Apr 28, 2016  This code can be deciphered as such: the first three numbers are the day of the year (082x = 82nd day of the year) the carburetor was built. The last number is the year (0828 = 82nd day of 1968). All Quadrajet fuel inlets are in one of two positions. Carburetors intended for. International / Farmall - Fits: Cub, 154 (WITH ENGINE SERIAL NUMBER 312390 and up) , 184, 185; Replaces: 70949C91, 70949C92, 71523C93 Carb Mfg #'s -: 13781, 13794 On older Cub tractors, you may need to use 2 ABC543 carb gaskets to clear the butterfly. This is a New Zenith replacement carburetor.

SU carburettors are a brand of carburettor of the constant depression type. The design remained in quantity production for much of the twentieth century.

The S.U. Carburetter Company Limited also manufactured dual-choke updraught carburettors for aero-engines such as the Rolls-Royce Merlin and Rolls-Royce Griffon.^[1]

Invention and development[edit]

Herbert Skinner (1872-1931), pioneer motorist and an active participant in the development of the petrol engine,^[2] invented his Union carburettor in 1904.^[3] His much younger brother Carl (Thomas Carlisle) Skinner (1882-1958), also a motoring enthusiast, had joined the Farman Automobile Co in London in 1899.^[4] He helped Herbert to develop the carburettor.^[4] Herbert's son could remember his mother sewing the first leather bellows.^[5] It would be given on loan to The Science Museum, South Kensington in 1934.^[3][6] In 1905 Herbert applied for a patent^[5] which was granted in early 1906.^[7] Later Carl sold his interest in footwear business Lilley & Skinner and became a partner in G Wailes & Co of Euston Road, London, manufacturers of their carburettor.^[4] Herbert continued to develop and patent improvements through to the 1920s including the replacement of the leather bellows by a brass piston though he was a full-time director and divisional manager of Lilley & Skinner.^[3][8][9]

S. U. Company Limited[edit]

S. U. Company Limited —Skinner-Union—^{[10][11][12][13]} was incorporated in August 1910^{[note 1]} to acquire Herbert's carburettor inventions and it began manufacture^[2] of the carburettors in a factory at Prince of Wales Road, Kentish Town in North London.^[14] Sales were slow. Following the outbreak of war in 1914 carburettor production nearly stopped with the factory making machine gun parts and some aircraft carburettors. With peace in 1918 production resumed but sales remained slow and the company was not profitable so Carl Skinner approached his customer, W. R. Morris, and managed to sell him the business.^[11] Carl Skinner (T C Skinner) became a director of Morris's privately held empire and remained managing director of S.U. until he retired in 1948 aged 65.^[4] Production was moved to the W R Morris owned Wolseley factory at Adderley Park, Birmingham. In 1936 W R Morris sold many of his privately held businesses including S. U. to his listed company, Morris Motors.^[15][16]

The S. U. Carburetter Company Limited[edit]

Manufacture continued, now by The S. U. Carburetter Company Limited which was incorporated 15 September 1936^[17] as part of the Morris Organization, later known as the Nuffield Organization.^[15] The S. U. Carburetter Company Limited of 1936^[17] was voluntarily liquidated in December 1994.

In 1996 the name and rights were acquired by Burlen Fuel Systems Limited of Salisbury^[14] which incorporated an entirely new company with the name The S.U. Carburetter Company Limited^[18] which continues to manufacture carburettors, pumps and components mainly for the classic car market.^{[citation needed]}

Applications[edit]

S.U. carburettors were widely used not only in Morris's Morris and MG products but Rolls-Royce, Bentley, Rover, Riley, Turner, Austin, Jaguar, Triumph and Swedish Volvo, Saab 99automobiles for much of the twentieth century. S.U. also produced carburettors for aircraft engines including the early versions of the Rolls-Royce Merlin, but these were of the conventional fixed-jet updraught type rather than the firm's patented constant-depression design.

