Factory Carburetors and Injection systems – An Overview

Factory Carburetors

Having now been involved with the development and maintenance of Classic and Neo-Classic BMWs since the mid-nineties, we have had the pleasure of dealing with literally hundreds of carburetor inducted BMWs from the 60s, 70, and 80s. We all know that the quality of BMWs during these periods is high – engines, transmissions and overall build quality are generally very good, with most components lasting far beyond their original design life.

It appears however, all these vehicle have the same Achilles heal – the factory fitted carburettor.

Whether you have Zeniths, Solexs or Pierburg carburettors fitted to your car, one rule seems to be constant:- By the time your vehicle reaches 80k-90k, the factory fitted carburettor will start to give problems, the basic underlying issue being one of quality.

All the factory carburettor we have ever come into contact with, suffer from ‘free-air’ ingress. This is where unmetered, external air is pulled into the venturi via worn carburettor casings, resulting in weak overall fuel mixtures and high hydrocarbon emissions. The biggest culprit appears to be worn butterfly spindle supports in the aluminium casing.

Manufacturers such as Weber, often use phosphor bronze bearings to support the butterfly spindles, as they open and close with throttle actuation. These bearings are self-lubricating and don’t wear as quickly as the aluminium casing around them. With BMW’s own factory carburettors, the butterfly spindles tend to be supported by the cast aluminium material of the carburettor body. Over time and use, these pivot points ovalise and wear, allowing outside air to be pulled in.

Rather than talking about each individual model and carburettor type separately, we can safely impose a blanket rule regarding all these vehicles: – If the car still has a factory carburettor fitted – replace it with a Weber conversion. It is the best single modification you will ever make to your vehicle. Fuel economy, reliability and performance will all be improved by the application of these ‘better’ quality carburettors.

Don’t even entertain trying to rebuild any of the factory fitted carburettors – the success rate is extremely low, and you will waste time and money discovering the very situations described above.

Factory Injection systems


Fitted mainly to the 2002Tii, the mechanical Kugelfischer system has seldom caused us much trouble here at Fritz, when fitted to standard engines. The components appear to be of good quality overall, and last well, as long as servicing and maintenance schedules are followed correctly. Fuel filters need to be changed every 40K, along with cleaning of the fuel inlet strainers on the tank pick up and distribution pump.

The positive displacement distribution pump does have a bit of a tendency to seize up internally when left for years without use, but a simple strip down and lubrication seems to bring them back to life in most cases. If you ever have the need to replace this pump, either due to failure or internal wear, be careful to acquire the exact same model for the replacement, as there are several different versions, which are not inter-compatible with each other. These differences are mainly due to throttle linkages and throttle body type, as BMW slowly evolved the system throughout the life of the Tii model. Check out 2002tii.org for more information on this subject.

The Kugelfischer system, we have found, is not too sympathetic to variations in engine specification. Introductions of up rated camshafts or higher compression pistons can play havoc with fuelling across the engine’s rev range, and often will require alterations in fuel delivery pressure or an up rated distribution pump in order to balance things out.

Bosch D-Jetronic

First fitted to E3 and E9 models from 1972 , it was Bosch’s first proper attempt at electronically governed fuel injection. The overall system is pretty reliable, with most components having lasted pretty well over the last 40 years.

The system uses an inlet manifold vacuum unit to measure engine load, which then transmit’s the information to the ECU (often positioned underneath the back seats) which operates the injectors and fuels accordingly. The main issue we have noted here at Fritz, is that the vacuum unit seems to slip out of adjustment over time, resulting in a rich mixture throughout the rev range. Looking at the unit, there appears to be no real method of adjustment available – however it is possible. At one end of the unit, there is an aluminium blanking plug, with a groove cut into it, much like the head of a basic wood screw.

With a drill and 6mm drill bit, carefully drill through the plug, which is about 8mm thick. Once through the aluminium, a brass adjustment screw will be revealed. Adjust the screw, with the car running on a gas analysers, and you should be able to alter the CO content of the emissions back to sensible levels. Remember also to check the CO levels at higher revs too, usually setting up the mixture at a static 3000rpm gives a good indication that the fuelling will be right across the rev range.

Apart from the above issue, failing temperature sensors, miss adjusted throttle bodies, split induction pipe work and the odd dirty electrical connection are the only other faults that we ever see with this system. In terms of coping with engine modifications, the D-jetronic system, because of the way that it interoperates engine load, is quite forgiving. Available adjustments on the system allow you to effectively balance the increase in fuelling necessary when increasing displacement or general specification.

Bosch L-Jetronic

First introduced in the Mid-70s, L-Jetronic was Bosch’s first ‘Air Flow’ controlled injection system. Utilising a sprung loaded flap meter to measure the amount of inlet charge being drawn into the engine, the signal is then transmitted to the ECU, which in turn dwells 2 banked sets of injectors by use of a reference table contained on a hard wired ROM chip on the ECU motherboard. These early systems seem to be quite robust, only really suffering from splits in vacuum tubing, the same as most other negative displacement systems, and failing temperature sensors.

