The Zenith-Stromberg CD carburettor: principle, SU settings and adaptation to the 2 CV

In this document

The Zenith-Stromberg type CD carburettor: principle and operation

The document describes a new Zenith-Stromberg type CD carburettor, operating "according to the constant depression principle", similar to English SU carburettors p. 25. A tapered needle integral with a piston regulates the jet orifice; the piston is actuated by the intake manifold vacuum, applied to a bell via a diaphragm ensuring a seal p. 25.

The detailed operation describes the fuel circuit (tube, needle valve, floats, bowl, jet), the rise of the piston via the vacuum chamber, and the role of the metering needle p. 25. A "dash-pot" controls the descent of the piston and ensures, in the absence of an accelerator pump, the enrichment of the mixture during acceleration p. 25.

The Cold start and Idle sections explain respectively the role of the choke lever coupled to the device (6)(20) and the absence of a specific idle circuit, the richness being adjusted by a screw (13) and the speed by a screw (3) p. 25. The text specifies that this carburettor "is fitted as standard on the Triumph TR 4" p. 25.

Fig. 7 offers four cross-sectional views numbered (1 to 29) illustrating these mechanisms p. 23.

SU setting tables (Morris, Triumph)

The document provides the settings for the semi-downdraught SU HS 2 carburettors for the Morris Minor and the Mini Cooper 1.100, as well as for the Triumph Spitfire 4 p. 24:

The text specifies that the Spitfire 4 engine is derived from the Triumph Herald 1,147 cm³ engine, but with a compression ratio of 9 to 1 p. 24.

Adaptation to the Citroën 2 CV AZL

The document indicates that "since January 64, Citroën has equipped the 2 CV AZL saloon with a Zenith 28 IN 4 carburettor" p. 25, operating in a similar way to the 28 IN model of the AZU vans, but supplemented by a "dash-pot" intended to prevent too abrupt a return to idle, which "would cause the engine to stall" due to the presence of the auxiliary clutch on the saloon p. 25. The mechanism (lever, plunger, ball-valve piston, spring) is detailed and illustrated by Fig. 8 p. 25.

Context

The 2 CV is an iconic Citroën model, produced from 1948; the auxiliary clutch mentioned here corresponds to a device specific to certain versions of the saloon intended to facilitate low-speed manoeuvres — this point is general knowledge and is not detailed in the document.

SU carburettors and their constant depression derivatives (including the Stromberg CD) equipped many British vehicles of the 1950s-1960s, notably at Triumph and Morris/Austin (BMC group), manufacturers cited in this document.

Archivist's perspective

This documentary piece, probably extracted from a manual or a technical journal from 1964, seems to constitute a reference source gathered by Paul Magès to compare constant depression carburetion technologies — British (SU, Stromberg CD) and French (Zenith fitted to 2 CV) p. 23-25.

The interest in this type of carburettor, an alternative to injection, is part of a broader technical monitoring that can be found in other pieces of the collection devoted to carburetion and injection, such as the handwritten divider "Carburation" or the handwritten cover page of the file on carburetion and injection, which suggest a comparative and documentary approach followed over several years.