when fitting or removing the fuel pipe brass securing nut.  (Note that this tank is a special BMH tank for fuel injection with a second lower union to provide fuel feed and spill return.

The 1976 on tanks lost that format of fuel pick-up pipe to one that became part of the sender unit that is also on the right side of the tank, so it becomes accessible and more easily replaced if needed.  Both types of pick-ups had a clip on plastic filter screen on the end of the pipe near the bottom centre inside the tank to draw as much of the tanks content, sometimes aided by some internal tank baffling. Note that the filter screen mesh can over time become blocked, perforated or become dislodged and is not replaceable.

Perforated fuel filter pick-up. What might this be doing to your fuel pump? 

Bearing in mind that the fuel storage tanks at the filling stations you may be using are not always spotless inside there is a good chance that some small muck may have been drawn from the filling station tank and dumped in yours, and clearly the more years and miles under the car's wheels, the greater the potential!  (Note that this debris may have sat for many years in a stable state at the bottom of the tank until the arrival of ethanol which has a tendency to disturb such deposits.)

This was drained out of an MGB that had a new tank after only covering a couple of thousand miles, just to illustrate what can come into it from petrol station tanks!

Over time the plastic pick up filter screen does see its flow capability reduce and any destabilised debris will be drawn to the filter screen that will reduce the flow further or even block it. (Hint: when driving up a steady hill and the engine loses power giving no further engine response with a wider throttle, but still works with a lighter throttle then this is traditionally associated with a weakening fuel pump unable to deliver the increased fuel demand needed to power the car up the hill. However, keep in mind the possibility of debris being picked up against the filter screen in the tank. Check this out by stopping; going to the back of the car, and then bounce the rear to slosh the fuel in the tank. Wait five minutes to allow any debris that was drawn to the filter screen to drift away as there is no more pump draw, and then try driving up the same hill again. If noticeably better then look at a tank clean before spending out on a new pump that may not show improvement.)

From 1976 the fuel pick up and sender unit was combined into one.  Here, the clipped on filter basket/screen can be seen on the end of the pick-up pipe.

Note that the MG RV8 used a variation of the 1976- ’80 tank pick up that had to have an open pipe end as there wasn’t sufficient fuel flow volume with a filter screen and so far these cars haven’t shown any major problem with blockages, but debris can cause problems such as seen in the image below which was very simple to remove, but getting at the pick up to remove the object required the tank to be drained. The RV8 also introduced a pre-pump filter to provide some filtering of the fuel before it reaches the more sensitive fuel injection fuel pump, something I shall come back to later in respect to retro fitting to MGBs.

This is a combined sender/pick-up from an RV8 that because of the fuel injection flow needs didn’t have a filter screen on the end of the pick-up pipe. However this item got into the tank is unknown, but the injection pump did a good job of sucking this up the pipe and stopping progress!

This sender is the 1965 to 1976 format and was in a tank that was full when the car was parked in a heated garage, but when it came out nearly twenty years later, there were just five litres of rank fluid left, and rust was extensive.

Watch this space! In part 2 coming soon, we will be moving on to covering the fuel pump, pipes & hoses.  

Please Note: The information provided in this blog is intended as a general guide only. While we aim to ensure accuracy, classic vehicles can vary, and procedures may differ depending on model and condition. Always consult your vehicle’s official workshop manual before carrying out any work. If in doubt, seek the services of a qualified professional.

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