They remained on production cars through to 1993 in the Mini and the Maestro by which time the company had become part of the Rover Group.

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 interchangeable.^{[citation needed]}

Operating principle[edit]

A similar description of the below content can be found here: ^[19]

SU carburettors featured a variable venturi controlled by a piston. This piston has a tapered, conicalmetering 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 exact dimensions of the taper are tailored during engine development.

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 pushed 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 rate of air delivery.

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 (20W Grade) 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 when the throttle is suddenly opened, thus increasing the richness of the mixture.

SU carburettors do not have a conventional choke flap, which in a fixed-jet carburettor enriches the mixture for starting the engine from cold by restricting the air supply upstream of the venturi. Instead a mechanism lowers the jet assembly, which has the same effect as the needle rising in normal operation - namely increasing the supply of fuel so that the carburettor will now deliver an enriched mixture at all engine speeds and throttle positions. The 'choke' mechanism on an SU carburettor usually also incorporates a system for holding the throttle plate slightly open to raise the engine's idling speed and prevent stalling at low speeds due to the rich 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 (26TPI for most pre-HIF versions). 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 carburettors and substituted constant-depression carburettors, 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.

SU carburettor types[edit]

SU carburettors were supplied in several throat sizes in both Imperial (inch) and metric (millimetre) measurement.

The carburettor identification is made by letter prefix which indicates the float type:

'H': introduced in 1937 in which the float bowl has an arm cast into its base, which mounts to the bottom of the carburetor with a hollow bolt or banjo fitting. Fuel passes through the arm into the carburetor body. The bolt attaches to the carburetor body just behind the main jet assembly.

'HD': introduced in 1954 with the float bowl mounted with its arm fastening directly below, and concentric with, the main jet. The arm has a flange that fastens with 4 screws to the bottom of the carburetor, and sealed with a rubber diaphragm integral with the main jet.

'HS': introduced in 1958 the float bowl can be rigidly or rubber mounted to the main body, fuel is transferred by an external flexible pipe to the jet. The jet moves down to richen the mixture for cold starting,when the 'choke' linkage is pulled.

'HIF': (1972) the float bowl is horizontal and integral (hence the name) Horizontal Integral Float. Example:^[20] 1972-1974 MGB.

'HV' (1929), 'OM' and 'KIF' types also exist but were less commonly employed.^[21]

The Imperial sizes include 1-1/8', 1-1/4', 1-1/2', 1-3/4', 1-7/8', and 2', although not every type (H, HD, HS, HIF) was offered in every size.

There were also H models made in 2-1/4' and 2-1/2', now obsolete. Special purpose-built carburetors (Norman) were made as large as 3'.

To determine the throat size from the serial number:If the final number (after one, two or three letters, beginning with H) has 1 digit, multiply this number by 1/8', then add 1'. For example, if the serial number is HS6, the final number is 6: 6/8 = 3/4', add 1, total is 1-3/4', etc.

If the final number has 2 digits, it is the throat size in mm. For example, if the serial number is HIF38, the final number is 38, size is 38 mm etc.

SU fuel pumps[edit]

In 1929 SU introduced the Petrolift electric fuel pump which could be fitted as a substitute for the vacuum type pumps common at the time. This was superseded in 1932 by the L type fuel pump which used a solenoid to operate a diaphragm pump.

See also[edit]

Notes[edit]

1. ^S. U. Company Limited, 386—388 Euston Road, N.W. Capital £5,000 in £1 shares. Formed to acquire from G. H. Skinner certain inventions relating to carburettors for motorcars &c
   New Companies Registered. Private Companies. The Automotor Journal, 24 September 1910

References[edit]

As of this edit, this article uses content from PESWiki, a source licensed under the terms of the GNU Free Documentation License which was imported into Wikipedia before November 2008 and is therefore validly licensed for use on Wikipedia. All relevant terms must be followed. The original article was at 'PowerPedia:Carburetor'.

