A choke valve is a valve that lifts up and down a solid cylinder (called a "plug" or "stem") which is placed around or inside another cylinder which has holes or slots. The design of a choke valve means fluids flowing through the cage are coming from all sides and that the streams of flow (through the holes or slots) collide with each other at the center of the cage cylinder, thereby dissipating the energy of the fluid through "flow impingement". The main advantage of choke valves is that they can be designed to be totally linear in their flow rate.
A choke valve is sometimes installed in the carburetor of internal combustion engines. Its purpose is to restrict the flow of air, thereby enriching the fuel-air mixture while starting the engine. Depending on engine design and application, the valve can be activated manually by the operator of the engine (via a lever or pull handle) or automatically by a temperature-sensitive mechanism called an autochoke.
Choke valves are important for carbureted gasoline engines because small droplets of gasoline do not evaporate well within a cold engine. By restricting the flow of air into the throat of the carburetor, the choke valve raises the level of vacuum inside the throat, which causes a proportionally greater amount of fuel to be sucked out of the main jet and into the combustion chamber during cold-running operation. Once the engine is warm (from combustion), opening the choke valve restores the carburetor to normal operation, supplying fuel and air in the correct stoichiometric ratio for clean, efficient combustion.
Chokes were nearly universal in automobiles until fuel injection replaced carburetion in the late 1980s. Choke valves are still extremely common in other internal-combustion applications, including most small portable engines, motorcycles, small prop-powered airplanes, and carbureted marine engines.
Reference Link : http://en.wikipedia.org/wiki/Choke_valve