Early EDM non-cat 4A-GE 16v Bluetop Fails emissions (high co)

[Rascal21]

Ok, went and picked up EFI parts that is for another AE86 that I bought (more on that later), same ECU, harness, sensors etc etc.
So thats the THIRD ECU swap on forum recommendations

Changed green water temp sensor (same part#), still runs rich.
Changed MAP sensor (same #), still runs rich.
Changed injectors (also grey 182cc same # as everything else I have), still runs rich.
Changed ECU (yep, same #), still runs rich.
Changed potmeter (..), still runs rich.

Timing, fuel pressure etc is all good, but as usual adjusting the potmeter makes no change to idle RPM or rich condition (whereas on that Portugese AE86 guys Youtube video, the one who had his car stolen, potmeter adjustment makes an immediate change on idle)

I had a look at fuel return, I`m using the old 2T fuel line for return to the fuel catch tank (around 3 or 4 mm inner diameter), can feel the hose that goes from FPR to return line pulsing a bit hard (then again I dont know what to compare to), so maybe return line is a bit too small?
Am I perhaps looking at too much fuel back pressure here?
Whats the recommended pipe diameter?

Oh, also changed harness, still runs rich
Fully charged battery (in case injectors dont get the correct voltage), still runs rich.