REFERENCE has already been made to the practice, for fuel economy reasons, of tuning the carburettor to give a weaker mixture over the cruising range. This practice naturally tends to affect the acceleration characteristics, and many carburettors are now provided with a somewhat modified arrangement of accelerating pump. These pumps are of what is usually termed the delayed‐action type, and their function is still to augment the fuel supply from the slow running and/or main jet systems during acceleration; and whereas the discharge from a fixed type pump synchronises with the movement of the throttle, the supply from a delayed action pump also continues for a period after the operation has ceased to move the throttle, i.e., whilst the engine is attaining the r.p.m. corresponding to the extent to which the throttle has been opened. A good example is given in Fig. 39, which shows the delayed‐action pump which is fitted to certain models of Strombcrg carburettors. As shown by the figure, this consists of an inverted cylinder having a stem at the upper end which is operated from the throttle spindle. Within the cylinder is a piston which slides on a hollow stud screwed into the main body casting. The upper end of the stud is shaped like a poppet valve with several holes in its seating leading into the central hole. The valve scat is in the piston, which is held up against the valve by a spring. From the stud a passage is taken to the main jet system. As the throttle is opened rapidly the cylinder is moved and the pressure of the fuel above the piston forces it down, thus opening the valve so that fuel under pressure is forced out to the main choke tube. The spring then forces the piston up, so the fuel discharge continues even after the throttle has ceased to move. If the throttle remains in that position the piston soon reaches the valve seat and stops all fuel How through the accelerating system. Upon closure of the throttle fuel is drawn into the cylinder through the clearance space which exists between the piston and cylinder.
Stokes, F.C. (1936), "Carburation Practice in Aero‐Engines: A History of the Development of Carburettors for Special Aircraft Requirements", Aircraft Engineering and Aerospace Technology, Vol. 8 No. 7, pp. 194-196. https://doi.org/10.1108/eb030068
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