What is a SU Carburetor? How does it Work?

The SU carburetor is a type of carburetor that was widely used in British cars from the 1930s to the 1980s. “SU” stands for “Skinners Union”, the company initially designed and manufactured the carburetor.
In this article, I will discuss the full details of the SU carburetor.
SU carburetor


SU carburetor

SU carburetor is an example of a constant vacuum-type carburetor. The SU carburetor operates on the principle of variable venturi, which means that the diameter of the carburetor’s throat can be varied to control the amount of air that enters the engine. It consists of a single jet in which a tapered needle operates. The area of the throat is varied by means of a piston that slides up and down. The trapped hand is connected to the accelerator. 
SU carburetors are known for their simplicity, reliability, and ease of tuning, making them popular with both car manufacturers and enthusiasts. They are often used in classic British cars such as MG, Triumph, and Jaguar.

SU carburetor working – 

SU carburetor working.
When the accelerator is operated, the piston moves up and down in the throat controlling the supply of air, and the needle moves up and down to the jet controlling the fuel supply. The carburetor accomplishes this by controlling the flow of air and fuel into the engine.
The throat increases when the piston moves up, and more air passes through it. The needle also moves up increasing the annular area in the jet to pass more fuel. When the piston moves down, the throat area decreases, and less air passes through it. The needle also moves down decreasing the annular area in the jet to pass less fuel. 
The piston and tapered needle are so designed that they maintain the correct air-fuel mixture at different operating conditions of the engine. By changing the tapered needle and adopting a suitable one, the same can be used on different engines to some extent. 
The SU carburetor’s simplicity and effectiveness make it a popular choice among car enthusiasts, particularly for classic British cars.

Fundamentals of SU carburetor- 

The upper side of the piston is connected to the throttle passage through a slot cut in the piston. The lower side is exposed to atmospheric pressure. Thus, the position at any instant depends upon the balance of its own weight (downwards) against the vacuum force(upward). As the weight of the piston is constant, the vacuum also remains constant. 
The jet can be moved bodily up and down with respect to the tapered needle, by an adjusting screw fitted at the bottom of the screw. This is done for adjusting the mixture strength. There is no separate idling or slow-speed system and no accelerating pump.

Advantages of SU carburetor- 

The main advantage of this type of carburetor is the rapid response during acceleration hence it is fitted in racing cars and in most scooters and motorcycles. There are several advantages to using SU carburetors:
  • SU carburetors have a relatively simple design, with few moving parts, which makes them easy to maintain and repair.
  • SU carburetors are use to tune. The piston and butterfly valve design of the SU carburetor makes it easy to adjust the air-fuel mixture to suit different engine loads and speeds.
  • SU carburetors operate smoothly. It provides a smooth and consistent delivery of fuel and air to the engine.
  • SU carburetors result in better engine performance and reduced emissions.
  • SU carburetors were widely available at those times. SU carburetors were widely used in British cars, which
    means that they are still widely available and easy to source.

Disadvantages of SU carburetor-

The SU carburetors have some more disadvantages- 
  • The SU carburetors have limited air-flow capacity. It has a relatively small throat diameter, which can limit its airflow capacity. This can limit the carburetor’s ability to provide adequate fuel and air to high-performance engines.
  • SU carburetors have some problems with altitude changes. SU carburetors rely on a fixed jet size to regulate fuel flow, which can make them difficult to tune for altitude changes. This can result in poor performance in atmospheric pressure.
  • SU carburetors were designed primarily for use in British cars, which can make them less compatible with other types of engines. This can make it difficult to use them in custom or modified engines.


SU carburetors are basically used in British cars. This type of carburetor is simple in design. Overall, the SU carburetor is a reliable and efficient carburetor that provides good performance and ease of maintenance, which makes it a popular choice among classic car enthusiasts.
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