Part two in a four part series covering many of the common problems and some of the solutions often found within the MGBs fuel system, and many of which can also apply to most other MGs running SU carbs.
Fuel pump, Pipes and Hoses
Fuel exits the tank through the pick-up pipe under the draw created by the SU fuel pump that is mounted to the car in one of two places, on the back of the right side bulkhead behind the right hand seat, forward of the axle and next to the right hand battery box for chrome bumper cars. This was moved to the boot bulkhead behind the rear axle on rubber bumper cars, and interestingly has the electrical end of the pump poked through that bulkhead into the boot to the right of the spare wheel with a protective steel box cover inside the boot.
The MGB up to 1976 saw the fuel pump mounted on the axle side of the bulkhead on the right side of the car
Obviously, detail differences are found with the make-up and runs of pipes and hoses carrying fuel from the tank to the pump, and pump to engine. A simple approach can be adopted for all fuel hoses is to say that if it looks hard, aged or showing any cracking or other degradation then just replace them all with appropriate new E10 resilient hosing that is carried by MGOC Spares.
We should all now be aware of the damage that ethanol in fuel does to some materials and unsuitable fuel hoses. The hose shown was new four years before fuel started leaking and once removed it simply broke up.
Being as most MGBs will have possibly had several pump replacements over the years you must not assume that the pump fitted is an SU fuel pump with original fuel lines and hoses, SO do check what is fitted to your car before ordering any new parts, as often owners order a new pump only to find that the hoses connecting the currently fitted pump on the car do not fit the new pump! A simple referral back to the MGOC Spares parts catalogue will be your guide on what was specified by the factory.
MGB SU fuel pump 1968 to 1980
This chrome bumper MGB has had a new fuel pump but the hoses are not correctly fitted.
The 1977 to 1980 MGB has the fuel pump relocated so that the electrical end of the pump sticks through into the boot. This is not an original SU pump to illustrate that you need to know what you have before ordering replacements.
Pump replacement often follows a complete pump failure to deliver any fuel into the engine bay with a fuel feed hose removed from a carb and pointed into a container when the ignition is switched on, but remember the check I mentioned in Part 1 related to possible fuel tank pick-up filter screen being blocked, or if the pump draws air it runs like a demented jack hammer because it is only pumping air, check before going out and spending on a new pump only to fit it and have the same situation. (Note that a pump running like this can quickly cause internal damage and this can render a new pump defective immediately.)
Whilst a simple approach applies to hoses there are more options in respect of fuel pumps that create a need for additional considerations. The original pumps fitted to an MGB are SU ones that have the very important characteristics of a sympathetic fuel delivery that works well with the sensitivities of an SU carburettor. When the ignition is switched on after the car has stood for a few hours the SU pump generates the very traditional rapid tick, tick, tick, as fuel refills the carbs float chambers, then slowing to a tick every ten to twenty seconds once the float chambers are full. If it is an SU pump and it continues to tick at a faster rate then do check for signs of leaking fuel under the carbs, or on the ground under them, or if no signs near the carbs look under the car. Carb float chamber overflow is an issue I will expand on in the next episode. (Note that some non SU pumps also deliver ticking sounds so don’t assume a tick means it is an SU pump!)
If an SU pump operation is very slow then check the white power feed wire connected to the pump is showing 12v+, or certainly within 0.2v of the current battery voltage, (i.e. over 11.5v and ideally towards 12.5v that equates to a charged battery,) when the ignition is on, which is to confirm there is a good electrical connection to the pump. You also need to check the earth path which is probably easiest by using a spare piece of wire connected between the metal body of the pump and a good clean part of the car's body. Any issue with the normal earth path would be corrected by the temporary connection and will deliver an immediate clear improvement in pump activity.
If an SU pump doesn’t tick when the ignition is switched on then a very long standing roadside if temporary cure was, and still can be, a sharp tap on the main metal body with a suitably hard object for example the head of a wood or plastic screwdriver or small pin hammer. This sharp tap sends a shock wave into the SU pumps points to vibrate the points as these can stick together and was a common reason for non SU pump operation with success displayed by the pump starting to run. In recent decades points have largely been replaced by electronic switching in the same vein as electronic ignition conversions replacing distributor points, and with no points to stick this tap doesn’t help here, but, with another possible issue being stuck one way valves inside the pump this can free them. Tapping a non-functioning SU pump may be traditional but has always been a temporary, get-you-home method and is always a warning to sort or change the pump.
It’s also worth noting that the sticking one-way valve problem is usually caused by dried fuel deposits acting as an adhesive that do not readily dissolve when in contact with fresh fuel, so putting a brake hose clamp on a hose between the tank and pump to stop any syphoning of fuel from the tank to pump when you then remove the pump hose connections. Then give both inlet and outlet sides of the pump a good spray of carb cleaner, as this fluid instantaneously dissolves dried fuel deposits. Note if there is an issue here then it is very likely that there will be similar problems at the carbs that I shall mention in the next instalment.
If the pump is still non-functioning after these checks then whilst you can buy individual spare parts for an SU pump, generally speaking, you get longer service from a new pump, but there are also other considerations. I suggest retaining a points switching type is not an ideal choice when well-proven electronic switching has been around for so many decades. There are other considerations related to the age of the car as MG used positive earth polarity up to 1968 and then moved to negative earth. Very many positive earth cars have been converted to negative earth in the intervening decades, so before ordering a new pump you must confirm the car's polarity unless buying a dual polarity pump. This should be easy by noting which terminal the earth strap cable connects to on the battery, the one that bolts to the car's body next to the left side 6v battery, actually connects to. Battery posts should be marked + and – but in lieu of clear marking look at both battery posts and note that the positive post is always clearly fatter.
