Greetings.

I'm working on a 1990 Jeep YJ, 4.2L I6, that won't idle.  From 1200RPM
up, everything's fine.  It runs smooth under power.  It won't idle at
all.

The previous owner replaced the carb with an older non-computer
controlled unit.  He made the decision to use the older carb because it
was cheaper and simpler and someone knowledgeable (?) told him to.  I
believe it never ran right while he owned it.

I have:

1. Checked for vacuum leaks.
2. Made sure the EGR valve is completely closed.
3. Replaced spark plugs and checked cap and rotor (old plugs were worn
but tan).
4. Verified functional vacuum advance function.
5. Verified electric choke function.
6. Tried various settings of the idle bleed screws.  Currently 2 turns
out.

No help.

Since it has an ECM controlling spark but no electronic carb, I figured
the computer controlled spark could be an issue.  So, I picked up a new
spark control module for a non-ECM jeep and wired it up to the
distributor following the wiring diagram for an older jeep.  Now there
is no computer control whatsoever.

No help with idle, but the throttle response seems a bit crisper.

It appears to be running lean at closed-throttle because it will idle
if I manually close the choke flapper.  This leads me to believe that
the problem is not the common "idle tubes" issue which causes a rich
condition.  It really feels like a bad vacuum leak but I can't find it.

I'm completely out of ideas.  In all honesty, I'm a lot more
comfortable working on fuel injection.  A grizzled Jeep veteran could
probably whack it with a hammer and it would purr like a kitten.  Any
advice?  

Thanks!

-rev