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Carburetor Classics: The Humble SU

To know them is to love them

Ask many former owners of 1960s British sports cars about problem areas on their steeds, and they will likely digress into horror stories concerning the dreaded SU carburetors. In most cases though, they'd be wrong.

The SU carburetor, far from being high maintenance or temperamental, is one of the best designed carburetors of all time, and it simply thrives on neglect. In fact, the principal reason it is blamed for so many maladies is that it is conveniently perched beside the engine where it can be easily misadjusted by ham-fisted mechanics often unaware that the real gremlins usually live in the worrisome Lucas "Prince of Darkness" electrical systems common in older British iron. Those in the know realize that the humble SU can be made to feed anything from economical grocery-getters to fire-breathing racing cars. Some lemon!

Two Brothers

The SU Company was formed in 1910 by two brothers, George Herbert and Thomas Carlyle Skinner, who named their company Skinners Union. They had been very successful in the family shoe and leather business, but were captivated by the primitive cars starting to rattle down English lanes. The carburetors of the time were Rube Goldberg devices, requiring separate hand controls for varying gas flow and air mixtures.

George Skinner came up with an alternative in 1904, introducing a carburetor with a floating piston mounting a long tapered needle that moved up and down inside the fuel jet. A leather bellows that responded to engine vacuum actuated the piston. This carburetor was well-received, even though it was expensive to produce.

Then WWI intervened, and SU made military equipment and did general engineering work until 1925 when the company introduced a new carburetor, built on the same principals but without the leather bellows. In its place was a bell-shaped aluminum "dashpot" on top of the carburetor in which the piston slid up and down. This setup was to become one of the most successful carburetor designs of all times. It was used by the fabulously expensive Bentleys on both road and track (where they won Le Mans five times), and by low-priced marques like Wolseley and Morris as well.

Road and Track

In 1926 the growing company was bought by Morris Motors, which expanded production and soon added a line of fuel pumps. During WWII, SU carburetors fed the Hurricanes and Spitfires that won the Battle of Britain over fuel-injected Messerschmitts.

By the 1950s, SU was making 60,000 carburetors and fuel pumps per week in their spacious factory. These were installed on everything from tiny Mini Coopers to booming Aston Martins. The winning Jaguar at Le Mans in 1951 wore SU carbs, as did hundreds of amateur racing cars around the world.

The SU's small, round fuel bowls were extremely resistant to starvation in hard cornering, and they were easy for even amateur racers to modify for more performance. Although less tunable than the more intricate (and expensive) Weber carbs from Italy, the SU was easier to set up and less likely to need adjustment.

Most Americans saw their first SU carbs during the sports car boom of the 1950s. They were mounted on MGs, Triumphs, Austin-Healeys and Jaguars, and usually delivered in pairs on 4-cylinder and 6-cylinder engines. They appeared in triplicate on hotter sixes like the Jaguar XKE, Aston Martin and some Healeys.

The twin carbs were connected by a simple linkage system that could be easily adjusted with a screwdriver. This ease of adjustment led amateur mechanics to attempt to correct any engine malady from an intermittent miss to catastrophic crankshaft failure by turning one or more of the funny little carb screws. When this didn't work, they blamed SU. Not fair.

Audible Tuning

With a little experience, SU tuners found they could do a passably good job of synchronizing multiple carbs with a simple listening device. While holding a long rubber hose between the intake hole on each SU and their ear, they turned the screws until the sound of rushing air was the same on both carbs.

Not so innocent was the SU fuel pump, which had a disturbing tendency to go on strike with no warning (usually late at night during a torrential rainstorm). Ninety percent of the time the cure was to do just what one would want to do anyway, which is to bang on the offending pump with a solid object (the culprit was almost always sticking points in the electric fuel pump motor).

Like other famous carburetors from Weber and Holley, the SU was doomed by the introduction of fuel injection in the 1970s. The final SUs were special emission-controlled models that caught fire on a regular basis. The end came in the early 1980s when SU, now a part of the emaciated British Leyland empire, was renamed Austin Rover Fuel Systems. Thankfully, Burlen Fuel Systems in England took over production of the classic SU carbs and parts. Thousands of classic cars around the world still rely on the amazingly simple and effective SU carburetor.