Some Carburetor Theory – Part 2

A carburetor basically consists of an open pipe, a “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, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve called the 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, thereby regulating 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 stream through small holes at the narrowest part of the venturi. Fuel flow in response to a particular pressure drop in the venturi is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path

Idle Circuit

When the throttle valve is closed or nearly closed, the carburetor’s idle circuit is in operation. The closed throttle reduces the airflow through the venturi to a level which cannot overcome the resistance to flow of the fuel, but it also means that a fairly significant vacuum occurs behind the closed butterfly valve. This manifold vacuum is sufficient to pull fuel through small openings placed after the butterfly valve (and in SU and similar sidedraft carburetors to pull the piston and metering rod up).

Only a fairly small amount of air and fuel can pass through in this manner. Since this small volume of fuel/air mixture can generate so little force to keep the engine turning, keeping it running at idle is more difficult than keeping it running at higher speeds. Since the airflow is too low for the carburetor to respond at all, it cannot compensate for fluctuations;instead, idle airflow is set manually by the technician or mechanic, adjusting a screw which opens the throttle a tiny fraction to allow a minimal amount of air to pass, and another screw which serves as a valve in the idle fuel circuit to adjust the volume of fuel delivered. These adjustments interact with each other, as well as affecting manifold vacuum which affects distributor spark advance which in turn affects idle speed, so that adjusting the idle to optimum (highest manifold vacuum at the specified engine idle speed) is not a completely trivial operation. While experts often claim the ability to set the idle perfectly by ear, most individuals do a better job using a tachometer and vacuum gauge. Since the advent of emissions controls on production automobiles, the idle fuel flow is typically set at the factory on the “lean” side of optimal, by restricting fuel flow so that idle speed falls by 100 — 150 rpm from where it was when optimally adjusted, in order to reduce unburned hydrocarbons and carbon monoxide with some slight loss in reliable and smooth idling; the idle fuel adjustment is typically sealed at the factory to prevent tampering, so that adjustment when age and wear cause a large deviation from proper operation requires drilling out a plug over the adjusting screw or some similar modification to gain access.

Off-idle Circuit

As the throttle is opened up slightly from the fully closed position, the throttle plate uncovers additional fuel delivery holes slightly higher in the carburetor throat; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to metering fuel flow through the regular open throttle circuit.

Main Open-throttle Circuit

As the throttle is progressively opened, the manifold vacuum reduces since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli’s principle (i.e. as the velocity increases, pressure falls). The venturi (sometimes a second or “booster” venturi is placed inside the venturi shaped into the carburetor throat to increase the effect) raises the air velocity, and this high speed and thus low pressure sucks fuel into the airstream through a nozzle located in the center of the venturi.

As the throttle is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain this fuel flow, and the idle circuit takes over again, as described above.

Power Valve

For open throttle operation a richer mixture will produce more power, prevent detonation, and keep the engine cooler. This is usually addressed with a spring loaded “power valve”, which is held shut by engine vacuum. As the throttle opens up, engine vacuum decreases and the spring opens the valve to let more fuel into the main circuit.

Accelerator Pump

Similarly, the greater inertia of liquid gasoline, compared to air, means that if the throttle is suddenly opened, the airflow will increase more rapidly than the fuel flow, causing a temporary “lean” condition which causes the engine to “stumble” under acceleration (the opposite of what is normally intended when the throttle is opened). This is remedied by the use of a small mechanical pump (often just a simple plunger which pushes down through a small tube filled with gasoline which feeds into the carburetor throat) which injects an additional amount of fuel as the throttle is opened to cover this lean period; this is usually adjustable for both volume and duration by some means, sometimes just bending the linkage. Often the seals around the moving piston parts of the pump wear out, so that pump output is reduced; this loss of accelerator pump action causes the characteristic stumbling or bogging under acceleration often seen in old, well worn engines until the seals on the pump are replaced. Specialized aftermarket kits are widely available for this purpose. Other variations of pump also exist, such as diaphragm based pumps.


When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a richer mixture (more fuel to air) is required to start and run the engine until it warms up.

To provide the extra fuel, a choke is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls fuel through the venturi to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.

In addition, the choke is connected to a “fast idle cam” or other such device which prevents the throttle from closing fully, which could starve the venturis of vacuum and cause the engine to stall. This also serves as a way to help the engine warm up quickly by idling it at a higher than normal speed. In addition, it increases airflow throughout the intake system which helps to better atomize the cold fuel and smooth out the idle.

In older carbureted cars, the choke was controlled by a cable connected to a pull-knob on the dashboard (GB — facia) operated by the driver. In most carbureted cars produced from the mid 1960s onward (mid 1950s in the United States) it is usually automatically controlled by a thermostat employing a bimetallic spring, which is exposed to engine heat. This heat may be transferred to the choke thermostat via simple convection, via engine coolant, or via air heated by the exhaust. More recent designs use the engine heat only indirectly: A sensor detects engine heat and varies electrical current to a small heating element, which acts upon the bimetallic spring to control its tension, thereby controlling the choke. A choke unloader is a linkage arrangement that forces the choke open against its spring when the vehicle’s accelerator is moved to the end of its travel. This provision allows a “flooded” engine to be cleared out so that it will start.

