The Zenith Carburetter Company Limited was a British company making
carburettors in Stanmore Middlesex founded in 1910. In 1965 they
joined with their major pre-war rival
Solex Carburettors and over time
the Zenith brand name fell into disuse. The rights to the Zenith
designs were owned by
Solex UK (a daughter company of
While better known for their much later products, Zeniths were
standard equipment on some very early, brass era automobiles,
including the Scripps-Booth.
2 See also
4 External links
The big products of Zenith were the Zenith-Stromberg carburettors used
in MGs, 1967-1975 Jaguar E-types, Saab 99s, 90s and early 900s,
1969-1972 Volvo 140s and 164s, and some 1960s and 1970s Triumphs.
non-UK German Pierburg (Stromberg) carburettor in a Saab 90
non-UK German Pierburg (Stromberg) carburettor dashpot
British made Zenith/Stromberg carburettors as installed on a 1969
Jaguar E-type 6cyl 4.2l engine
Triumph Spitfire for instance used Zenith IV carburettors in the
North American market. In Australia the CD-150 and CDS-175 models were
fitted to the high performance triple carburettored Holden Torana
Designed and developed by Denis Barbet (Standard Triumph) and Harry
Cartwrite (Zenith) in order to break SU's patents, the Stromberg
carburettor features a variable venturi controlled by a piston. This
piston has a long, tapered, conical metering rod (usually referred to
as a "needle") that fits inside an orifice ("jet") which admits fuel
into the airstream passing through the carburettor. Since the needle
is tapered, as it rises and falls it opens and closes the opening in
the jet, regulating the passage of fuel, so the movement of the piston
controls the amount of fuel delivered, depending on engine demand.
The flow of air through the venturi creates a reduced static pressure
in the venturi. This pressure drop is communicated to the upper side
of the piston via an air passage. The underside of the piston is in
communication with atmospheric pressure. The difference in pressure
between the two sides of the piston creates a force tending to lift
the piston. Counteracting this force is the weight of the piston and
the force of a compression spring which is compressed by the piston
rising; because the spring is operating over a very small part of its
possible range of extension, the spring force approximates to a
constant force. Under steady state conditions the upwards and
downwards forces on the piston are equal and opposite, and the piston
does not move.
If the airflow into the engine is increased - by opening the throttle
plate, or by allowing the engine revolutions to rise with the throttle
plate at a constant setting - the pressure drop in the venturi
increases, the pressure above the piston falls, and the piston is
sucked upwards, increasing the size of the venturi, until the pressure
drop in the venturi returns to its nominal level. Similarly if the
airflow into the engine is reduced, the piston will fall. The result
is that the pressure drop in the venturi remains the same regardless
of the speed of the airflow - hence the name "constant depression" for
carburettors operating on this principle - but the piston rises and
falls according to the speed of the airflow.
Since the position of the piston controls the position of the needle
in the jet, and thus the open area of the jet, while the depression in
the venturi sucking fuel out of the jet remains constant, the rate of
fuel delivery is always a definite function of the rate of air
delivery. The precise nature of the function is determined by the
tapered profile of the needle. With appropriate selection of the
needle, the fuel delivery can be matched much more closely to the
demands of the engine than is possible with the more common
fixed-venturi carburettor, an inherently inaccurate device whose
design must incorporate many complex fudges to obtain usable accuracy
of fuelling. The well-controlled conditions under which the jet is
operating also make it possible to obtain good and consistent
atomisation of the fuel under all operating conditions.
This self-adjusting nature makes the selection of the maximum venturi
diameter (colloquially, but inaccurately, referred to as "choke size")
much less critical than with a fixed-venturi carburettor.
To prevent erratic and sudden movements of the piston it is damped by
light oil in a dashpot (under the white plastic cover in the picture)
which requires periodic topping up.
One of the major drawbacks of the constant depression carburettor is
in high performance applications. Since it relies on restricting air
flow in order to produce enrichment during acceleration, the throttle
response lacks punch. By contrast, the fixed choke design adds extra
fuel under these conditions using its accelerator pump.
Solex for the
SU Carburettor, works with the same principle.
Zenith (other) for other companies named Zenith.
^ Harold George Castle (1950). Britain's Motor Industry. Clerke &
Cockeran. p. 146.
^ Zenith Carburettor Prospects. The Times, Monday, Apr 26, 1965; pg.
16; Issue 56305
Listing of application of Zenith carburettors
A very comprehensive SU and Stromberg Carb Needle Selection and