1. ^The Two RsFlight magazine, p 577, 7 May 1954
2. ^ ^abObituary. Mr. G. H. Skinner. The Times, Wednesday, Jan 06, 1932; pg. 12; Issue 46023
3. ^ ^abcE A Forward, Handbook of the Collections illustrating Land Transport, II. Mechanical Road Vehicles, Science Museum South Kensington, 1936
4. ^ ^abcdObituary. Mr. Thomas C. Skinner The Times, Saturday, Nov 15, 1958, Issue 54309, p.10.
5. ^ ^abH. Jones: Herbert Wakefield Banks Skinner. 1900-1960. Biographical Memoirs of Fellows of the Royal Society, Vol.6, (Nov. 1960), pp.259-268, publisher: The Royal Society
6. ^The Science Museum's (1965) carburettor, sectioned
7. ^3257 G H Skinner, Carburetting apparatus. reported in page 152, AutoMotor Journal 3 February 1906
8. ^'Lilley & Skinner, The Times, Tuesday, Jun 02, 1896; pg. 14; Issue 34906
9. ^Skinner-Union Carburettor Improvement. The Commercial Motor, 24 August 1920, p.28.
10. ^Profile Professor Herbert Skinner, The New Scientist 14 November 1957
11. ^ ^abJames Leasor, Wheels to Fortune, Stratus, Cornwall 2001 ISBN0755100476
12. ^Colin Campbell, Tuning for Economy, Springer 1981 ISBN9780412234903
13. ^Nick Meikle, Malloch's Spitfire: The Story and Restoration of PK350, Casemate 2014 ISBN9781612002521
14. ^ ^abBurnett, John (2012). SU Carburettor Company catalogue. Salisbury: Burlen Fuel Systems.
15. ^ ^abJames Leasor, Wheels to Fortune, Stratus, Cornwall, 2001 ISBN0-7551-0047-6
16. ^Herbert Wakefield Banks Skinner. 1900-1960 H Jones, Biographical Memoirs of Fellows of the Royal Society 1960
17. ^ ^abCompanies House company number 00318520
18. ^Companies House company number 03285338
19. ^SU Carburetors Owners Workshop Manual by Don Peers: Part 1 Chapter 2 and Part 2 Type H, Haynes Publishing Group, Spsrkford YEOVIL Somerset England. Copyright JH Haynes and Company Limited 1976
20. ^https://mossmotors.com/mgb/fuel-intake-emissions/carburetors/hif4-carburetor
21. ^SU technical pages

External links[edit]

From Wikipedia, the free encyclopedia

SU Carburetters (named for Skinners Union, the company that produced them) were a brand of carburetter usually of the sidedraught type but downdraught variants were used on some pre-war cars. They were widely used in British (Austin, Morris, Jaguar, Triumph, MG) and Swedish (Volvo, Saab 99) automobiles for much of the twentieth century. Originally designed and patented by George Herbert Skinner in 1905, they remained on production cars through to 1995 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 carburetters based on the SU design which were used on the Datsun 240Z, Datsun 260Z and other Datsun Cars. While these look the same, they are different enough that needles (see below) are the only part that fits both.

The History of S.U.

The First Carburetters

Over the next few years, Thomas Carlyle Skinner (known as Carl), who was Herbert's younger brother and who had a practical ability, had begun to try some of his brothers concepts on a Star motorcar that he then owned. The idea was to place the fuel jet in an air channel that could be varied in size, in accordance with the demand of the engine, thereby giving a constant depression and air velocity. Herbert was granted a full patent (no. 3257) for this device in January 1906. At this stage a tapered metering needle, to vary the flow of petrol, had not been thought of. The two brothers continued to work together and their first carburetters were made at the premises of George Wailes & Co. at Euston Road, London, where Carl became a partner with George Waile's son.

