Carburetors are either:
- Fixed Choke (Venturi) - the varying depression in the venturi alters the mixture
- Constant depression - the jet is varied to alter the mixture.
The most common, variable choke (constant depression) type carburetor is the SU, which was simple in principle to adjust and maintain. This rose to position of domination in the UK car market for that reason.
The carburetor must:
- Deliver the correct ratios of fuel and air across the operating range
- Mix the two finely and evenly
The fundamental function of a carburetor is fairly simple, but the implementation is fairly complex. A carburetor must provide the proper fuel/air mixture under a wide variety of different circumstances and engine speed range.
- Cold start
- Idling or slow-running
- Acceleration
- High speed / high power at full throttle
- Cruising at part throttle (light load)
Most carburetors contain equipment to support several different operating modes, called
circuits.
A carburetor basically consists of an open pipe, the carburettor's "throat" or "barrel", through which the air passes into the inlet manifold of the engine. The pipe is in the form of a
venturi - it narrows in section and then widens again. Just after the narrowest point is a butterfly valve or
throttle - a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver. This in turn affects the engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.
Fuel is introduced into the air through fine calibrated holes, referred to as
jets.
Idle circuit
When the butterfly valve is closed or nearly closed, the carburetor's
idle circuit is in operation. The closed throttle means that a fairly significant vacuum occurs behind the closed butterfly valve. This manifold vacuum is sufficient to pull fuel and air through small openings placed after the butterfly valve - and in SU carburetors to pull the piston and metering rod up. Only a fairly small amount of air and fuel can pass through in this manner.
Off-idle circuit
As the throttle is opened up slightly from the fully closed position, the side of the rotating "plate" that moves forward as it swings open uncovers extra openings similar to that of the idle circuit. These allow more fuel to flow as well as compensating for the reduced vacuum at slight open throttle.
Main open-throttle circuit
As the throttle is progressively opened, the manifold vacuum reduces since there is less restriction on the airflow. This reduction in vacuum reduces the flow through the idle and off-idle circuits, so another method of introducing fuel into the airflow is needed.
This is where the venturi shape of the carburetor throat comes into play. The Bernoulli effect shows that as the velocity of a gas increases, its pressure falls. The venturi (sometimes two venturi nested in the same barrel) makes the air reach a higher velocity at the middle than at the ends, and this high speed and thus low pressure in the middle sucks fuel into the airstream through a nozzle (a "jet") located in the center of the throat.
The main circuit requires a reasonable airspeed through the carburetor throat to function, and thus ceases to function during idle, where the idle circuit steps in.
Accelerator pump
If the throttle is rapidly opened, it can be seen that all of the above circuits will fail to function. The idle circuit will not work, since the throttle is open and the manifold vacuum has fallen off. The main circuit will also not work, since there is not yet sufficient airflow. Thus, there needs to be a supplemental method of fuel delivery that will "bridge the gap" between the idle circuit stopping and the main circuit kicking in.
This is the accelerator pump, driven from the accelerator linkage, which delivers a squirt of fuel under low pressure when the throttle is rapidly opened. The size and duration of this need to be adequately tuned so that the gap is bridged and a transition from the idle to main circuit is achieved smoothly.
Choke
When the engine is cold, ignition and combustion happens less readily, and some of the fuel vapor condenses on the cold intake manifold and cylinder walls. Thus, a
richer mixture - more fuel to air - is required. To achieve this, a "choke" is used. This is a device that restricts the flow of air at the entrance to the carburetor. This functions similarly to the throttle being closed, except for the fact that it is closed off before both idle and main circuits. Here, the low pressure caused by the restriction sucks fuel through all the fuel circuits - idle, off-idle, and main. The choke may be automatically controlled by a
thermostat, or manually operated. The choke may also be known as a
strangler for older vehicles.
Some carburetors do not have a dedicated air restriction valve but instead use a mixture enrichment device. Typically used on small engines, notably motorcycles, it works by opening a secondary fuel circuit. The outlet of this circuit is located behind the throttle valve and when engaged, delivers its extra fuel when the throttle is closed and vacuum is high. As the throttle opens the vacuum falls at the opening and it supplies less fuel. This self regulation makes it possible to operate the engine sooner.
