How carburetors work

The carburetor is used to create a fuel-air emulsion, depending on the conditions and specified operating modes of the engine cylinders

We will consider the operation of carburetors using the example of a VAZ-21083 carburetor.

Main dosing system

Fig. 1. Scheme of the main dosing systems

Fuel is fed through the strainer 4 (Fig. 1) and the needle valve 6 into the float chamber.

From the float chamber, the fuel enters through the main fuel jets 9 into the emulsion wells and mixes with the air leaving the holes of the emulsion tubes 1, which are made integral with the main air jets.

Through sprayers 2, the fuel-air emulsion enters the small and large diffusers of the carburetor.

Throttle valves 8 and 10 are interconnected in such a way that the second chamber starts to open when the first one is already open by 2/3 of the amount.

Idling system

Fig. 2. Scheme of the idle system and transitional systems

Takes fuel from the emulsion well after the main fuel jet 7 (Fig. 2).

Fuel is supplied to the fuel jet 2 with an electromagnetic shut-off valve 1, at the outlet of the jet it mixes with air coming from the flow channel and from the expanding part of the diffuser (to ensure the normal operation of the carburetor when switching to idle mode).

The emulsion exits under the throttle valve through a hole regulated by screw 9 for the content of carbon monoxide (CO) in the exhaust gases.

Transition systems

When the carburetor throttles are opened before the main metering systems are turned on, the air-fuel mixture flows:

  • - into the first mixing chamber through the idle jet 2 and the vertical slot 8 of the transition system, located at the level of the throttle valve edge in the closed position;
  • - into the second mixing chamber through the outlet 6, located just above the edge of the throttle valve in the closed position.

Fuel comes from jet 4 through a tube, mixes with air from jet 5 coming through the flow channel.

Power modes economizer

Fig. 3. Scheme of econostat and economizer of power modes

It works at a certain vacuum behind the throttle valve 5 (Fig. 3).

Fuel is taken from the float chamber through ball valve 8.

The valve is closed as long as the diaphragm is held by vacuum in the intake pipe.

With a significant opening of the throttle valve, the vacuum drops somewhat and the diaphragm spring 7 opens the valve.

The fuel passing through the economizer jet 9 is added to the fuel that passes through the main fuel jet 4, enriching the combustible mixture.


Operates at full engine load at speeds close to maximum, with wide open throttles.

Fuel from the float chamber through jet 3 enters the fuel pipe and is sucked out through the injection pipe 13 into the second mixing chamber, enriching the combustible mixture.

Accelerator pump

Fig. 4. Diagram of the accelerator pump

Diaphragm, mechanically driven by cam 6 (Fig. 4) on the axis of the throttle valve of the first chamber.

When the throttle is closed, the spring pulls the diaphragm 3 back, which leads to filling the pump cavity with fuel through the ball valve 8.

When the throttle is opened, the cam acts on lever 5, and diaphragm 3 pumps fuel through ball valve 2 and nozzles 1 into the mixing chambers of the carburetor, enriching the combustible mixture.

The performance of the pump is not adjustable and depends only on the profile of the cam.

Semi-automatic launcher

Fig. 5. 21083–1107010-31 semi-automatic carburetor starter diagram

Improves vehicle control and reduces exhaust gas toxicity during engine start and warm-up (Fig. 5).

When starting a cold engine, the bimetallic spring of the starting device (not shown in Fig. 5) with the help of levers and rod 8 keeps the air damper 7 closed.

After starting the engine, the damper with the help of diaphragm 6 opens slightly to clearance A, which is adjusted by screw 11 of the stem 12 of diaphragm 6 of the starting device.

Fig. 6. 21083–1107010-31 semi-automatic carburetor hose connection diagram

As the engine warms up with the coolant circulating through the liquid chamber 4 (Fig. 6) of the starting device, the bimetallic spring also heats up, which ensures the opening of the air damper through the levers of the starter drive and thrust 8 (see Fig. 5).

When the engine is warm, the air damper is fully open with a bimetallic spring.

Forced idle economizer

Disables the idle system during forced idle (during engine braking, downhill driving, gear shifting), reducing fuel consumption and hydrocarbon emissions.

In the forced idle mode, at a crankshaft speed of more than 2100 min -1 and with the carburetor limit switch closed to ground (pedal released), the shut-off solenoid valve turns off, the fuel supply is interrupted.

When the speed of the crankshaft at forced idle speed drops to 1900 min-1, the control unit turns on the electromagnetic shut-off valve (although the limit switch is turned on to ground), fuel begins to flow through the idle jet , the engine gradually idles.