Introduction to Zenith-Stromberg Carburetors
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, as 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 CDSE 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 CD5T 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, self-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 (ie.
less polluting) combustion, and lowering the manifold vacuum which
caused the overly-rich condition.
(The next Under the Bonnet article will deal with maintaining, troubleshooting, and tuning Zenith-Stromberg carburetors.)
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