Idle adjustment question, Idles high or slowly stalls Options

[BillC]

I have an idle adjustment question for all you D-Jet gurus:

I can adjust the idle air screw on the throttle body so the car idles at 1600 RPM, and it will happily idle at 1600 all day long. If I turn in the screw just a little bit, like 1/4 turn, the car will slowly drop idle speed down slower and slower until it stalls. I can blip the throttle to keep it running, but it just wants to slow down and stall. This is all when the car is fully warmed up.

How do I get the car to idle steady around 1000-1200 RPM? 1600 is just too high.

Car is a '73 2.0 with factory D-Jet. I installed new Standard Products FJ67 injectors, and the fuel pressure is 29 psi. There are no leaks in the intake side, and the idle adjustment screw is about 2 turns out (so there's plenty of adjustment available in both directions). The MPS hasn't been messed with, and the ECU looks like the correct unit. All new vacuum lines, fuel lines, pump & filter, and fresh gas. New @JeffBowlsby engine and alternator wiring harnesses.

Known issues: it has the wrong CHT sensor (it's the one that's commonly available) and I think the TPS board has worn traces. I finally found the right CHT and ballast resistor, and I also have a 914Rubber replacement TPS board, and I will install both of them when I get a chance. However, I don't think either of those should keep it from idling at a reasonable RPM.

[BillC]

I did more testing this morning, and I'm coming (back) around to the vacuum leak theory.

First, I took the car out for a good 30 minute drive, to get it warmed up. Then, I tried putting the Mityvac on the MPS. With the vacuum set to 17", the car idled nice and solid, but high-ish. And, when I screwed in the idle bypass screw, the car still idled at 1150 RPM even with the screw bottomed out. So, I'm guessing the issue is a vacuum leak that the car can't overcome below 1600 RPM. And, based on past smoke testing, I'm leaning toward the throttle body being the culprit -- it would also explain why it would idle better near startup but get worse as heat migrates through everything (aluminum throttle body expands faster than the steel throttle shaft, letting in more air, and the leak is right at the vacuum retard port, reducing the signal to the distributor).

I also did a leakdown test on all four cylinders. They are all at 5% or less. So, the engine seems good and solid.

I have a good used throttle body on the way (Thanks @brant !). Once it arrives, I'll swap it in and see if it makes any difference, and then I'll send my current one out for rebuild.