After many months of mysterious overheating, I replaced cracked head gasket -- which was allowing air into the coolant and collant into a cylinder. Now, for the first time in the past five years the car idles very roughly at 800 rpm. (However, it does run smoothly when driven.) I took back to mechanic and he said that the roughness may be due to "carbon build up" and that it should shortly go away. I just had new plugs installed, as well as the $125 "trigger" switch which attaches to one of the plugs. Why is the idle rough? Could it have something to do with the activities of replacing the head gasket. Or, are there other possible causes? Thanks for any help. Idling rough in KY.