This is the last 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.
Carburettor Wear
Moving parts in the carbs will always develop wear, but since the arrival of unleaded fuels, some fuel metering metal parts wear rate has increased. I have already mentioned the float valves, but the ‘biased’ (spring) needles in later HS and all HIF carbs also wear. These are designed to see the needles' spring-loaded effect rub on the engine-facing side of the main jet, so as the piston rises and falls with different engine loads, the constant rubbing generates visible oval wear over higher mileages. In previous decades, the ‘lead’ in leaded fuel would provide lubrication for the carb components just as it did for the engine's valves and valve seats, although the increased wear rates in the carbs using unleaded fuel is not huge.
Nevertheless, when doing the Carb Spring Clean (See Part 3), look closely at the main jets and if your eye can see any ovalization of the main jet hole, then that is actually significant wear and fuel metering will be negatively compromised. This is more of an issue with the spring-biased needles as they are designed to rub the same side of the main jet all the time, and wear rates have increased since the retirement of leaded petrol. Naturally, you will now need to replace the main jet, but you must also replace the needle with the correct one. All of these spring-biased SU needles have an individual three-letter code, whilst the earlier shorter fixed needles have one or two letters/numbers, which identifies the individual taper of those needles, and you find this ID engraved on the fatter upper shank of the needle. This obviously needs the needle to be removed to read this, so do note the correct fitting position. On the fixed needles it is simpler with the shoulder of the needle being level with the base of the piston, whilst for the later spring biased needles, the tube shaped needle securing collars has to be level with the line of the bottom of the piston, which leaves the bottom of the needles brass sticking out a miniscule amount.
Twin carbs need to be balanced in a number of ways to ensure that the engine runs smoothly and efficiently. Here the screwdriver is pointing to one jet that has not been correctly fitted in the piston.
Note; my experience has shown some carbs are fitted with different needles (different codes), a definite no-no and begs the question of what else may have been messed with? With access to the needles currently present, it’s an opportunity to remove both needles and confirm what is fitted. MGOC Spares stocks a wide range of needles to cater for original and performance fuel metering requirements. If you’re unsure as to what should be fitted, give MGOC Spares a call to discuss your requirements, noting any performance upgrades which demand increased fuel delivery for good combustion.
Spring-biased needles are noticeably longer than ‘fixed’ needles and will automatically sit in the same correct place and do not require jet centring as is needed with earlier ‘fixed’ needles, which have to sit centrally in the main jet's hole. (Fixed needles if not centred will see the piston not readily fall to its rest position onto the bridge of the carb and will be a major upset of engine running. A simple test with fully assembled carbs but with air filters removed and both carb dashpot damper pistons undone is to lift both carb pistons to their maximum height with your fingers and then release both at the same time. They should fall at equal speed, and effectively, both should deliver a simultaneous clunk as they contact the bridge. Hearing two separate clunks is a signal that more checking and adjustment inside the carbs is needed – see the Workshop manual for guidance.)
The longer biased needle was introduced as it is able to impart better fuel control to promote more consistent exhaust emissions over high mileages. Being able to meet stricter exhaust emissions was the forced driver for many of the 1970’s SU carb changes, which forced the HS carb into retirement with it being replaced by the HIF, and even that wasn’t enough for US market cars that were eventually strangled by a single Zenith Stromberg carb and in some States a catalyst as well.
Another common area of wear on all types of SU carburettors is around the throttle spindle, which carries the throttle disc that has to have very precise contact inside the carb body for accurate and consistent operation. An indication of excessive wear comes when, during driving, the throttle is released and the idle speed settles to different levels with different throttle releases, demanding a quick tap of the throttle pedal to settle a high idle speed to normal. This often signifies wear in the throttle spindle bush and the contacting area of the spindle. This will require both to be renewed and is quite deep work, and if this has to be done professionally - and often this the better route - then the higher cost raises the question of what else needs to be done and would it then be better overall value to go for a professionally fully reconditioned pair of carbs? Certainly, reconditioned carbs are well under half the cost of a pair of brand new if stocks are available, although brand new should be the best option. MGOC Spares offers replacement carburettors to suit on exchange and new, along with carburettor rebuild kits and service kits.
AUD325P - HS4 Carburettors MGB 1969>71 (Available on an exchange-only basis)
The worn throttle spindle area and other wear areas around the carbs and inlet can be easily highlighted by the use of carb cleaner or similar brake cleaner sprays when the engine has just started from cold. Spraying around any area where there is an air leak, throttle spindles, gaskets, loose unions, cracks etc., will see some of that spray being drawn into the engine and burnt, which very clearly alters the engine running sound and often its rpm. This can be repeated time and time again for confirmation of the problem and the best time for this check is when the engine is cold, as any gaps will be wider as heat expansion has yet to take place.
My final wear area to look at with the carbs is the operation of the choke, officially called the cold start enrichment. In the HS and earlier SU’s, the choke function is a simple lever system that moves the main jet lower so effectively widening the space between its inner hole and the needle, allowing more fuel to pass. As the external choke linkage creates the downward movement of the main jet, a cam system connecting the choke and throttle linkages causes the throttle to be opened slightly to raise idle RPMs to ensure that the richer mixture doesn’t cause the engine to stall. This is an almost entirely reliable system of operation that rarely needs any attention.
