Working of SU carburetor with diagram

Working of SU carburetor with diagram
Diagram of SU carburetor

In previous articles, we have seen Solex carburetor and Carter carburetor which are generally choke type carburetors. In this article, we are going to see the construction and working of SU carburetor which completely different from the choke type 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.

Working of SU carburetor:

  • SU carburetor consists of a float chamber, piston, piston rod, piston rod guide, helical spring, suction disc, suction chamber, throttle valve, 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 the bottom side. Taper jet needle is inserted into the main jet which is connected to the float chamber. The up and down movement of the piston and piston rod controls the air passage.
  • The air passage is connected is to the air chamber with the help of suction air entrance as shown in 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 in the SU carburetor is controlled by the up and down movement of the piston.
  • Pressure in the suction chamber is directly proportioning to the opening of a butterfly type throttle valve whereas the pressure at suction disc is atmospheric.
  • SU carburetor contains only one jet. There is 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, a rich mixture of air and fuel is very important. This mixture can be obtained by pulling the jet in a downward direction with the help of a lever. Opened throttle valve 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, in the SU carburetor, a constant air-fuel ratio maintained at different engine speeds, unlike other modern carburetors.

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.


We have learned the working of SU carburetor with its diagram. If you have any doubts regarding this topic please use our comment section.

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