This condition allows engine vacuum to draw proportionally more fuel than normal from the jet. The extra rich mixture required for acceleration is produced by the upward motion of the air valve being retarded by the oil damper in the piston guide rod. Since both of these elements operate together in direct accordance with the requirements of the engine, expressed by the engine vacuum, a properly balanced air-fuel mixture is maintained throughout the engine’s operating range with an absolute avoidance of complexity. This amounts to having a carefully calibrated variable jet to match the variable venturi – a classically elegant design. 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.

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