Chokes

A choke valve is sometimes installed in the carburetor internal combustion engines. Its purpose is to restrict the flow of air so as to enrich the fuel-air mixture while starting the engine. more...

The choke valve used to be a standard item in car engines until the 1970s. The valve could be activated manually, by a lever or pull handle on the dashboard, or automatically by a heat-sensing mechanism.

This device fell into disuse after the development of fuel injection and electronic ignition systems, but is still found in carbureted motorcycles and other special-purpose engines.

Read more at Wikipedia.org