1958 MGA - H4 SU CARBURETOR OPERATING PRINCIPLES - ANIMATIONS SHOW VARIABLE VENTURI AIR-FUEL CONTROL 

This episode of Classic Cars - Driven By Data presents the basic operations of the H4 SU Carburetor as fitted to a 1958 MGA Mk1 with MGB 1800 engine. The key principles of operation are made clear with animations that show the process by which the SU carburetor achieves control of air:fuel mixture ratio over a wide range of engine load, engine speed and ambient conditions. The SU carburetor is a variable choke or variable venturi fuel air metering device. All adjustments are described, along with choke or cold start controls, accelerator or throttle controls as well as mixture adjustment. The need for, and operation of the dashpot or viscous damper are also described in detail, emphasizing the importance of the dashpot for transient operations, such as rapid changes in throttle or accelerator pedal position.
The motion of the main control piston is controlled by the differential pressure between the underside and topside of the piston. The source of these pressures is tracked through the hardware and fully explained.