Some carburetors do not have a choke but instead use a mixture enrichment circuit, or an enrichener. Typically used on small engines, notably motorcycles, enricheners work by opening a secondary fuel circuit below the throttle valves. This circuit works exactly like the idle circuit, and when engaged it simply supplies extra fuel when the throttle is closed.

Classic British motorcycles, with side-draft slide throttle carburetors, used another type of “cold start device”, called a “tickler”. This is simply a spring-loaded rod that, when depressed, manually pushes the float down and allows excess fuel to fill the float bowl and flood the intake tract. If the “tickler” was held down too long it also flooded the outside of the carburetor and the crankcase below, and caused a few fires in the process.

The main idea behind these devices is that extra fuel (a rich condition) is necessary to get a “cold” engine started and running for a short period of time. Either the air is restricted (choke), or more fuel is added (enrichener and tickler).

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. Various air bleeds (often chosen from a precisely calibrated range, similarly to the jets) allow air into various portions of the fuel passages to enhance fuel delivery and vaporization. Extra refinements may be included in the carburetor/manifold combination, such as some form of heating to aid fuel vaporization.

Fuel Supply And Float Chamber

To ensure a ready supply of fuel, the carburetor has a “float chamber” (or “bowl”) that contains a quantity of fuel at near-atmospheric pressure, ready for use. This reservoir is constantly replenished with fuel supplied by a fuel pump. The correct fuel level in the bowl is maintained by means of a float controlling an inlet valve, in a manner very similar to that employed in toilet tanks. As fuel is used up, the float drops, opening the inlet valve and admitting fuel. As the fuel level rises, the float rises and closes the inlet valve. The level of fuel maintained in the float bowl can usually be adjusted, whether by a setscrew or by something crude such as bending the arm to which the float is connected. This is usually a critical adjustment, and the proper adjustment is indicated by lines scribed into a window on the float bowl, or a measurement of how far the float hangs below the top of the carburetor when disassembled, or similar. Floats can be made of different materials, such as sheet brass soldered into a hollow shape, or of plastic; hollow floats can spring small leaks and plastic floats can eventually become porous and lose their flotation; in either case the float will fail to float, fuel level will be too high, and the engine will not run well unless the float is replaced. The valve itself becomes worn on its sides by its motion in its “seat” and will eventually try to close at an angle, and thus fails to shut off the fuel completely; again, this will cause excessive fuel flow and poor engine operation. Conversely, as the fuel evaporates from the float bowl, it leaves sediment, residue, and varnishes behind, which clog the passages and can interfere with the float operation. This is particularly a problem in automobiles operated for only part of the year and left to stand with full float chambers for months at a time; commercial fuel stabilizer additives are available that reduce this problem.

Usually, special vent tubes allow air to escape from the chamber as it fills or enter as it empties, maintaining atmospheric pressure within the float chamber; these usually extend into the carburetor throat. Placement of these vent tubes can be somewhat critical to prevent fuel from sloshing out of them into the carburetor, and sometimes they are modified with longer tubing. Note that this leaves the fuel at atmospheric pressure, and therefore it cannot travel into a throat which has been pressurized by a supercharger mounted upstream; in such cases, the entire carburetor must be contained in an airtight pressurized box to operate. This is not necessary in installations where the carburetor is mounted upstream of the supercharger, which is for this reason the more frequent system. However, this results in the supercharger being filled with compressed fuel/air mixture, with a strong tendency to explode should the engine backfire; this type of explosion is frequently seen in drag races, which for safety reasons now incorporate pressure releasing blow-off plates on the intake manifold, breakaway bolts holding the supercharger to the manifold, and shrapnel-catching ballistic nylon blankets surrounding the superchargers.

If the engine must be operated in any orientation (for example a chain saw), 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 moves 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.

Multiple Carburetor Barrels

While low performance carburetors may have only one barrel, most carburetors have more than one venturi, or “barrel”, most commonly a two barrel, with 4 barrels being common in higher performance larger displacement engines, to accommodate the higher air flow rate with larger engine displacement. Multi-barrel carburetors can have non-identical primary and secondary barrel(s) of different sizes and calibrated to deliver different air/fuel mixtures; they can be actuated by the linkage or by engine vacuum in “progressive” fashion, so that the secondary barrels do not begin to open until the primaries are almost completely open. This is a desirable characteristic which maximizes airflow through the primary barrel(s) at most engine speeds, thereby maximizing the pressure “signal” from the venturis, but reduces the restriction in airflow at high speeds by adding cross-sectional area for greater airflow. These advantages may not be important in high-performance applications where part throttle operation is irrelevant, and the primaries and secondaries may all open at once, for simplicity and reliability; also, V configuration engines, with two cylinder banks fed by a single carburetor, may be configured with two identical barrels, each supplying one cylinder bank. Similarly, in the widely seen V8 and 4-barrel carburetor combination, there are often two primary and two secondary barrels.

Similarly, multiple carburetors can be mounted on a single engine, often with progressive linkages; three two barrel carburetors were frequently seen on high performance American V8s, and multiple four barrel carburetors are often now seen on very high performance engines.

Some great references can also be found in the Books Reference page

Comments are closed.