What carburettors do and how they work.
Mixing fuel and air in the right proportions at the right time is key to the operation of an engine — although petrol is a volatile liquid, it needs to be mixed with air in the right quantities to ignite. To be clear, the air-fuel ratio in an engine is the ratio of air to fuel in terms of mass. The ‘perfect’ ratio (defined as comprising the right amount of air to completely burn all of the fuel) is known as a stoichiometric mixture.
For petrol, this is 14.7:1, which means, for every gram of petrol, 14.7 grams of air is required, not that classic Porsches have to run with such a mixture. It’s really only necessary for proper catalytic converter operation, though it is something to be aware of for the purposes of this article. It’s also worth noting lean operation is when the air-fuel ratio is higher than 14.7, while rich operation is when the ratio is lower.
The carburettor is an ingenious mechanical design that actually pre-dates the automobile itself. Its job is to combine fuel and air in the right ratio and deliver it to an engine for combustion. To understand carburettor operation, imagine a simple cylindrical tube (the carburettor barrel). The top is open to the atmosphere and the bottom is attached to a petrol engine’s inlet manifold. This design is what’s known as a downdraught carburettor, the only type used in classic Porsches.
Just above the manifold, at the bottom of this pipe, is a circular disc rotating on its axis to control airflow through the barrel. This is a butterfly valve, referred to as the throttle valve, and is opened and closed by a linkage connected to the accelerator pedal. Here’s the clever bit.
The barrel is full diameter at the top and bottom, but features a narrower section in between, usually with a smooth transition from one to the other. As air flows through this restriction, it speeds up, which reduces what’s referred to as static pressure. This is known as the Venturi effect and is why this part of a carburettor is referred to as a venturi. The reduced air pressure in this section of the carburettor sucks fuel from small holes in
the circumference of the barrel (fed by a fuel reservoir) into the airflow and therefore into the engine. Note that there’s no direct link between the fuel supply and the accelerator pedal. This is why the latter is often referred to as a ‘throttle pedal’ — all it does is open and close the throttle valve, hence the design of the air holes from the fuel reservoir to the barrel are critical to the carburettor’s operation. You may already know they’re known as ‘jets’. We’ll return to them momentarily.
THE JET SET
Incidentally, the fuel reservoir is an important aspect of the carburettor’s design. If the jets were directly linked to a pressurised fuel supply from the tank, there would be nothing stopping fuel flowing at all times. Instead, the jets are connected to a reservoir of petrol with the top open or vented to atmospheric pressure. In other words, fuel will only flow into the barrel of the carburettor when the pressure within the venturi drops below atmospheric pressure. The reservoir itself features a float on an arm that, much like a cistern float in a toilet, opens or closes a valve allowing fuel to top up the reservoir. The supply to this valve comes from the fuel tank via a simple electric pump.
So far, we’ve described an extremely basic carburettor and, if an engine operated at a fixed speed and load within a narrow temperature window, we could probably leave our lecture there, but a car engine obviously has to cope with a range of conditions, and the carburettor has to cope with them all. Cold-starting, for example, is a particular challenge. In this scenario, the engine components are cold, meaning evaporation of the fuel isn’t as efficient and, generally, a richer mixture is required to get the engine to fire up. That’s where the choke comes in. It takes the form of an extra butterfly valve in the barrel, this time on the outside air side of the venturi, which ‘chokes’ the flow. This has the effect of reducing the pressure in the venturi further, increasing fuel flow and enriching the air-fuel ratio. The choke can be manual or automatic, the latter controlled by temperature without needing electronics. As we’ll outline later in this Classic Porsche article, however, not all carburettors use chokes.
Before we look at the carburettor designs fitted to air-cooled Porsches, it’s worth considering a few other characteristics of a carburettor. Once the engine is started and up to temperature, it must be capable of idling at relatively low speed with the throttle more or less closed. This means the Venturi effect in the barrel is of no use. Instead, the low pressure below the throttle
valve causes air to be sucked in through an idle port, open on one end to the atmosphere and also joined to the main fuel reservoir jet. An idling screw adjusts the idle speed by controlling the flow of air, while another adjustment screw alters the air-fuel ratio by adjusting the amount of fuel that is simultaneously sucked into the incoming airflow. A further refinement of this behaviour includes bypass jets joining the air/fuel line before it is restricted by the idling screw to the barrel of the carburettor. These jets are uncovered by the throttle valve as it moves from closed to open, smoothing the progression from idle to part-throttle while the main jet in the venturi may still not be operating fully.
The final aspect of a carburettor worth mentioning is the acceleration circuit. Petrol is denser than air, meaning it reacts slower to a change in pressure. Consequently, when the throttle is opened quickly, there’s the potential for there to be a momentary excess of air until the fuel supply catches up. This can cause a ‘flat spot’ in performance, or a hesitation in acceleration. To counter this, most carburettors include an accelerator pump, which sends more fuel into the airstream when the accelerator pedal is pushed down. When the pedal is released, it primes the accelerator pump once more, ready for the next time the pedal is pressed. Obviously, this feature isn’t required during steady-state driving.
There have been many additional tweaks to the design of the carburettor through the years, each development intending to optimise operation for a wide band of conditions. Multiple barrels spring to mind, as do multiple fuel jets within the barrels, but we’ve covered the basics of carburettor design and operation for now.
