The Twin Choke Carburettor
Big capacity V8 engines produced by American manufacturers cause the problem of getting a large choke area to pass all the required air, and simultaneously allow sufficient depression at low speeds. The difficulty is met by using four choke carburettors. These are, in fact, a pair of twin choke instruments in a single block, the twin choke being two carburettors of which one becomes effective only at wider throttle openings. The other provides the starting, slow running and compensating systems, as well as all the mixture at lower throttle openings. The Weber 28/36 DCD, as was fitted to Fiat and Ford GT engines, is a good example of a highly efficient modem multi jet twin choke carburettor. It satisfies the varying demands of relatively high performance engines and aids economy by ensuring that the first throttle has opened about half way before a lost motion linkwork on the throttle spindles begins to open the second throttle below the parallel second choke. The two carburettors then work together to deliver mixture for the higher power demands. More complex Weber carburettors may be fitted with automatic, thermostatically controlled chokes, pneumatically controlled differential opening of the throttles, systems for part throttle weakening for economy, means for heating the idling speed mixture, means for compensating for altitude and full air pollution control devices. All new Mercedes models nowadays, do not use this technology any longer, rather relying on direct injection systems.