The first measure taken to decontaminate the exhaust gas, is the equipping of carburetor engines with a complicated nozzle-jet system (see above figure on the left beside the choke funnel).
During warm idling, the throttle valve, in exhaust gas optimised carburetors, is closed. The air is led through a channel around the throttle valve. It is no longer a pure air-duct, because through the idling air-jet, (the right one of the three jets) and an additional mixing tube, fuel is added. The idling speed can be efficiently regulated by an adjusting screw with a large diameter more precicely and for a longer period. Due to the already added fuel, the mixing proportions are not changed all that much any more when adjusting the idling speed. After this adjusting screw is the fuel channel, to which, through the idling fuel-jet, (the left one of the three jets) air has already been mixed in. A small adjusting screw determines the mixing proportions when idling, thus also the CO amount. Because this can only determined by using a suitable test-device, it is sealed with a cap. 01/11