Carburettors for early 356s were supplied by French manufacturer, Solex. Instead of holes in the wall of the venturi, the Solex carburettor featured a tube in the centre of the barrel fed fuel by the reservoir. If you look down into one of these carbs you can clearly see it, topped by an adjustment screw. The fuel, when the engine is running above idle, exits through little holes in the tube’s circumference, aligned with the restricted width of the venturi. The diameters and positions of these holes are matched to the engine to optimise the air-fuel ratio depending on the designer’s requirements.
FOR THE 1957 ‘SUPER’ FLAT-FOUR, THE STUTTGART BRAND TURNED TO ZENITH, A BRITISH MANUFACTURER
The Solex design also dispenses with the choke valve described earlier. Instead, Solex came up with a clever starter jet system it called the bi-starter. This features a bypass of the main venturi and throttle valve for the air, through a narrow passage. Within that is a fuel jet fed by the main reservoir. With the throttle valve closed, the cranking of the engine causes the pressure within this bypass passage to reduce, sucking fuel into the airflow, in a much richer air-fuel ratio than the stoichiometric process mentioned earlier, thereby aiding starting.
The amount of fuel allowed in is controlled via a flat disc on the side of the carburettor. The disc has three positions: it blocks the fuel outlet when off and has two different sized holes — the large hole allowing more fuel in when cold-starting, the smaller hole reducing the enrichment. The disc is turned by a lever and a cable running through to the cabin. The ‘choke’ cable in the car (it isn’t a choke, but is often referred to as such) has three positions, including pushed-in fully for normal driving and for when the engine is fully up to temperature. The mid setting is for idling after the car has started as the engine warms up (or indeed restarting the engine if it is not fully up to temperature), and the fully-out position is for cold starts. This feature was discontinued from 1955, leaving 356 owners to rely on enriching the cold-start air-fuel mixture by pumping the throttle pedal to operate the carburettor’s accelerator pump before start-up. Even so, these cars can be fiendishly difficult to start if their carburettors are not properly maintained.
Porsche continued its relationship with Solex for many years, though for the 1957 ‘Super’ flat-four, the Stuttgart brand turned to Zenith, a British carburettor manufacturer. Zenith produced a double-barrel carburettor for this larger capacity engine, with one barrel for each cylinder (there was a carburettor for each bank of the host flat-four). Two barrels in a single carburettor allow finer control of the air-fuel ratio across the operating range of the engine, along with individual tuning for each cylinder. This design was, ultimately, replaced by a Solex double-barrel carburettor using a single shaft for both throttle valves. Later, a split shaft
design was included, which enhanced the operating range of the carburettor, but made tuning very difficult. Eventually, Zenith was absorbed into Solex.
The arrival of the 911 and its new six-cylinder ‘901’ engine signalled the next major change in carburation for Porsche. According to Excellence was Expected, the Porsche bible written by Classic Porsche contributor, Karl Ludvigsen, Porsche engineering whizz, Ferdinand Piëch, disagreed with his uncle, Ferdinand ‘Ferry’ Porsche, on the specification of the carburettors for the then new six-cylinder model. While both men recognised higher cornering speeds and forces rendered the traditional float carburettor inadequate, Ferry specified the 901 should use the Solex ‘overflow’ system, while Piëch preferred Weber’s simpler triple-barrel carburettors.
Naturally, the boss had his way, and the 901 flat-six was equipped with a triple-barrel Solex carburettor for each bank of cylinders. The set of three shared a common fuel reservoir with a float in it, fed from the fuel tank by an electric pump (as outlined earlier). However, the supply jets in the venturis of the carburettor did not take fuel directly from the float reservoir. Instead, mechanical pumps, driven by the camshafts, lifted the fuel from the reservoirs into special passages in the carburettors, ready to be sucked into the venturis when needed. Unused fuel was allowed to drain back down into the main reservoirs. Porsche even designed the castings of parts of the engine to fit in with this system. It appeared Piëch wasn’t having his way, but even before the 901 engine went into production, work began on the 906 (Carrera 6) street-legal sports prototype race car, which was to use a specific development of the 911’s then new flat-six powerplant.
The 906’s engine was blessed with triple-barrel Webers, the company having already proven its ability to produce carburettors suitable for racing. Traditionally, these items were much larger in bore than the carburettors used in the 911, but
Piëch surreptitiously commissioned Weber to produce the 906’s carburettors, with an instruction to adopt the same dimensions as those of the 911. This allowed him to easily replace the Solex carburettors in production 911s, which he did in 1966, just in time for the arrival of the 911 S, which featured Weber carburettors with larger bores than the standard 911’s equivalent components. These Weber carburettors used two tightly packaged fuel reservoirs and floats, one either side of the central barrel. They were, in effect, three carburettors in one, sharing two reservoirs, with no choke. This became the default configuration (with the exception of Zenith carburettors for the entry-level 911 T in 1970 and 1971) until Porsche abandoned carburation completely in favour of fuel injection.
Porsche’s adventures with fuel injection technology also involve the 906 as a star player. We’ll cover the story in a forthcoming issue of Classic Porsche, but for the time being, let’s all pay tribute to the carburettor, which should be remembered for being an elegant solution to a complex problem decades before computer control made us take such things for granted.