Other elements
The interactions between each circuit may also be affected by various mechanical or air pressure connections and also by temperature sensitive and electrical components. These are introduced for reasons such as response, fuel efficiency or automobile emissions control. Extra refinements may be included in the carburetor/manifold combination eg electrical heating to compensate for a cold engine.
Fuel Supply
Float chamber
To ensure a ready supply of fuel, the carburetor has a "float chamber" (or "bowl") that contains a quantity of fuel ready for use. It converts fuel from fuel pump pressure to atmospheric pressure. This works similarly to a toilet tank; a float controls an inlet valve. If the float drops, the inlet is opened allowing the fuel to flow under the fuel pump's pressure. Usually, special vent tubes allow air to escape from the chamber as it fills.
Diaphragm chamber
If the engine must be operated in any orientation (i.e. chain saws), a float chamber cannot work. Instead a diaphragm chamber is used. A flexible diaphragm forms one side of the fuel chamber and is arranged so that as fuel is drawn out into the engine the diaphragm is forced inward by ambient air pressure. The diaphragm is connected to the needle valve and as it moved inward it opens the needle valve to admit more fuel, thus replenishing the fuel as it is consumed. As fuel is replenished the diaphragm moves out due to fuel pressure and a small spring, closing the needle valve. A balanced state is reached which creates a steady fuel reservoir level, which remains constant in any orientation.
Carburetor systems
The carburetor supplies the engine with the correct mixture to suit all operating conditions, from idling to high speed. To do this it has a number of systems.
With the engine stopped, the throttle valve is closed but fuel is still held in the bowl. With the engine running, fuel is held at a set level by the float and a needle valve. Fuel supplied to the engine when it is running is replaced by fuel from the tank.
A passage from the air horn to the float bowl balances air pressure between the air cleaner and the float bowl. The idle air passage and the main discharge nozzle are above the level of the fuel.
The first stage of the idle system uses the idle and low-speed circuit, plus an idle adjustment screw.
The throttle valve is almost closed, so air-flow through the carburetor is very small. The action of a piston creates a low-pressure area below the throttle, and this is concentrated at the edge of the throttle valve as the air passes the idle port. Fuel flows from the float bowl, through the idle passages and into the carburetor below the throttle valve at the idle port. The air bleed lets air enter the fuel on its way from the float bowl. This helps aerate the fuel before it reaches the idle port.
Engine idle speed is set by 2 different adjustments. The amount of fuel is adjusted by the mixture adjustment screw at the idle port.
The amount of air is adjusted by changing the throttle stop screw.
Second-stage idling starts as the throttle valve opens. This is similar to idle, but with the low-speed port uncovered. Both ports discharge fuel to mix with incoming air. Low-speed ports help the transition from idling, to low-speed, to high speed. Without them, the engine tends to hesitate until the main system comes fully into action.
The main or high-speed system comes into action above fast idle, as airflow through the venturi increases. A main metering jet in the fuel in the bowl meters fuel passing into the discharge nozzle.
How much fuel leaves the nozzle depends on the pressure difference created by the air flow through the venturi. As the throttle opens, and air flow increases, and speeds up, more and more fuel is drawn from the discharge nozzle. However, the mass of air doesn’t increase in proportion with the speed, and as a result, high speeds can produce a mixture that is too rich. To correct this, more air can be added. This is called compensation by air correction.
As the throttle opens and engine speed increases, the level in the jet well falls, exposing air bleed holes in the discharge tube. Air can now mix with the fuel and stop the mixture becoming too rich. As the throttle opens further, the fuel level falls too, exposing more air holes. More air bleeds in, to maintain the correct mixture.
The size of the main jet is selected to provide the best mixture for economy, under cruising conditions. When the throttle is open wide for maximum power, a richer mixture is required. The extra fuel is provided by a power jet, with a vacuum piston and rod opening it as it is needed.
At low speeds, intake manifold vacuum is transferred through a passage to the vacuum piston. This holds the piston up, and keeps the power valve closed.
