<P> In an internal combustion engine, a choke valve modifies the air pressure in the intake manifold, thereby altering the ratio of fuel and air quantity entering the engine . Choke valves are generally used in naturally aspirated engines with carburetors to supply a richer fuel mixture when starting the engine . Most choke valves in engines are butterfly valves mounted in the manifold upstream from the carburetor jet to produce a higher partial vacuum, which increases the fuel draw . </P> <P> In heavy industrial or fluid engineering contexts, a choke valve is a particular design of valve with a solid cylinder placed inside another slotted cylinder . </P> <P> 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 . </P> <P> Choke valves are important for naturally - aspirated 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 reduces the pressure inside the throat, which causes a proportionally - greater amount of fuel to be pushed from the main jet 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 . </P>

How does a choke work on a lawn mower
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