The AFM (Air Flow Meter) do tend to suffer a bit after large mileages, the swinging contact arm, under the black cover on the AFM, do have a tendency to wear the resistance track over time, resulting in ‘flat spots’ during normal driving, as the load signal to the ECU is lost at certain points – usually from just above tick-over to around 3500rpm is the normal range to experience such a fault.

Injector operation frequency is triggered by the low tension (switch side) of the coil, with the signal being delivered to the ECU by way of a separate grey ‘jumper’ wire to the main ECU plug. If this additional spade connection to the ECU becomes corroded or loose in its fitment, it is common to lose the signal, resulting in a total loss of injector actuation. Check and clean this connection regularly, especially on M30 powered E12s and early E24 models.

Bosch LE and LH Jetronic

The ‘next’ generation of Bosch’s L-Jetronic family was introduced on to the BMW range of vehicles in early 1982. LE-Jetronic found its home on the M10 1.8 and M20 2.0 and 2.3 engines fitted to newly introduced E30 and E28 models. Using a new smaller ECU to operate the injectors, impulse frequency was now taken from a phase sensor mounted internally in the distributor, rather than the coil, as with earlier L-Jetronic systems.

As with previous Bosch Jetronic systems, the main components of the system are fairly robust and reliable, but due to they system’s over complicated recirculating breather and air bypass system, vacuum leaks are a constant menace, with split and perished rubber pipe work being the main protagonists. The cold start system can also cause problems, due to the thermostatically controlled Air Bypass Valve, which is hidden under the inlet manifold. These valves tend to fail in the ‘shut’ position, making the vehicle start properly from cold, but resulting in ‘lumpy’ running and poor fuel economy when the engine is up to standard running temperature.

The LH-Jetronic system, fitted to the 2.5 and 2.8 M30 engines from 1982 on, in E23, E24 and E28 models, is marginally easier to live with than the LE-Jetronic system, due to less unnecessary rubber pipe work and a solenoid operated air bypass valve, improving reliability so much that the system was used throughout the 1980s on all these engines.

Neither of these systems allow you to remap or change the fuelling aspects through the ECU’s electronics, as both rely on size of injector and distributor speed to manage the fuel mixture. It is possible however to balance the fuelling of a modified engine, by altering the delivery pressure in the fuel rail and recalibrating the AFM internal spring tension, although the range of ignition advance curves available from the factory distributors does hamper serious engine tuning.

Bosch K-Jetronic

If we were being particularly unkind, we would say that K-Jetronic is probably Bosch’s worst attempt at an injection system, but unfortunately we weren’t about in the mid-70s with any better ideas, so we will keep our opinions to ourselves! Especially as the system seems to have worked so reliably on other manufacturer’s vehicles, with VW, Mercedes and Porsche all using the system to good effect throughout the 70s and 80s.

The system is a non-electronic, vacuum operated set up with a pressurised fuel distribution head to supply each cylinder separately. The engine pulls vacuum through the throttle body, operating a ‘trap door’ type arrangement on the fuel distributor, which in turn acts upon a plunger valve which controls the fuel flow into the distributor, and then on to each injector.

Due to the mechanical actuation, the induction is greatly compromised by the fuel distributor, restricting the speed of the inlet charge, and as a result, reducing the available torque from the engine. The fuel distributor itself, is prone to wear and in cases where the car has been stored for some time, the plunger valves seize in to their housings.

The cold start sequence is controlled by a thermostatically actuated enrichment unit on the engine block. These units, like any physically controlled thermostat unit, have a tendency to fail in time, resulting in huge amounts of over fuelling, or even no additional fuelling at all when the engine is cold. Moreover, they are also disproportionally expensive to replace.

Thankfully, this system was only used on a few BMW models, E21 318i and 320i (M10) and 323i, as well as some late E12 520i and early E28 520i models. From what we at Fritz, have experienced over the last 15 years, the 4 cylinder versions seem to be a lot more reliable than the 6 cylinder incarnations, but for no real reason that I can quantify, perhaps under bonnet heat has a part to play.

Tuning the system for greater power output is difficult and can be a bit hit and miss. I would recommend ‘How to tune Bosch fuel injection system’ by Ben Watson (ISBN 0-87938-570-7) as a guide to playing with the K-Jetronic system. Mixing and matching K-Jetronic components from other manufacturers cars can be done, but it can also reduce power outputs if you don’t understand the theory behind the system. For example, using larger fuel distributors housings from either Porsche or Mercedes vehicles will not give you any advantage on standard BMW engines, because in the main they have been designed to run with higher compressions and displacements, so applications of such items actually slow the inlet charge speed, reducing power and torque outputs.

