Editing Carburetor (section)

== Fuel circuits ==
[[File:CarbNomenclature.jpg|thumb|upright=1.2|Nomenclature for a single-barrel carburetor]]

===Main metering circuit===
The main metering circuit usually consists of barrel/s which reduces to a narrow part where the air is at its highest speed, forming a [[Venturi tube|venturi]]. Fuel is introduced into the air stream at that narrow part through small tubes leading from the main jet.<ref>{{cite book |last1=Lind |first1=Wallace Ludwig |title=Internal-combustion engines; their principles and applications to automobile, aircraft, and marine purposes |date=1920 |publisher=Boston, Ginn |page=71 |url=https://archive.org/details/internalcombust01lindgoog/page/n80/mode/2up |access-date=19 November 2022}}</ref>

Downstream of the venturi is a [[throttle]] (usually in the form of a [[butterfly valve]]) which is used to control the amount of air entering the carburetor. In a car, this throttle is usually mechanically connected to the vehicle's throttle pedal, which varies engine speed.

At lesser throttle openings, the air speed through the venturi may be insufficient to maintain the fuel flow, so then the fuel may be supplied by the carburetor's [[#Idle circuit|idle]] and [[#Off-idle circuit|off-idle circuits]] which will work even with a low volume of air because the narrow gap between the edge of the butterfly plate and the body gives sufficient local air speed at those jets.

At greater throttle openings, the speed of air passing through the venturi increases, which lowers the pressure of the air and draws more fuel into the airstream.<ref>{{cite web |title=Carburetor Basics - Tech Article - Chevy High Performance Magazine |url=https://www.motortrend.com/how-to/83118-carburetor-basics/ |website=MotorTrend |access-date=28 October 2022 |language=en |date=1 June 2002}}</ref> At the same time, the reduced manifold vacuum results in less fuel flow through the idle and off-idle circuits.

==== Choke ====
During cold weather fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making [[Cold start (automotive)|cold starts]] difficult.  Additional fuel is required (for a given amount of air) to start and run the engine until it warms up, provided by a ''[[choke valve]]''.

While the engine is warming up the choke valve is partially closed, restricting the flow of air at the entrance to the carburetor. This increases the vacuum in the main metering circuit, causing more fuel to be supplied to the engine via the main jets. Prior to the late 1950s the choke was manually operated by the driver, often using a lever or knob on the [[dashboard]]. Since then, automatic chokes became more commonplace. These either use a bimetallic [[thermostat]] to automatically regulate the choke based on the temperature of the engine's coolant liquid, an electrical resistance heater to do so, or air drawn through a tube connected to an engine exhaust source.  A choke left closed after the engine has warmed up increases the engine's fuel consumption and exhaust gas emissions, and causes the engine to run rough and lack power due to an over-rich fuel mixture.

However, excessive fuel can [[flooded engine|flood]] an engine and prevent it from starting. To remove the excess fuel, many carburetors with automatic chokes allow it to be held open (by manually, depressing the accelerator pedal to the floor and briefly holding it there while cranking the starter) to allow extra air into the engine until the excess fuel is cleared out.

Another method used by carburetors to improve the operation of a cold engine is a ''fast idle [[Cam (mechanism)|cam]]'', which is connected to the choke and prevents the throttle from closing fully while the choke is in operation. The resulting increase in idle speed provides a more stable idle for a cold engine (by better atomizing the cold fuel) and helps the engine warm up quicker.

=== Idle circuit ===
The system within a carburetor that meters fuel when the engine is running at low RPM. The idle circuit is generally activated by vacuum near the (near closed) throttle plate, where the air speed increases to cause a low-pressure area in the idle passage/port, thus causing fuel to flow through the idle jet. The idle jet is set at some constant value by the carburetor manufacturer, thus flowing a specified amount of fuel.

===Off-idle circuit===
Many carburetors use an off-idle circuit, which includes an additional fuel jet which is briefly used as the throttle starts to open. This jet is located in a low-pressure area caused by the high air speed near the (partly closed) throttle. The additional fuel it provides is used to compensate for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition from the idle circuit to the main metering circuit.

===Power valve===
In a [[four-stroke engine]] it is often desirable to provide extra fuel to the engine at high loads (to increase the power output and reduce [[engine knocking]]). A 'power valve', which is a spring-loaded valve in the carburetor that is held shut by engine vacuum, is often used to do so. As the airflow through the carburetor increases the reduced manifold vacuum pulls the power valve open, allowing more fuel into the main metering circuit.

In a [[two-stroke engine]], the carburetor power valve operates in the opposite manner: in most circumstances the valve allows extra fuel into the engine, then at a certain engine [[RPM]] it closes to reduce the fuel entering the engine. This is done in order to extend the engine's maximum RPM, since many two-stroke engines can temporarily achieve higher RPM with a leaner air-fuel ratio.

This is not to be confused with the unrelated [[Two-stroke power valve system|exhaust power valve]] arrangements used on two-stroke engines.

==== Metering rod / step-up rod ====
A metering rod or step-up rod system is sometimes used as an alternative to a power valve in a four-stroke engine in order to supply extra fuel at high loads. One end of the rods is tapered, which sits in the main metering jets and acts as a valve for fuel flow in the jets. At high engine loads, the rods are lifted away from the jets (either mechanically or using manifold vacuum), increasing the volume of fuel flow through the jet. These systems have been used by the [[Quadrajet|Rochester Quadrajet]] and in the 1950s [[Carter Carburetor|Carter]] carburetors.

===Accelerator pump===
While the main metering circuit can adequately supply fuel to the engine in steady-state conditions, the inertia of  fuel (being higher than that of air) causes a temporary shortfall as the throttle is opened. Therefore, an accelerator pump is often used to briefly provide extra fuel as the throttle is opened.<ref>{{cite journal |author1=Packer, Ed |title=Know Your Carburetor- what it is, what it does |journal=Popular Mechanics |date=July 1953 |page=183 |url=https://books.google.com/books?id=ENwDAAAAMBAJ |access-date=19 November 2022 |publisher=Hearst Magazines |language=en}}</ref> When the driver presses the throttle pedal, a small [[piston pump|piston]] or [[diaphragm pump|diaphragm]] pump injects extra fuel directly into the carburetor throat.<ref>{{cite book|last1=Hillier |first1=V.A.W. |last2=Pittuck |first2=F.W. |year=1966 |title=Fundamentals of Motor Vehicle Technology |chapter=Section 3.6 |edition=Second |publisher=Hutchinson Educational |isbn=9780091107116}}</ref>

The accelerator pump can also be used to "prime" an engine with extra fuel prior to attempting a [[Cold start (automotive)|cold start]].{{citation needed|date=July 2023}}