Adjusting an SU carburettor

Working of SU carburetor

Unlike other types of carburettor , which have fixed jets , the SU has just one jet, therefore the mixture setting affects the engine throughout its speed range. Some SUs even have exhaust emission-control devices to comply with anti-pollution laws. Adjustment of those is described in Adjusting an emission-control carburettor. Before tuning the carburettor, carry out all the checks in Checking and cleaning an SU carburettor , and bring the engine up to normal working temperature.

Also check the level of oil within the dashpot – the domed cylinder at the top of the carburettor. The oil is there to slow the movement of the piston. The delay enriches the mixture briefly when the throttle is opened suddenly. SAE 20 engine oil is that the correct grade to use once you top up. On older SU carburettors, which have a vent hole within the screw-in plastic top of the dashpot, the oil level should be i in. (13 mm) above the highest of the piston tube.

On dustproof SUs, which have a hole within the dashpot neck rather than the top, the oil should be i in. (13 mm) below the piston top. If the engine is slow to reply to sudden throttle openings, or responds fluffily, correcting the dashpot oil level may cure the matter , in order that you are doing not got to retune.

However, if correcting the oil level doesn’t resolve the problem, you’ll got to clean the carburettor (See Checking and cleaning an SU carburettor ) before tuning it. Whenever possible, make adjustments with the air cleaner in place removing it alters the partial vacuum inside the carburettor. On cars where the carburettor isn’t very accessible, you’ll need to remove the air filter to succeed in some parts, then perform further fine adjustments to get correct engine running afterward.

Checking the mixture

Use the piston lifting pin at the side of the carburettor to see the fuel-air mixture. With the engine running at working temperature, hook your finger under the pin and lift the piston about 1/32in. (1 mm). If, while the piston is raised, the engine speed increases briefly then returns to normal, the mixture is correct. If it rises and stays high, the mixture is just too rich. If it falls and the engine tends to die , the mixture is too weak.

To correct the mixture, move the jet adjuster nut one hexagon flat at a time. Screwing it up – anti-clockwise as seen from above – makes the mixture weaker. Screwing it down – clockwise makes the mixture richer. Each time you progress the adjuster, wait for about ten seconds, then check again with the lifting pin to see whether the mixture is now right. Take care that the engine temperature doesn’t rise above normal – which it’ll if you’re taking too long. Such an increase will end in a false, overweak mixture setting which can show when the temperature returns to normal.

Before and after adjustment, make sure the jet needle is central within the jet. Stop the engine, use the damper to lift the piston to the top of its travel and let it drop. It should fall smoothly with a pointy click. If it doesn’t , the jet is out of line and you want to centralise it. An off-centre jet can also score or bend the tapered needle. don’t plan to pack up or straighten a badly scored or bent needle. It must be replaced (See Checking and cleaning an SU carburettor ).

Centralising the jet

Remove the air cleaner and the dashpot screw-in top and damper. Use a screwdriver to boost the piston. Turn the jet adjuster nut up as far because it will go, or until the jet is level with the bridge inside the carburettor. Slacken the massive locking nut above the adjuster nut on the jet where it enters the carburettor body.

Now use a pencil or soft metal rod, like a stick of solder, to push the piston right down. Hold it down and tighten the locking nut. Check that the piston drops with a click – if it doesn’t , repeat the centralising process. Screw the jet adjuster nut down two full turns, which should bring it near enough to the right setting for the engine to be ready to run.

SU Carburetor

SU carburetor full form is Skinners Union carburetor. Skinners Union is a carburetor company established in London in 1910. SU carburetor is a constant depression or constant vacuum type of carburetor. It has an automatic variable choke rather than a simple choke. We are going to see the construction and working of SU carburetor which completely different from the choke type carburetor.

Working of SU carburetor:

  • SU carburetor consists of a float chamber, piston, piston rod, piston rod guide, helical spring, suction disc, suction chamber, accelerator, taper jet needle, and jet.
  • A suction disc is nothing but a flat portion above the piston. A piston rod is connected to the taper jet needle at rock bottom side. Taper jet needle is inserted into the most jet which is connected to the float chamber. The up and down movement of the piston and piston rod controls the air duct .
  • The air duct is connected is to the air chamber with the assistance of suction air entrance as shown within the figure. The suction chamber is situated above the suction disc. An air rectifier hole is used to provide atmospheric air to the suction disc.
  • The air-fuel ratio within the SU carburetor is controlled by the up and down movement of the piston.
  • Pressure within the suction chamber is directly proportioning to the opening of a butterfly type accelerator whereas the pressure at suction disc is atmospheric.
  • SU carburetor contains just one jet. there’s no separate acceleration pump or an idling jet-like Solex carburetor and Carter carburetor.

Starting of SU carburetor:

  • At the time of starting the engine, an upscale mixture of air and fuel is extremely important. This mixture are often obtained by pulling the jet during a downward direction with the assistance of a lever. Opened accelerator allows more air to flow under more suction causes the piston moves upward direction and increases the jet area in SU carburetor.
  • This increased jet area allows more engine fuel to flow into the mainstream. Therefore, within the SU carburetor, a continuing air-fuel ratio maintained at different engine speeds, unlike other modern carburetors.

Types of SU Carburetor

SU carburettors were provided in several “throat sizes” in both inch and millimetre measurement. These carburetors are identified by letter prefix which indicates the float type. “H”, “HD”, “HS”, “HIF”, “HV”, “OM” and “KIF”.

“H” types have float bowl with an arm cast into its base. it’s mounted to rock bottom of the carburetor with a hollow bolt. Fuel passes in carburetor body through the arm. “HD” it’s the float bowl with its arm fastening directly below, and concentric with the jet. The arm features a flange that tightened with four screws at rock bottom of carburetor. A sealed rubber diaphragm is integral with the jet. “HS” the most body is roofed with rubber, fuel is supplied by an external pipe to the jet.

“HIF” it’s horizontal Integral Float. “HV”, “OM” and “KIF” types less commonly used.

Vehicles using SU carburetor:

SU carburetor is another type of modern carburetor that is widely used in Bentley, Austin, Jaguar, Rover, Rolls-Royce, Saab automobiles and Volvo.


The main advantage of SU carburetor is that the rapid response during accelerating. Hence it’s fitted with racing cars and in most of the scooters and motorcycle.

SU Carburetors were widely utilized in automobile vehicles from much of the 20 th century. These also are utilized in aircraft engines including the first versions of the Rolls Royce.


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