Bosch Motronic Systems

Motronic First Generation and V1.1

First fitted to E23 and E24 M30 powered models at the start of the 1980s, it was Bosch’s first attempt at a combined fuelling and ignition control system, which was revolutionary at the time. Using the signals from position and speed sensors placed in the gearbox bell-housing and reading from reference points on the flywheel, the Motronic ECU controls the switching speed to the coil to actuate advance and retard sequences whilst the engine is running. The duty sequence of the two banked sets of injectors is triggered by the same sensor information, with the dwell time controlled by the engine load signal produced by the AFM. Cold start enrichment and air bypass are governed by temperature sensitive sensors positioned in the thermostat housing, and the same solenoid activated air bypass valve found on the LH-Jetronic set up.

Version 1.1 was released about 18 months later, using pretty much the same components, but with a revised and more compact ECU design. Both systems suffer with the same faults overall, speed and position sensor failures result in loss of spark, or in some cases when it is just the speed sensor that fails – fuelling and spark generation happens at the wrong time, causing ‘backfiring’ and loss of power. The failure of the temperature sensor in the thermostat housing, which supplies the signal to the ECU can also be a common issue, resulting in massive over-fuelling when the engine is up to normal running temperature. The ECU themselves, can be a little fragile and prone to internal condensation, mainly due to the ridiculous places that BMW saw fit to mount them, especially on the E23 – fitted behind the right hand A-pillar, behind the speaker – genius!!

Bearing in mind that the First Generation was only available for a very short time, and Motronic 1.1 was phased out in late 1984, replacement ECUs are becoming few and far between, and can be very expensive. However, it is easy to upgrade the Motronic 1.1 to the Motronic 1.2 system if you get stuck. The early ECUs are unfortunately not remap able, as the referenced ROM chip on the motherboard is actually soldered in – another situation which can be remedied by an upgrade to Motronic 1.2.

Motronic 1.2

Motronic 1.2 appeared in late 1984 across Europe, using many of the same components as the previous Motronic systems, but with a revised AFM, ECU and air bypass system. The breather and air bypass pipe work were marginally simplified, making maintenance and fault diagnosis a bit easier. The revised design and positioning of the new solenoid Air Bypass Valve, improved cold starting greatly, as bypass airflow was now metered more accurately, with use of a new temperature sensor which offered greater sensitivity and a larger signal range.

Motronic 1.2, as well as replacing the previous Motronic systems on the larger M30 engines, now found its way on to the 2.7 M20 ETA engine (which previously had a very odd version of Motronic 1.1) and the newly released E30 325i, to great effect. The revised design of the Motronic 1.2 ECU offered a removable reference ROM chip for the first time, initially a 24pin EPROM and after 1986, a 28 pin version. Additionally the 1.2 ECU offered ‘multi-mode’ capabilities, when using the factory EPROM, with the adjustment of a potentiometer, through a hole in the ECU casing.

The potentiometer offers 4 modes, from left to right –

Position 1 – Standard fuelling and ignition map Position 2 – +6% ignition advance Position 3 – +6% ignition advance and +10% injector dwell Position 4 – -6% ignition advance (when compared to standard)

Please note :- If you are planning to change the ECU mode, remember to re-tune the AFM afterwards.

Motronic 1.2 suffers with very similar faults to previous versions of the Motronic systems, with ignition sensors and temperature sensors being the most common faults. The Air Bypass valve also benefits from regular cleaning, as oil vapour and sludge tends settle around the control flap, jamming it up eventually.

Motronic 1.3

Introduced in late 1987/ early 1988 in Europe, BMW fitted this new revised version to its entire range of 4 and 6 cylinder engine ranges. Now simplified even further, a single TDC sensor now supplied the crankshaft speed and position data to the ECU by referencing a castellated front vibration damper, thus doing away with the troublesome bell-housing mounted sensors, improving overall reliability.

The revised ECU now offered fault code retention, so that primitive diagnostics were now possible, highlighting faults with individual components, by way of return signal failure to the ECU. The vast majority of the other components on the 1.3 system remained the same as the previous 1.2 system. The ECU retained the removable ROM chip system from the 1.2 version, allowing for easy re-mapping, making it possible to modify both fuelling and ignition to suit physical engine variations.

Failing temperature sensors and sticking Air Bypass valves are still the main issues with long-term reliability, as well as the perennial split and perished vacuum and breather hoses. If you have this later system fitted to an M20 engine of any type, the connection between the injector harness and the rest of the engine loom (plug located under the inlet manifold) can cause major issues – mainly due to corrosion of the pins contained within the plug. This can manifest itself as a total loss of injector activity, or, sustained dwelling of the injectors, resulting in poor ‘warm’ start and over-fuelling of the engine during normal operation.