Car Parts and Accessories

The fixed-jet (or fixed-choke) carburettor incorporates various jets and an accelerator pump to alter the mixture’s strength according to engine needs.

As the airstream through a carburettor’s venturi speeds up, the air becomes ‘thinner’ and without some compensating device the mixture would become progressively richer until it was too rich to burn.

The fixed-jet carburettor solves this problem by air correction—mixing some air with petrol before the petrol is drawn into the venturi. On most carburettors, air correction is by means of a perforated tube which emulsifies the mixture. The main jet supplies petrol to a spraying well which contains the perforated tube, closed at the top by a calibrated air-correction jet. As the engine speed rises and the petrol level in the well falls, an increasing amount of air is drawn through the series of holes in the tube, automatically weakening the mixture.

The alternative method is to put in a compensating jet in addition to the main jet. As the fuel level drops in a well alongside the float chamber, air is drawn into the compensating jet so that a mixture of air and petrol, instead of petrol alone, reaches the main discharge point. The weakness of the mixture from the compensating jet cancels out the increasing richness of the mixture supplied by the main jet.

The size of the main jet is usually designed to give the relatively weak mixtures necessary for economic cruising.

To give the richer mixtures needed for full throttle, the fixed-jet carburettor has an additional jet feeding the main discharge well. This supplements the main jet which can be kept small for economy.

Varying the mixture for different Vehicle Speeds

When an engine is started from cold pulling out the choke on the dashboard of cars fitted with manual chokes closes a spring-loaded butterfly valve and opens the throttle valve slightly. This restricts the air flow and increases suction from the main jet into the venturi, so giving the richer mixture needed for starting. When the engine fires and speeds up, the additional air drawn in forces the strangler flap to open partially, weakening the mixture, to prevent crankcase oil dilution and cylinder bore wear.

When the engine is hot, the movement of the pistons on starting causes a partial vacuum in the inlet manifold. Since the throttle is closed, this partial vacuum becomes effective at the ‘idle‘ discharge hole, and draws fuel from the float chamber via the main jet through the idling system of the carburettor.

Air is drawn into this fuel through the pilot air-bleed, to emulsify the mixture. As the fuel flows through the idle circuit, the fuel level drops in the main emulsion well, uncovers some of the emulsion holes and allows air to mix with the fuel.

Opening the throttle by pressing the accelerator pedal adds to the air flow through the air-correction jet, and the increased partial vacuum at the venturi causes the emulsified petrol/air mixture to rise in the well and discharge into the main airstream, through the venturi.

Simultaneously, the partial vacuum at the idle discharge decreases and the flow of fuel at this point ceases.

To avoid any undue weakening of the mixture, which would cause flat spots, during this transition stage, it is usual to provide one or more ‘progression’ holes in the idle circuit.

To give the additional fuel needed for acceleration and sudden throttle openings, an accelerator pump is provided. This consists of a well, filled with fuel and fitted with a spring-loaded plunger or diaphragm, linked to the throttle. When the throttle is opened, the fuel in the well is discharged by the plunger through a separate passage into the venturi.

On some carburettors the plunger stroke can be adjusted to provide increased fuel delivery for winter conditions, or less for summer motoring.

Present-day engines and motoring conditions have resulted in a wide variety of fixed-jet carburettors with a complex arrangement of fuel passages, jets and discharge holes.

Their great advantage is the absence of moving parts; all the metering of fuel and air is controlled by fixed jets and venturi throats of fixed size.

Choke increases mixture strength

A RICH MIXTURE with an air/fuel ratio of between 1 : 1 and 3:1 is needed to start the engine in cold weather. This is provided by closing a choke flap or strangler, operated automatically or by a choke control on the instrument panel.

Because the carburation system is cold, and because the air speed through the venturi is low, only part of the petrol will be able to vaporise.

When the engine fires, the inlet manifold starts to heat up and the mixture can then be weakened to between 8:1 and 10 : 1. This weakening is essential to avoid dilution of oil, and undue wear of the cylinder bores caused by liquid fuel running down the cylinder walls. A mixture of 14 : 1 is usually sufficient once the engine is warm.

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