How a Carb Works
(By Fabio Dougie)
What Does a Carburettor Do?
The engine needs a supply of combustible material to burn to produce power, the carb combines fuel and air creating a combustible mixture.

The action of the carb is linked to the rider via the throttle, by varying the throttle position the rider can vary the action of the carb to provide the correct supply of fuel/air mixture.

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A Dell'orto slide type carb


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A Keihin CV type carb


So....the carb also regulates the ratio of air and fuel to provide the optimum level of combustion at every throttle, and in effect, the action of the carb controls the engine's speed.
How the Carb Works: An Overview
A vacuum is created by the engine "turning-over" that draws air through the carb, and also primes fuel to be drawn from the petrol tank. The airflow through the carb causes fuel to be drawn through the carb, mixing with the air, and then the resulting air/fuel mixture progresses on to the engine to be combusted. The amount of fuel mixed into the air to obtain the required air to fuel ratio is controlled by the venturi (the venturi is the part of the carb where the airflow becomes restricted hence creating pressure changes and allows the successful control of the mixture, see picture).

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Diagram showing fuel and air flow through a slide type carb

When air flows through the venturi its speed increases and the pressure drops. This causes the fuel to be sucked into the air stream from a hole or jet. Fuel is delivered to the carb via the fuel-tap (and/or fuel pump in some cases) and is stored in the fuel bowl which sits at the bottom of the carb. To keep the level of fuel stored in the bowl constant under all conditions a float system is used (very similar to the method used in a toilet cistern/tank).
Carb vs. Fuel Injection
Some scooters utilise a fuel injection system to provide the air/fuel mixture. The primary difference between a carb and fuel injection is that fuel injection atomises the fuel by forcibly pumping it through a small nozzle under high pressure, while a carb relies on low pressure created by intake air rushing through it to add the fuel to the air-stream. In the Fuel Injected system this allows the air/fuel mixture to be more atomised and hence more combustible. The primary advantages being smoother and more dependable engine response during quick throttle transitions, easier and more dependable engine starting, and better operation at extremely high or low ambient temperatures.
Scooter Carb Types
The are quite a few types of carb that are used in differing applications; the 2 types that I have experience of are (and I think that these types are the most common in our scooter applications too)...
Slide Carb (may also be known as a Cable Carb, or a Mechanical Carb)
Slide carbs have a "slide" that is attached to the linkage that is driven by the throttle cable, so twisting the throttle raises or lowers the slide, which opens or blocks the air-stream flowing through the venturi. This type of carb is one of the simplest to understand and to work with.

The slide houses a needle that open or closes the Main Jet that provides the fuel supply, as the slide lifts then so too does the needle unblocking the Main Jet and allowing more fuel into the airstream.

So at "full throttle" the air-stream will be stronger with it not being blocked by the slide, its now fully open and the needle is in effect fully out of the Main Jet... maximum air and maximum fuel.

At mid-range throttle openings, the slide is not so high and the needle is partially blocking the Main Jet, so reduced fuel and reduced air-flows are provided; the quantity of both are controlled by the position of the throttle within the range.

At low throttle openings the needle closes the Main Jet, and fuel is provided via a smaller jet known as the Idle Jet. This jet is able to pass fuel at very small openings until the needle starts to lift out of the Main Jet. The slide even when fully closed allows a certain amount of airflow to pass due to the shape of the bottom of the slide being concave. This combined with the positioning of the Idle Jet allows air/fuel mixture to be provided when very little air-stream is present.

Throttle openings and effective carb components...
From idle to about 1/4 throttle - Idle Jet (AKA Pilot jet)
From 1/4 throttle to about 3/4 - Needle position.
From 3/4 to Wide Open Throttle (WOT) - Main Jet.
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Diagram showing the throttle ranges that the various fuel systems effect

Constant Velocity Carbs (also known as a Constant Vacuum Carb, or CV Carb)
This is very similar in operation to the slide carb, but the effective difference is that the throttle cable controls a large valve in the venturi "throat" called a Butterfly Valve (also known as a Choke Valve, or Throttle Valve) which opens or restricts the air-flow through the carb.

The slide on this type of carb is lifted by the vacuum created as air flows through the venturi and over a hole situated in the end of the passage to the vacuum chamber above the slide's diaphragm. As the airflow speed increases, the vacuum increases, and the slide/needle is lifted higher hence opening the jet.

The amount of "opening" is proportional to the volume of air-flowing at that time so in effect the carb in self managing all of the air that it can combust.... the spring returns the slide back to closed when the air flow is reduced.

This allows the carb to "regulate" itself by using the vacuum that is created to lift the slide as opposed to the rider manually lifting it with the throttle cable.

The CV is better than the slide carb because when you twist the throttle on a slide carb the slide goes up and the engine takes on all the fuel and air it can take and more. With a CV carb the amount of fuel is controlled by the vacuum, hence the engine only gets the amount of fuel and air it can handle and no more.
Three Fuel Circuits
When the engine is cold, at idle or at rapid acceleration there is not enough air passing through the venturi to draw fuel, so to overcome these problems other systems are used...
Idle Circuit (also known as Slow Jet, or Pilot Jet)
Because the carb slide is almost closed at idle, a special jet called the Idle Jet supplies fuel to the carb when the scooter is idling or at very low revs (up to about 1/4 throttle). The amount of fuel/air has to be re-balanced at these small openings and this is done by use of the Air/Mixture Screw. This screw can be adjusted to richen or lean-out the mixture provided to the carb at these settings. There is also another screw called the Idle Screw (AKA Idle Position Screw) which sets the revs that the engine will idle at.
Main Jet and Needle
At 3/4 throttle to wide open throttle (WOT), the fuel that is required by the carb is provided by the Main Jet circuit un-hindered by the needle.

At throttle openings in-between 1/4 and 3/4, the needle attached to the slider rises and falls inside the Main Jet to open and block (and all points in-between) the jet hence metering the fuel provided.

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View from beneath a Dell'orto slide type carb showing the fuel jets

Auto-choke (also known as Bystarter, or enrichment device)
Your scooter may have a manual or electric choke that richens the mixture while the engine warms up. When your engine is cold it needs more fuel to start, the auto-choke is used to slowly block off a fuel enrichment circuit as the engine warms up. Your scooter may have a Power Jet (also known as the Choke Jet) that supplies the fuel enrichment circuit.
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View from underneath a CV type carb showing the main fuel circuits

Fuel Supply
The jets of the three circuits sit in the fuel bowl and when a vacuum is attached the relevant jet draws fuel from the bowl which is then replenished from the fuel-tank.
Further Reading
There are some notes here: 2-Stroke Carburetor Set-up Technique on carb setup and tuning, the article refers to 2 Stroke carbs but the majority of principles apply to 4 Stroke carbs too.

Carburettor Mixture Adjustment
Carb and Jet Cleaning 125/150/200 engines.

And Finally...
I found the meaning of the word Carburettor on the Wikipedia website:
Word origin - The word carburetor comes from the French carbure meaning "carbide". Carburer means to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon (and therefore energy) content of a fuel by mixing it with a volatile hydrocarbon.
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