I just got an 89 Accord, auto, carburated. When the engine is cold (first thing in the morning, 70ish F) I start the engine and it idles up to about 1500 rpms. It stays this way until A) it warms up and eventually, in a matter of seconds it shoots up to about 3000-4000 rpms or B) I bump the throttle to 2500 rpms and it drops back to 2000 rpms. A couple of times when it is in the 2000 rpm range I can tap the throttle and get it back down to 1500, but not any lower. When it is in the 3-4k range, there isn't anything I can do to drop the rpms but shut it off. When this happens and I shut it off, the engine has trouble shutting down, it "diesels" for a bit. I also notice, while driving that the brakes have to be on the floor to stop the car. I'm guessing it is because the engine is pushing the tranny so hard. I'm a diesel mechanic (heavy equipment) not a car guy, so what you may think are general, everyone should know terms are going to be greek to me. I have went through and checked out the really horrible mess of vac lines (diesel engines don't use those...) and haven't found anything obvious. I thought maybe brake issue and engine issue there could be a common denominator in the brake can (vacuum can brake assist thingy) but after leaving it for about a half an hour, I pulled the hose off of it and I heard a slight rush of air. I'm assuming it was holding a vacuum.
I have researched this topic all day and have tried covering the bases as best as I can. I burped the air out at the thermostat housing. I did notice that when it is cold and running at about 1500 rpms, if I took the hose off from the top of the valve cover to the air filter box and plugged the elbow that comes off of the valve cover, I can get the idle to drop about 300 rpms, down to 1200 or so. Anyway, that is all I really know. You will have to "dumb down" any answers you might have because like I said before, I'm not a gas guy. Thanks alot.

 

remove the air cleaner housing , plug hoses that will have a vacc leak, start the car ,chk to see if anything is hanging up ,
cables, throttle plate, chocke blade , idle adjustment
accell cable too tight?