Problem with low RPMs?

Hey guys, I'm having a problem with a 1987 Subaru Legacy wagon (5 spd). I took it in a while back to my friend so he could swap out snow tires for all seasons. He said he also turned up the idle setting since it was idling low.

Now, 2 months later there's a problem keeping the car running in neutral. I'm going to throttle up the idle again tonight, but does anyone else know what might cause this?

You most likely have a vacuum leak. That's the most common cause, but it could be just about anything man. With the information you gave us, we're shooting blind.