If you'd have asked me that about 20 years ago, I'm sure I could have told you what was wrong.
There was a time when I fixed those things by the dozen, but I haven't been inside one since 1986...
It's powered by an Onan CCK or CCKA engine, and it's almost certainly made by Miller.
Most of the Airco gas drives from that era were re-badged Millers. I think those were the Miller AEA200 or AEAD200 or something like that.
I remember the idle problem wasn't uncommon, but I don't remember the fix for it and I gave away all my Miller and Lincoln Parts and Service books about 10 years ago.
Miller always had (hopefully still has) excellent service. Maybe they can help you cross the Airco Model/Serial numbers to Miller. If so, I'll bet they can still provide schematics for it.
http://www.millerwelds.com/Can you post the Airco model and serial #'s, or PM them to me?