HOW TO CARBURATOR WORKS

The latest i.c engine is has an air pump; it pumps air in and air out.The first things performance is to get more in you have to get more out. Performance of exhaust is useless if you don’t increase the air and fuel mixture coming into the engine. 
OK you know that, so you also know bigger is to be better, a bigger carburetor allow more air to enter the engine with a free breathing air cleaner. This is true but when another factors are taken into account like, jetting, also the type of cams.
 “Venture Effect” of the carburetor, is basically a large tube or “Venture”. Vacuum occurred by the downward stroke of a piston causes the higher pressure inside the float chamber to force the fuel and air mixture through the fuel and air outlet ports in the carburetor venture and vaporized,occurred  by the fast moving air stream. A needle stay inside the hole to operate the fuel flow. The throttle cable operate the needle up and down to operate the fuel in the air stream.
all carburetors are not as simple as the one above, some have more hole for fuel to be drawn from. Mostly all-modern carburetors has a idle speed adjustment screw it's actually a valve; the end of the adjustment screw basically shaped like a needle and it operate fuel flow when the engine is running at low speed.
In higher speed operation a needle valve is attached with the slide, it is operated by vacuum ( cable). In this cases the high-speed needle is attached with the slide, the slide is raised and lowered by the throttle cable line. When throttle is open then needle is raised up out of the emulsion tube allowed more fuel to mix with in increased air allowed by rising slide. when the throttle is closed then slide and needle come down reduced air and fuel.
Mostly carburetors used on modern motorbikes  it also have a moving flap inside the venture to operate the airflow,by a butterfly throttle valve. On most carburetors this is where the throttle cable is joint. if the throttle plate is opened it allowed  more air into the engine. Fuel is fed through a fixed orifice the amount of fuel in addition to the idle port.

Some of the more costly carburetors and most of the newer original carburetors has additional circuits or passageways and ports for fuel to enter the air stream. In addition to the idle circuit, a transfer, mid range or air circuits may be used to fine-tune the fuel mixture. Remember the idle port will provide fuel during the entire operation of the carburetor. As the air speed increases the needle or main jet will add fuel in addition to the idle circuit.

The most latest carburetors are Vacuum controlled by Constant Vacuum type. A vacuum operated diaphragm operated the opening and closing of the slide. The Constant Vacuum Carburetor also has a butterfly throttle valve.
by the throttle plate closed and the air stream cut off, idle speed is measured by fuel metered via the slow jet. Air from the slow air jet mixes with fuel and it is delivered to the idle port at the vacuum side of throttle plate. in low speed as throttle plate is cracked open the transfer ports are also exposed to vacuum side of throttle plate and additional fuel is directed to the barrel of carburetor. by throttle plate cracked open a limit of fuel and also enters the air stream by the needle jet. The idle and transfer ports supplied additional fuel to the carburetor barrel to assist during transition period from idle to mid range.

The venture opening is reduced by low position of vacuum piston. This enables initial air stream velocities to be higher than normally attainable with fixed venture carburetors. The higher air stream velocities provide greater quantities of fuel necessary for good acceleration.

As the throttle plate is opened, airflow increases through the carburetor and the pressure drop in the venture near the needle jet increases.
 

The low pressure in the venture travels via the vacuum port in the vacuum piston to the chamber above the diaphragm. The chamber beneath the diaphragm is vented to atmospheric pressure by a port from the chamber to carburetor inlet. The higher pressure at the underside of the diaphragm overcomes spring pressure and moves the vacuum piston upward in proportion to the pressure different between  the chambers.

As throttle plate is opened further, the pressure difference is between the chambers above and below the diaphragm increases and the vacuum piston moves again upward. also venture opening increases and the needle is lifted also out of the needle jet. The quantity of fuel and the volume of air are increased and metered to the proportions of the engine demanded by the variable venture and also needle lift.

The accelerator pump is used to supply fuel when sudden or rapid throttle openings are occurred. It quickly injects fuel inside the venture; then extra fuel provides for soft acceleration and reducing throttle hesitation.
 

when cold starting carburetors are equipped with a choke. The choke is a flap valve also like to the butterfly valve. It is stay on the inlet side of the carburetor for reduces airflow making the fuel mixture richer. As the engine warms up the choke is opened up and the carburetor returns to normal operation. An enrich is a separate fuel circuit, manually controlled by a cable line when it is engaged it provides additional fuel making the idle mixture richer.