i got the 4.2 running but it idles at 3000 rpms. the carbs idle screws are all the way out. what is the problem?  

wow...vacuum leak, linkage stuck, cable frayed & stuck, hot idle screw turned in too far, timing advanced too far, just some ideas, maybe someone else has some specific 4.2 ideas  

If its a Carter...CHUNK IT!!! The first one I had would idle at 2500 to 3000 rpm. Couldn"t find a thing wrong with it. So I got another one. It wouldn't idle at all. It was a leak. Traded it to my brother in law for a winch.....I got the better end of that deal. I installed a Motorcraft 2100. He put the Carter on and chunked it a few months later for the Weber.  I know this doesn't answer your question, but maybe it will help.  

the distributor gear on the cam could be worn it was on my yj but i did not know till after i put tbi in.i would start small with the things you talked about.like jeepnut sed the carb youhave is known to be a pain  

Help...just bought a '85 CJ-7 (2.5L) with 22,000 actual miles (used on farm), no rust!.  It also has a Carter car and has problems.  Idles Ok, but back fires 70% of the time when I shut it off and sounds like its missing on inclines in 3rd and 4th gear.  Can I assume it obviously the timimg.  All direction is greatly appreciated.  

Do a google search for "jeep tfi upgrade".  It's a pretty cheap way to greatly improve idle and power.  I thought my carb was the problem on my 85 cj7, but it turns out the weak ignition system was really holding me back.