In 1908 Herbert was granted another patent (no. 26,178) for a carburetter having a 'collapsable chamber' and a 'fuel needle valve', which was located in an adjustable block (i.e. a jet). Herbert's inventive genius had therefore devised the basic principles of the later 'constant vacuum' S.U carburetter. In August 1910, 'The S.U. Company Ltd' (S.U being a contraction of 'Skinners Union') was formed and some time later moved onto premises at 154 Prince of Wales Road, Kentish Town, London. By 1913 the company's accounts showed that they were supplying 'Sloper' carburetters (so called because the suction chamber and needle assembly was positioned at an angle from vertical, in order to reduce the fluctuations of the chamber when driving over the rough, unmetaled, roads of the period) to Wolseley Motors and the Rover Co.

The First World War

During the First World War, the S.U Company Ltd. became engaged on munitions contracts, which included making carburetters for Aero engines, with a staff of about 250. Normal production resumed after the War but the company had few customers and it showed a loss for 1919 and 1920. Although the S.U 'Sloper' had the advantage of automatically adjusting the flow of petrol to the engine, it was expensive and it had no proper means of providing a rich mixture for cold starting, other than 'flooding the float chamber'. This deficiency was tackled by Wolseley Motors Ltd, who added another jet and a physically operated needle. Wolseleys patented their modification (no. 119187) in 1918, for which they received royalties from S.U.

Takeover

The general depression of the 1920s saw the company resorting to general engineering work but some development continued and the alloy suction chamber and piston replaced the leather bellows used on earlier types. When the leather bellows were first introduced to control the movement of the tapered needle, there was much skepticism that they would soon perish and fail, but the family connection with the shoe business allowed access to a supply of the finest glace kid leather which worked very well and did not perish. These bellows were made by hand by Herbert's wife Mabel right through to 1928 - presumably, by this time for spares, not production.

In 1925 S.U introduced the 2m, which is significant because it set the general pattern for S.U carburetters thereafter. Financial matters came to a head in 1926 when G. H. Skinner withdrew from the company. Carl Skinner was now in a difficult position and approached W. R. Morris (Lord Nuffield) and following negotiations the S.U company was purchased by Morris for £100,000. This was a considerable sum for a loss making concern but Morris obviously saw potential in the product-retaining Carl as manager. Soon after purchase, S.U was relocated to a part of the old Wolseley Plant in Adderley Park in Birmingham. Carl was given £17,000 for plant and equipment needed to prepare for mass production of carburetters, to meet the demand of 1,000 units a week for Morris Ltd alone. From this point, growth and development were the order of the day with new products such as the S.U Petrolift in 1929, an Aero carburetter in 1932 and the now familiar S.U Electric pump in 1934 along with improvements and additions to the carburetter range. In 1936 the S.U Carburetter Company Ltd was formed with Carl Skinner as Managing Director.

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The Second World War

After the out break of World War 2 in 1939, realising the importance of S.U carburetter production to the war effort, work was started at the Riley Plant in Coventry to duplicate production. S.U manufactured the carburetters for the Rolls-Royce Merlin engines fitted to Hurricanes, Spitfires and many other aircraft, and when the factory was damaged by two air raids on Birmingham in 1940, the Air Ministry evacuated it to another factory in Highlands Road, Shirley. In 1941 the S.U Company acquired a shadow factory in Wharfedale, Yorkshire, and during these years a fuel injection pump was developed for use on Merlin aircraft engines.

Growth and Decline

In 1945 the production of car carburetters and petrol pumps resumed and the company moved to a site in Wood Lane in Erdington, Birmingham in July 1947. Later in the same year, in December, the link with the company's founder ended when Mr T C Skinner retired. With the end of its military requirement, the company's interest in fuel injection manufacture was sold to the Stirling Company of America. The company's market for carburetters and fuel lift pumps expanded considerably as Morris Motors Ltd merged with the Austin Motor Co. Ltd in 1952, to form the British Motor Corporation, and then still further when in 1965 British Motor Holdings was formed in association with Jaguar Guy and Coventry Climax. This period and the following few years saw the peak of production for the company ranging from 'H' through 'HD' to 'HS' types when in excess of or 30,000 carburetters were produced each week. In addition to carburetters, the company expanded its product range with the introduction of a Mechanical Fuel Lift Pump and an Automatic Cold Start Enrichment Device.

