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. The basic Zenith-Stromberg, as used on Triumphs in the mid-’60s. for example, is actually simpler and more reliable than the SU carburetor it replaced. 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.
Like the more familiar SU carburetors. Zenith-Stromberg carbs operate on the constant depression principle. (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. This condition is produced by the air valve (air piston) rising and falling in response to engine vacuum. 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. (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. When engine vacuum decreases, the weight of the air valve, aided by its spring, brings the air valve downwards.
Fuel is metered by a tapered needle mounted in the bottom of the air valve, and a fixed jet. 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. This amounts to having a carefully calibrated variable jet to match the variable venturi – a classically elegant design. 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.
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. This condition allows engine vacuum to draw proportionally more fuel than normal from the jet. Should there be no oil in the dashpot, only a normal running mixture can be produced, and acceleration will be very poor.
Model designation of Zenith-Stromberg carburetors can be a little confusing, as the cast series designation on the top covers are not always complete and accurate. The only way to accurately identify these carburetors is by the small square metal tag attached under one of the top cover screws. The number stamped on these tags is the unique specification number for a carburetor’s particular application.
First introduced in 1964, the original CD series incorporated no pollution control features, and provided cold start enrichment by means of a “starter bar”, which operated as a true choke. Various types followed, incorporating various pollution-control features. The CDSF series incorporated biased spring-loaded metering needles and cable-operated “starter boxes”, along with temperature compensators and bypass valves. The CDSEV series added a float chamber vent valve, which alters float chamber ventilation according to throttle position. The CDST series used a thermostatically operated “water choke”. CD4T and CDST carbs use the water choke, a “Downstream Discharge Idle Circuit”, which is adjustable independently from the non-idle operation of the carburetor, and internal emulsion jet temperature compensation.
The “add-on” devices found on most Zenith-Stromberg carburetors are probably the main cause of confusion, complaint, and problems, mostly because they are not understood. Careful study of them, and the understanding that they are discreet, sell-contained units makes dealing with them much easier.
The rotary “starter box” used on many of these carburetors has a series of holes of different diameters drilled in a circular plate. As the choke cable is pulled out. more of these holes are progressively lined up with a passage leading into the throttle body between the air valve and the throttle disc. Fuel is drawn from the float chamber through the holes in the disc to provide the required enrichment for starting.
The “water choke” units are very complex in construction and operation. They incorporate a bi-metallic coil which controls the fuel enrichment by means of a tapered needle valve assembly. However, in order to meet pollution control requirements, this action is not direct, but acts through a stepped cam, levers, vacuum piston, and throttle position sensor. All the driver has to do is to momentarily depress the accelerator pedal, and the automatic functions do the rest to establish the correct amount of fuel enrichment required.
The external temperature compensators prevent overly rich mixture when the engine compartment temperature, and therefore the fuel temperature, is high, particularly when idling in traffic, by bleeding additional air into the throat of the carburetor. Bypass valves overcome the mixture-richening effects of sudden deceleration and engine overrun. At a certain high level of manifold vacuum the bypass valve opens, allowing a metered bleed of air/fuel mixture to pass behind the throttle disc. This has the dual effects of maintaining efficient (i.e. less polluting) combustion, and lowering the manifold vacuum which caused the overly-rich condition.