Another consideration I mentioned earlier is that the fuel pump has to operate in sympathy with the SU carbs fitted and whilst volumes of fuel moved by any of the standard SU pumps per hour varies the pressure that it is delivered at is a very important consideration with the ideal delivery pressure being 2.5 to 3psi for earlier SU HS carbs (ones with external float chambers) as this type can’t cope with higher pressures as well as the HIF integral type SUs that can usually cope with up to 4psi.
When considering a non-original fuel pump you need to note its delivery pressure as SU carbs can’t cope with many non SU pumps delivery. This will usually show in repeated leaks from the float chambers even after new float valve are fitted. Rather than buy a new pump a more economical route is to fit a fuel pressure regulator and many provide a bonus of a replaceable filter too.
Whilst SU pumps are made to work with SU carbs many non-original fuel pumps that can be fitted often see too high operating pressure, or pressure pulses generated by the pump are enough to unseat one carbs float valve and see too much fuel into that specific float chamber. Note that excess fuel pressure will only affect one of the two carbs float valves as the weaker valve relieves the excess pressure on the other carbs float valve. This extra fuel will raise the fuel height in the affected float chamber and overly richen the mixture, especially at low rpms and low engine loads making correct carb tuning impossible. If the over-pressurisation is significant then it shows very quickly with overflowing fuel out of the affected carb overflow.
As SU pumps do operate sympathetically with SU carbs they are a very sensible option, although perhaps not so attractive on price. The post 1968 negative earth pump being the most common found on original MGBs (AZX1307 MGB Fuel Pump) with for only a modest extra £20 the electronic version of this pump (MGB Electronic Fuel Pump Negative Earth). There has always been a wide variety of alternative fuel pumps that are generally cheaper than an SU pump they have often had a wider range of operating pressures or other shortcomings and have always tended to generate more issues from too high delivery pressure. However, one non SU pump that has carved itself a justifiable positive reputation and a very good alternative to an SU pump is a Hardi pump. One example for 1968 onwards MGBs is this one (MGB Hardi Fuel Pump), which offers full electronic control, dimensions that are intended to allow simple substitution of the original SU pump, quite significantly increased volume that can even cope with a tuned larger capacity Rover V8 conversion on carbs, so any four cylinder MGB engine won’t present any problem, and best of all, proven reliability at a significantly lower price than an original spec SU pump.
MGOC Spares has worked with Hardi to develop an exclusive pump (MGB Hardi Fuel Pump) to deliver the required working pressure not found with other variants. Additionally, the HARDI pump is self-balancing, unlike the SU pump which requires a vent to atmosphere, (by way of washer tubing to that odd plastic Tee piece in the boot). On SU pumps mounted under the car this vent can be missing or blocked, preventing the internal pump diaphragm from moving back to its rest position.
This 1980 MGB has a Hardi electronic pump that doesn’t need the internal venting the SU original does so the small washer-size hose is left hanging...
AZX1307Q - Hardi Dual Polarity Fuel Pump suitable for MGB 1965-80 *MGOC Spares exclusive
…from the plastic tee piece that many owners wonder what it is for in the boot (arrowed).
My last comment around the fuel pump is to ensure that the rubber mounting block (grommet) between the pump and its mounting bracket has not gone hard as when it does its function to insulate pump noise from entering the car is compromised. For a current cost of under £3 a new grommet bought with any replacement pump is a good idea. (Note the HARDI pump from MGOC Spares includes a correctly sized fuel pump grommet for external mounting as standard (not required for rubber bumper- boot mounting).
Previous poor workmanship is an ongoing problem where on this car the battery was changed for a single 12v in the right-hand cradle and it was decided to fit the replacement pump in the now vacant left battery cradle. However, the routing of the fuel pipes and hoses was dreadful and at risk of being caught by the propshaft, and…
…hoses ran through metal holes with no grommet protection!
We now follow the fuel as it is pumped towards the engine into the underfloor fuel pipe. I have mentioned how to view hoses but now we consider the underfloor metal fuel pipe that in standard form is made from mild steel that usually had no specific additional anti-corrosion protection so it will rust especially in the hidden areas underneath clamps and covers that hide it from view. It is clipped to the underside of the right side floor panels and accompanies the main battery to the starter cable, the rear wiring loom and the front-to-rear brake pipe. When looking to replace steel fuel pipes with steel you need a forming tool to bend it to achieve smooth non-restricted bends. Without the tool, it is very difficult to form by hand without serious kinks, which can lead to flow restriction or cavitation – air bubbles forming in the fuel supply. This is why replacement fuel pipe has usually been to use complete fuel pipe kits offering a double benefit; the pipe is a copper alloy that is both corrosion resistant and quite easy to hand bend without kinking to follow the shape of the car. (Fuel Line Kit MGB HS4 is one example of the available kits.)
The original metal fuel pipes on an MGB were made of mild steel and so would rust. Replacing these with steel needs a forming tool to create non-kinked bends, whilst copper alloy replacements can be easily formed by hand without kinking and are corrosion and ethanol resistant. The copper alloy pipes on this MGB tank is for a fuel-injected engine and incidentally, a coil spring rear suspension so the pipe doesn’t get chopped by the leaf spring.
In part 3 we'll be covering the fiddly business of carburettors.
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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