Mitsubishi Repair: Idle Surge, mass air flow, cat back exhaust

Question
I have a 95 eclipse 2.0,GST,auto that runs fine but the idle pluses in park or neutral only. When I go to start the car the RPMs will climb to about 1800 then drop down to 1100 and back up to 1800 and down again. I've adjusted the fast idle air valve to stop the pluseing for it to only to come back after a five minute drive.The idle pluses when the car is cold or hot but is worse when hot.The TPS, mass air flow,and the IAC have all been replaced. I saw on another forum about a base idle speed set screw, but I don't know where it is. Also I have a cat back exhaust and performance down pipe on the car but with no CAT. I know it won't pass inspection but does the CAT effect the idle?I've checked for vaccum leaks, checked timing which is two degrees btc when computer overide button is pushed and 5-7 when in normal operation mode,i've check my fuel pressure which is constant and within spec. My fast idle air screw was loose in the threads and i've put RTV on the threads to tighten it up.

Answer
Alan,
Not having a CAT should not cause idle surge.  Usually this is caused by a faulty IAC or MAS, though you indicated you already replaced those.  You base idle setting screw (BISS) is located on top of the throttle body.  It's typically covered by a small rubber cap.  Remove the cap to get access to the BISS.  The following sites will give information for properly adjusting the BISS and additional info about idle surge:
http://vfaq.com/mods/BISS-2G.html

http://vfaq.com/mods/wayback/BradBauer/engine/surge.html

Good luck!