As the air valve rises, allowing more air to enter, the tapered needle also rises in the jet, allowing the proper amount of fuel to mix with the air. When engine vacuum decreases, the weight of the air valve, aided by its spring, brings the air valve downwards.įuel is metered by a tapered needle mounted in the bottom of the air valve, and a fixed jet. (It is interesting to note that the original SU models prior to World War I used thin leather diaphragms for this purpose.) What causes the air valve to rise is engine vacuum acting on the air above the air valve through the holes in the bottom of the air valve. While “modern” SU carburetors rely on carefully and expensively machined parts to produce a moveable vacuum seal at the top of their pistons, Zenith-Strombergs use a thin flexible diaphragm to effect this seal. This condition is produced by the air valve (air piston) rising and falling in response to engine vacuum.

(The “CD”in the Z-S series designation stands for “constant depression) Constant depression, also known as “constant vacuum”, “constant pressure”, “variable venturi”, or “variable choke”, simply means that the effective area of the carburetor venturi varies according to the engine requirements, producing constant air velocity and pressure differential across the jet orifice during normal running. Zenith-Stromberg carbs operate on the constant depression principle. While later Z-S models used a large number and variety of “add on” features to meet our pollution control requirements, the basic “heart” of these carburetors remained relatively simple. for example, is actually simpler and more reliable than the SU carburetor it replaced. The basic Zenith-Stromberg, as used on Triumphs in the mid-’60s. Fear of the unknown is probably the greatest stumbling block most people seem to have in dealing with Zenith-Stromberg carburetors – they are much maligned largely because they are not understood.

Zenith stromberg 175 cd 2 manual