How it works

Looking at the downdraught carburettors used on Land Rovers, first is the float chamber, which is nothing more than a fuel tank for the carburettor, supplying fuel to the emulsion tube. The fuel pump delivers fuel into that float chamber and, as the level rises, a hinged float touches against a needle valve, shutting off the fuel supply to the chamber. It’s a similar principle to a toilet ballcock. As fuel is used, the level (and float) lowers and the needle valve re-opens, thus constantly maintaining the crucial float height, and therefore fuel supply to the emulsion tube.

Parallel to this, sits the carburettor throat: a large circular hole through the carb where air enters at the top (drawn in by the suction created by the engine’s pistons on their intake strokes), and air-fuel mix is delivered at the bottom into the intake manifold. Jutting into the throat is the emulsion tube (which is nothing more than an outlet sitting in the middle of the throat), a venturi and, below that, the butterfly valve of the throttle plate.

Fuel sits in the emulsion tube at the same height as in the float chamber. As the engine runs, air enters the carb throat and passes through the venturi which constricts the air, creating negative pressure downstream, thereby sucking in the fuel from the emulsion tube. The fuel becomes atomised in the carb throat where it mixes with the air at a theoretical ratio of 14.7 parts air to 1 of fuel (known as the stoichiometric mixture), though the mixture will vary according to tuning and running conditions. The fuel/air mixture then enters the intake manifolds from where it’s drawn into the engine cylinders where the spark plugs ignite it in the correct sequence to drive the pistons and rotate the crankshaft.

An engine needs to rev above idle speed though, and that is where the throttle plate comes in. As the throttle is depressed, the throttle butterfly opens in the carb throat, allowing more air in, in turn sucking more fuel in, thereby engine revs increase.

The accelerator pump is an additional circuit linked to the throttle plate. Under rapid throttle increases, the accelerator pump squirts in a measured amount of extra fuel straight above the venturi. Being before the vacuum is formed, this enrichens the fuel mix ahead of the emulsion tube, aiding the engine to accelerate.