Why a very rich mixture is required for idling?

An idling,engine demands a rich mixture,which can be made leaner as the throttle is gradually opened.During idling,the pressure in the inlet manifold is about20 to25% of atmospheric pressure.At suction stroke,inlet valve opens and the product of combustion trapped in the clearance volume,expands in the inlet manifold.Latter when the piston moves downwards,the gases along with the fresh charges go into the cylinder.A rich mixture must be supplied during idling,to counteract the tendency of dilution and to get an ignitable mixture