 Also the air pressure in the manifold increases, decreasing the evaporation of the fuel, so less fuel vapour is sucked into the engine.This transient oversupply of air relative to the fuel causes a lean mixture, which makes the engine misfire (or "stumble")—an effect opposite to that which was demanded by opening the throttle.This is remedied by the use of a small piston or diaphragm pump which, when actuated by the throttle linkage, forces a small amount of gasoline through a jet into the carburetor throat.