 To meet increasingly stringent emission requirements, some cars that still retained manual chokes (from around 1980, depending on market) began to have choke opening automatically controlled by a thermostat employing a bimetallic spring, heated by the engine coolant.The 'choke' for constant-depression carburetors such as the SU or Stromberg does not use a choke valve in the air circuit but instead has a mixture enrichment circuit to increase fuel flow by opening the metering jet further or by opening an additional fuel jet for 'enrichment'.Typically used on small engines, notably motorcycles, enrichment works by opening a secondary fuel circuit below the throttle valves.