In the late 1960s the HIF Carburetter (Horizontal Integral Float) incorporating temperature compensation was introduced, leading to a need to expand the plant. With the various amalgamations and restructuring of the parent company over the next few years the name of the company was changed, in 1976 to S.U Fuel Systems as part of the S.U Butec division, and in 1983 to Austin Rover Fuel Systems as part of the Light Medium Cars division of British Leyland Cars. In 1984 the Design and Development Departments were transferred to Longbridge and S.U had become basically a manufacturing plant. Product development during this same period saw the introduction of 'Ball Bearing' piston slides to the range, and an electronically controlled cold start system for the HIF type. Carburetter manufacture declined with the introduction of Fuel Injection Systems for cars, and S.U started production of both Single Point and Multi Point Throttle Bodies. An alternative product line started in 1983 when Austin Rover's '01' Series Engines Oil Pump was transferred into S.U from the Morris Engines Plant in Coventry, which was being closed down. Other additional products were acquired in 1984/5 when the UK part of Solex went into liquidation and in conjunction with Burlen Fuel Systems, negotiated the manufacture and supply of Zenith Solex components required for spares support.

Administratively the period of 1976 to 1986 saw the company on a roller-coaster ride, during which time no fewer than ten top managers came and went. This lack of continuity at the top meant that the business declined. The only additional products added in this period being low volume throttle bodies for Rolls Royce and Lotus both of which were manufactured to the customers own design, In 1988 following the restructuring of Austin rover by Michael Edwards, S.U were sold to the Hobourn Group and the company name changed to Hobourn S.U and subsequent to this, in September 1989, the Hobourn Group was acquired by Echlin Inc of the USA. The Echlin Group's UK companies included Quinton Hazell and this gave S.U another route for sales.

A Design and Development facility capable of covering both oil pump and throttle body work was created allowing existing and potential customers to be approached with a comprehensive service. Product design and development over the next four years saw the company supplying Rover with oil pumps and a refined version of the HIF carburetter for the Rover K Series engines. This unit known as the KIF was to be the last new specification carburetter to be produced by the company. A major development exercise led to the manufacturer of a 'Plastic' throttle body for the Rover K series engines. The requirement by customers for larger unit assemblies saw S.U supplying induction manifold assemblies complete with throttle bodies to Rover, Rolls Royce and Land Rover. In spite of these initiatives the 'added value' of the in house content of the company's work suffered in line with the overall requirements of its major customers and manning reductions were made. In August 1999 Dana Inc of the USA (a major American car parts supplier with an increasing presence in Europe) took over the Echlin group and the company name changed once again to Dana-SU. Automotive.

Current and Future

The change to fuel injection in various forms for nearly all new vehicles saw carburetter manufacture decline to very small volumes, most of which were for the service market, and this led to S.U negotiating with Burlen Fuel Systems Ltd (its major parts customer and already a small volumes manufacturer in its own right) to take over the responsibility for manufacture and supply of all S.U carburetter units, pumps and spares, May 1999 saw the end, after ninety four years of the manufacture of carburetters by the descendant of the original S.U company, and the closure of Wood Lane, Birmingham; as a manufacturing plant. During the next three years, manufacture of the of all carburetters, fuel pumps and some throttle bodies passed to Burlen Fuel Systems Ltd and in 2002 Burlen acquired the intellectual property rights and the S.U trademark.

