SU Carburetor | Working of SU Carburetor | The Engineers PostS.u. carburetor differs compeletly from them being constant vacuum or constant depression type with automatic variable choke .
carburetor to shown .It consists of a sliding piston .The lower end of the piston is provided with a taper needle which is inserted into the main jet. When the piston is moved up and down the needle also moves up and down with the main jet .The upper end of the piston is given a flat form which is known as suction disc .The up and down movement of the piston and the suction disc is guided by means of piston rod and piston rod guide .The piston always remains loaded by a helical spring .The movement of the piston controls the air pressure .The portion above the disc is called suction chamber which connects the air passage by means of a slot provided in the piston .The main jet of the carburetor can be moved up and down along the taper needle by operating a lever from the dash board. This movement is required to adjust the mixture strength throughout the operating range of the carburetor .The carburetor consists of an ordinary butterfly type throttle valve .The lower portion of the suction disc is connected to the atmosphere by means of an air rectifier hole and the upper portion to the throttle air passage .The system does not have any separate idling slow running and accelerating system . 
The weight of the piston is constant and always acts down. The vacuum in the suction chamber always tends to move the piston upward . Therefore at a particular instant the position of the piston is balanced by  the weight and constant vacuum produced above the piston .In starting a rich mixture is needed by the engine which can be obtained by pulling the jet downwards with the help of the lever attached to it . Therefore as the throttle is opened more air is allowed to pass through the inlet due to upward movement of the piston .The upward movement of the tapper needle also ensured more flow of fuel from the main jet .Thus the air and fuel passage are varied with different engine speeds and velocities of the fuel and air remains consists in this system .

(This article was taken by IC engine )


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