What is a fixed-jet carburettor

fixed-jet carburettor

Here you can get Adjusting a fixed-jet carburettor, Changing from jet to jet and Typical fixed-jet carburettor ect.

A typical fixedjet carburettor. This is a Solex, and air flow is from top to bottom.The partial vacuum caused by increased air speed through the venturi sucks fuel through a jet to mix with the air. Similarly, air flow is controlled by a throttle flap linked to the accelerator pedal, to regulate engine speed.

A Fixed-jet Carburettor

The fixed-jet carburettor resembles the simpler variable-jet type (See How variable-jet carburettors work ) in having a venturi – a constricted neck – through which air flows on its way to the engine .

The partial vacuum caused by increased air speed through the venturi sucks fuel through a jet to mix with the air.

Similarly, air flow is controlled by a throttle flap linked to the accelerator pedal, to regulate engine speed.

Above the throttle a choke flap partially blocks the air flow, to give a richer mixture for starting. As in all carburettors , a float chamber provides a steady supply of fuel.

Changing from jet to jet

The fixed-jet carburettor has open jets to regulate fuel flow via them. Consequently there need to be several jets of different sizes to offer the one of a kind quantities of gas needed at any moment.

When the engine is idling, little or no gas is required. There is not a great deal air float via the nearly closed throttle – too little to draw any gas via the primary jet in the venturi.

But there’s a high vacuum underneath the throttle flap, wherein there is a tiny slow-running jet that forms a part of the often complex slow-running (idling)
circuit. The vacuum pulls a trickle of fuel through this jet to keep the engine idling.

When the throttle is opened, the air flow suddenly speeds up. An accelerator pump linked to the throttle provides a brief squirt of extra fuel to enrich the mixture temporarily to prevent a flat spot – a momentary hesitation – which is the inability of the carburettor to provide the correct mixture to meet the sudden power demand.

The pressure to supply this squirt comes from a rubber diaphragm open to the air on one side. Normal air pressure, higher than the partial vacuum inside the carburettor, pushes the diaphragm inwards against a piston , which pumps fuel.

Afterwards, the fast air flow sets up a vacuum in the venturi which draws fuel from the main jet. The faster the flow, the more fuel is sucked out. Most carburettors have one or more non-return valves , usually a small ball seating on a conical hole. This prevents wasted flow-back of fuel.


Set the throttle-stop screw so that the engine idles a little faster than normal. On most carburettors the volume screw regulates fuel flow — on this type the volume screw’s position is low on the carburettor body.

Turning it clockwise weakens the mixture, causing ‘hunting’ — a rhythmical rise and fall in engine speed. Turning it anti-clockwise enriches the mixture, producing ‘lumpy’, irregular running. An air volume-control screw is placed higher up. Turning it clockwise enriches the mixture; turning it anti-clockwise weakens the mixture.

Find the two points at which hunting and lumpy running begin. Set the screw halfway between them, unless some other setting clearly makes the engine run better. Note the screw setting. Reset the throttle-stop screw for a normal idling speed, and repeat the test with the mixture screw. Probably this will result in the same final setting.

Typical fixed-jet carburettor

The jets come into action as needed by changes in the vacuum inside the carburettor, caused by different engine speeds and throttle openings. Since the jets are fixed – not adjustable – the mixture is adjusted by directly regulating the fuel or air flow to them. On carburettors made before about 1974 you can adjust two settings. One is the volume of fuel (or on some types, air) going into the engine . The other is the engine’s idling speed, which is set by adjusting the throttle-stop screw.

The adjustment screws are located differently on different carburettors. Later carburettors have had to comply with anti-pollution laws concerning exhaust gases. They are called emission-control carburettors, and have sealed, tamper proof adjustments which generally you cannot change (See Adjusting an emission-control carburettor ).

Before you tune the carburettor, make sure that it needs tuning, by eliminating all other causes of bad running (consult Faultfinder). Certain adjustments are made with the engine running, so while it is cold, find the adjustment screws and work out how to reach them easily without burning yourself when working with the engine hot.

It may also help you to do the job without removing the air cleaner . Removing the air cleaner increases air flow and upsets the mixture. Bring the engine up to normal working temperature, then work quickly before it gets much hotter – which would also affect the mixture.


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