MGB GT Carburettor
MGB GT Carburettor
All MGB’s from 1963 to 1974 were fitted with twin 1.5 inch (38 mm) SU carburettors. US spec cars from 1975 used a single Stromberg 1.75 inch (44 mm) carburettor mounted on a combination intake/exhaust manifold. This greatly reduced power as well as creating longevity problems as the (adjacent) catalytic converter tended to crack the intake/exhaust manifold. All MGBs used an SU built electric fuel pump.
SU carbs were named after Skinners Union, the business that produced them. Skinners Union was founded in 1905 by brothers George and Thomas Skinner on Euston Road, London, the business having been acquired in 1926 by W. R. Morris they were widely used in his Morris and MG products and other British (Rolls-Royce, Bentley, Rover, Riley, Austin, Jaguar, Triumph) and Swedish (Volvo, Saab 99) cars for much of the twentieth century.
Originally designed and patented by George Herbert Skinner in 1905, they remained on production cars through to 1993 in the Mini and the Maestro by which time they had become part of the Rover Group. They are now manufactured by Burlen Fuel Systems Limited mainly for the classic car market. Hitachi also built carburettors based on the SU design which were used on the Datsun 240Z, Datsun 260Z and other Datsun Cars. While these appear the same, only their needles are interchangable.
SU carburettors featured a variable venturi controlled by a piston. This piston has a tapered, conical metering rod usually referred to as a “needle” that fits inside an orifice “jet” which admits fuel into the airstream passing through the carburettor. Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered, depending on engine demand.
The flow of air through the venturi creates a reduced static pressure in the venturi. This pressure drop is communicated to the upper side of the piston via an air passage. The underside of the piston is open to atmospheric pressure. The difference in pressure between the two sides of the piston lifts the piston. Opposing this are the weight of the piston and the force of a spring that is compressed by the piston rising. Because the spring is operating over a very small part of its possible range of extension, its force is approximately constant. Under steady state conditions the upwards and downwards forces on the piston are equal and opposite, and the piston does not move.
If the airflow into the engine is increased by opening the throttle plate usually referred to as the “butterfly”, or by allowing the engine revs to rise with the throttle plate at a constant setting, the pressure drop in the venturi increases, the pressure above the piston falls, and the piston is sucked upwards, increasing the size of the venturi, until the pressure drop in the venturi returns to its nominal level. Similarly if the airflow into the engine is reduced, the piston will fall. The result is that the pressure drop in the venturi remains the same regardless of the speed of the airflow – hence the name “constant depression” for carburettors operating on this principle – but the piston rises and falls according to the speed of the airflow.
Since the position of the piston controls the position of the needle in the jet and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. The well controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions.
This self adjusting nature makes the selection of the maximum venturi diameter colloquially, but inaccurately, referred to as “choke size” much less critical than with a fixed venturi carburettor. To prevent erratic and sudden movements of the piston it is damped by light oil in a dashpot, which requires periodic replenishment. The damping is asymmetrical: it heavily resists upwards movement of the piston. This serves as the equivalent of an “accelerator pump” on traditional carburettors by temporarily increasing the speed of air through the venturi, thus increasing the richness of the mixture.
The beauty of the SU lies in its simplicity and lack of multiple jets and ease of adjustment. Adjustment is accomplished by altering the starting position of the jet relative to the needle on a fine screw. At first sight, the principle appears to bear a similarity to that of the slide carburettor, which was previously used on many motorcycles. The slide carburettor has the same piston and main needle as an SU carburettor, however the piston/needle position is directly actuated by a physical connection to the throttle cable rather than indirectly by venturi airflow as with an SU carburettor. This piston actuation difference is the significant distinction between a slide and an SU carburettor.
The piston in a slide carburettor is controlled by the operator’s demands rather than the demands of the engine.
This means that the metering of the fuel can be inaccurate unless the vehicle is travelling at a constant speed at a constant throttle setting – conditions rarely encountered except on motorways. This inaccuracy results in fuel waste, particularly as the carburettor must be set slightly rich to avoid a lean condition which can cause engine damage.
For this reason Japanese motorcycle manufacturers ceased to fit slide carbs and substituted constant depression carbs, which are essentially miniature SUs. It is also possible, indeed, easy to retrofit an SU carburettor to a bike that was originally manufactured with a slide carburettor, and obtain improved fuel economy and more tractable low speed behaviour.
One of the downsides of the constant depression carburettor is in high performance applications. Since it relies on restricting air flow in order to produce enrichment during acceleration, the throttle response lacks punch. By contrast, the fixed choke design adds extra fuel under these conditions using its accelerator pump.