Burlen is now the world's sole manufacturer of genuine, new S.U product, and can supply new carburetters, pumps and spares for almost all models from 1930 to the present day. The company has invested heavily in new tooling where replacement has been necessary and to reintroduce models that have not been available for many years. Quality assured to BS5750 (ISO9002), Burlen has a policy to produce genuine, new S.U product to the original specification or better where new metallurgy, manufacturing techniques or materials allow. Now in 2007 the 'S.U Carburetter Company' is back and trading as a subsidiary and trading arm of Burlen Fuel Systems Ltd and along with the 'AMAL Carburetter Company' supplies fuelling products to vintage and classic cars and motorcycles around the world.

Operating principle

SU carburetters 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 carburetter. 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 tends to lift 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 carburetters 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 carburetter. To prevent erratic and sudden movements of the piston it is damped by light oil in a dashpot, which requires periodic topping up. The dampening is asymmetrical; it heavily resists upwards movement of the piston. This serves as the equivalent of an 'accelerator pump' on traditional carburetters 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 carburetter, which was previously used on many motorcycles. The slide carburetter has the same piston and main needle as an SU carburetter, 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 carburetter. This piston actuation difference is the significant distinction between a slide and an SU carburetter. The piston in a slide carburetter 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 which are rarely encountered except on motorways. This inaccuracy results in the wastage of fuel, particularly as the carburetter must be set slightly rich to avoid a lean condition, which when performed repeatedly can cause significant engine damage. For this reason Japanese motorcycle manufacturers ceased to fit slide carburetters and substituted constant-depression carburetters which are essentially miniature Japanese SUs. It is also possible - indeed, easy - to retro-fit an SU carburetter to a bike that was originally manufactured with a slide carburetter, and thereby obtain improved fuel economy and more tractable low-speed behaviour.

One of the downsides of the constant depression carburetter 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. The other downside is that, every time the piston rises and falls, the needle rubs against the jet. Over time, the needle gradually narrows through wear, thus allowing the fuel-air mixture to become richer and richer. Nornally, wear on the needle is compensated for by turning the adjusting screw under the carburettor when the engine is tuned. However, the needle does not wear evenly and it is good practice and costs little to replace the needle on older engines even before the carburettor requires a complete overhaul. The commonplace tendency to omit this regular maintenance is the main reason that older cars using SU carburetors, such as MGs and Volvos, are often considered 'gross polluters'.

SU Carburetter types

SU carburetors were supplied in several throat sizes in both Imperial (inch) and metric (millimeter) measurement.

The carburetor identification is made by letter prefix which indicates the float type:

'H': in which the float bowl has an arm cast into its base, which mounts to the bottom of the carburetor with a hollow bolt or banjo fitting. Fuel passes through the arm into the carburetor body. The bolt attaches to the carbureter body just behind the main jet assembly.

'HD': the float bowl mounts with its arm fastening directly below, and concentric with, the main jet. The arm has a flange that fastens with 4 screws to the bottom of the carburetor, and sealed with a rubber diaphragm integral with the main jet.

'HS': the float bowl is rigidly mounted to the carburetor body, but fuel is transferred by a separate external flexible line.

'HIF': the float bowl is horizontal and integral (hence the name).

The Imperial sizes include 1-1/8', 1-1/4', 1-1/2', 1-3/4', 1-7/8', and 2', although not every type (H, HD, HS, HIF) was offered in every size.

There were also H models made in 2-1/4' and 2-1/2', now obsolete. Special purpose-built carburetors (Norman) were made as large as 3'.

To determine the throat size from the serial number: If the final number (after one, two or three letters, beginning with H) has 1 digit, multiply this number by 1/8', then add add 1'. For example, if the serial number is HS6, the final number is 6: 6/8 = 3/4', add 1, total is 1-3/4', etc.

If the final number has 2 digits, it is the throat size in mm. For example, if the serial number is HIF38, the final number is 38, size is 38 mm etc.

See also

References

1. ^ ^abRobson, G. (2006). A-Z of British Cars 1945-80. Devon, UK: Herridge. ISBN 0-9541063-9-3.

External links