My 91 civic hatch dx has been doing this for a while now
its idles kinda rough-i'd say it's more like it vibrates-more than it used to anyway-when its stopped and at an idle. Sometimes it will idle really low and it feels like it's just about to stall, but it never does then it vibrates. It's extremely reliable still and never breaks down or doesn't start even though its been doing this for like 6 months *minimum* now.

Any idea what causes this and what I can do to fix it?