Editing Carburetor (section)

===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.