The HIF SU carbs do not have this method of fuel enrichment, although they do retain the same type of choke-to-throttle linkage to raise idle speed during choke operation. Because of the reliability seen in the earlier HS choke system, the alternative and quite hidden enrichment system for HIF is usually forgotten. In the HIF, fuel for fuel enrichment passes through a completely separate port inside each carb that feeds extra fuel into the incoming airstream near the main jet hole. The fuel flow is controlled by a rotating valve mounted inside the carb body and its rotation is controlled by choke linkage movement. This rotating valve is machined so that when in its off position, no fuel is allowed to pass, but as the choke is operated, this valve turns and exposes the machined part of the valve that allows extra fuel to flow. As the valve is turned more, then even more fuel is allowed to pass.
The rotating valve is sealed from allowing fuel to leak by way of small rubber O-ring seals and this works fine when all is in good condition. However, as I mentioned, this system is hidden and only in the case of an obvious leak of dripping fuel out of where the valve is inserted into the carb body would attention then focus. Unfortunately, these rubber O-rings on older carbs will, over time, degrade and that degradation will be accelerated further by ethanol in fuel. In most common fault cases, when the choke is in the fully off position, fuel can leak past worn O-rings into the engine. This is because there is suction from the running engine and this unmetered fuel will enrich the mixture, wasting fuel and making accurate tuning impossible.
Clearly, any issues here demand that the seals and O-rings be replaced, and two per carb, or find them in a full carb rebuild kit at a significant cost, as that includes throttle spindles, bushes, throttle discs, etc.. The wildly different cost means you should make an assessment of your carb's actual condition before ordering parts.
Of interest, small O-rings are also found on all HS float chamber to main jet pipe where it is held in place by the nut that screws into the base of the float chamber. These normally last a very long time and usually only leak after disturbance, which is often accompanied by damage due to incorrect refitting. HIF carbs with their integral float chamber do not have these, but they do have extra seals around the throttle spindles that HS types didn’t have.
A parts diagram of the SU HIF carburettor with the choke seal indicated by the yellow arrow as part of the sub assembly numbered 22.
A point to note is that there are two routes for MGB choke cables that predominantly fall into the earlier configuration of an underslung cable that comes up from under the carbs, with the cable exiting through a lower hole in the left side bulkhead. This compares to the late cars' overhead approach, where the cable emerges much higher up the bulkhead on the left side and comes in over the carbs.
SU HS4 and earlier HIF4 carbs had an underslung choke cable here indicated by the blue arrow, with the throttle cable shown by the red arrow.
This 1978 MGB shows the overslung choke cable shown by the blue arrow, whilst the throttle cable is indicated by the red arrow.
Another short divergence sees some MGBs have what are known as ‘overrun poppet valves’ in each carb's throttle disc. These were an emission-driven addition that sees the spring-loaded valve opened by a sudden lowering of pressure on the engine side of the throttle disc when the throttle is suddenly closed. The valves being drawn open allows small amounts of air to enter the engine, and this deters the creation of some toxic gases. The big downside is that the springs are often weak and can see the valves open when not needed, which simply makes for a high, fluctuating and uncontrolled idle speed. A very long-standing cure is to either renew the discs (WZX1329) or replace them with plain ones (no spring – WZX1323) or to solder the valve shut..
This HS4 carb has its throttle wide open to clearly show the overrun poppet valve with integral spring.
It's worth mentioning that the MGB didn’t have these poppet valves very often compared to the 1500 Midget, which was universally afflicted with them and so suffered much more from the use of these poppet valves in their twin HS4 carbs. They also suffered the use of Waxstat main jet control, ostensibly to maintain stricter fuel mixture control, as it was intended to offer automatic adjustment as temperatures changed. As a result, a sound and common conversion is to replace Waxstat jets with standard via conversion kit WZX978, from MGOC Sapres.
WZX978 - Midget 1500 Carburettor Conversion Kit
Following fuel from the filler cap to the point where it is being drawn into the airflow into the engine provides a summary of common problem areas of the MGB. It’s not fully exhaustive, although it covers most common aspects, but I will end with a tangential question that is often connected to worn carbs and something that demands the same detailed assessment as wear in carbs, especially if you intend to invest heavily in a carburettor rebuild or replacement. This is to remember the distributor, which, like the carbs, has a very fine detail control over engine function and often will have done the same mileage as the carbs and will also benefit from similar refurbishment. However, a detailed discussion on distributors and the rest of the ignition system is for another day.
I will end with a few words on the subject of winter lay-ups, which, for the few months that keep cars off UK roads, should not create any significant problems, but to provide some added insurance from fuel breakdown, I would suggest using a fuel treatment such as Etha-guard Plus. It can also be very good policy to run the level in the tank down, which makes draining the tank for winter prep easier if your tank has a drain plug. Tanks with no drain facility means it’s a much more tedious process, but the lower the level, the easier it is to dilute the stale fuel with fresh. MGOC Spares manufactures tanks with drains, making the process far easier today than in years gone by to lay-up your car.
There is also a suggested policy of disconnecting the fuel pump power supply so the engine can be run until the residual fuel in the float chambers is reduced to the point there isn’t enough fuel to run the engine that will help reduce dried fuel deposits that would need to be cleaned out next ‘Spring Clean’.
That rounds up our four-part series outlining the common issues with the MGB fuel system. To look back on previous episodes, follow the links below:
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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