With the throttle valve fully open for full engine power, the vacuum in the intake manifold falls. A spring pushes down the vacuum piston and rod, to open the power valve. Fuel flows through a bypass jet to enter the fuel well, and add to the fuel from the main jet. This provides the extra fuel needed to enrich the mixture for full power.
Extra fuel is also needed for accelerating. Suddenly opening the throttle increases the air flow, but fuel cannot flow from the discharge nozzle quickly enough to match it. An extra jet of fuel is needed. Depressing the pedal compresses a duration spring that exerts a force on the plunger of a small plunger pump. This pressurizes fuel below the plunger and closes off the inlet valve. Fuel flows through a bypass jet and enters the air stream from a discharge nozzle above the venturi. The duration spring extends the time for delivering the fuel.
Releasing the pedal lets the linkage move the plunger upwards. The bypass jet closes and the inlet valve opens, to let fuel refill the pump chamber from the float bowl.
When a cold engine is being started, little air flows through the venturi, and there is no heat to assist in vaporizing any fuel that is delivered. This makes the effective mixture of fuel and air too weak to be readily ignited by the spark plug. An excess of fuel must be supplied temporarily to ensure that the proportion of fuel that does vaporize will form an ignitable mixture. A choke valve is fitted to help. Closing the choke valve closes the carburetor intake. The pumping action of the pistons creates a low pressure area below the valve, even without venturi action. This low pressure causes fuel to flow from the discharge nozzle and from the idle and low-speed ports, and provides the rich mixture needed to start the cold engine.
The choke can be controlled manually by a cable that operates the valve.
Most are controlled automatically, so that the valve is closed when the engine is cold, and opens progressively as the engine warms up.
When the engine is warm, the fuel drawn into the manifold during starting vaporizes readily, and the engine can be started without the aid of a choke.
The choke should operate as briefly as possible. Overusing it produces rich mixtures that cause exhaust pollution, and increase fuel consumption.
Flooding a carburetor also produces rich mixtures. This can be caused by wear, or by dirt trapped in the needle and seat that causes the level in the float bowl to rise, and fuel to discharge from the nozzle - with little or no venturi action.
Carburetors can have a single barrel, or 2, or 4 barrels.
Extra barrels improve performance, particularly at high speeds, letting more air enter the cylinders than with a single barrel.
2 barrel-carburetors have 2 outlets to the inlet manifold, and have 2 basic designs.
One has a common float chamber, but each barrel has a complete set of all other circuits, and the throttles can open simultaneously. The float chamber may be straddled by 2 connected floats that almost surround the air passage.
This leaves it unaffected by cornering, climbing, accelerating or braking.
The other basic design has throttles that open in 2-stages. It combines the 2 barrels to act as a single carburetor. The 2 stages combine good low speed operation of a single-barrel design, with the extra air flow of 2 barrels.
One-half of the carburetor has all the circuits needed to supply mixtures for the whole range of operation. This is called the Primary side. The other barrel, the Secondary side, supplies extra mixture, but only at high speed or full throttle. It normally has a main metering system and a non-adjustable idling system. The primary side has a choke for cold starting.
When the engine is being started, the throttle on the secondary side is already closed, so a choke isn’t needed.
From idle to medium speeds, only the primary throttle is open. When engine speed rises to where additional breathing capacity is needed, the secondary throttle opens to admit more air-fuel mixture. By the time the primary throttle is wide-open, so is the secondary throttle. This can be controlled mechanically.
Or by a vacuum unit, connected by pullrod, to a lever on the secondary shaft. When air flows past ports in the venturis, it produces low-pressure areas. A hose transmits this low pressure to a diaphragm chamber. This low pressure acts on the diaphragm and opens the secondary throttle.
Large capacity V-8 engines may use a 4-barrel carburetor of 2-stage design - effectively two, 2-stage carburetors combined, with each side supplying 4 of the 8 cylinders.
A central molded plastic fuel bowl and suspended metering system can be incorporated into the design. This gives lower fuel temperatures with more precise fuel metering and closer control over air-fuel ratios.
