Idler Pulley? WHat does it do?

I dont have an idler pulley? Affects? I know I usually have some of the simplist/retarded questions but I didn thave the right size belt and couldt wait to get it running. Also the connector to my alternator is melted and mishaped. I managed to get it plugged in and working ok but I have a slight glith in the car when full throttle. I have full power then a split second it seams like a missed spark i checked all plugs and wires they are 100% what could it be?