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Choke valve
A choke valve is sometimes installed in the carburetor of internal combustion engines. Its purpose is to restrict the flow of air into so as to enrich 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.
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 carburetor, the choke valve causes more vacuum inside the carburetor and hence a greater amount of fuel to be sucked into the engine compared to normal 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.
