OK, a bit of follow up on the troubleshooting I did tonight.

1. Ignition timing: I moved the timing around from 20 deg down to 6 deg, eventually settling on the stock setting at 8 deg for now. While advancing the timing did slightly increase the idle, no setting seemed to affect the underlying rough idle.

2. Engine compression: Checked compression warm, 170 psi across all cylinders.

3. Ignition spark at each cylinder: Strong spark at each cylinder.

4. Injection at each cylinder: Pulled each injector and tested for spray pattern per green book instructions. All four had an absolutely perfect 35 degree spray cone. It actually surprised me they all looked so good.

In addition to those tests, I also used propane to test for vacuum leaks at the manifold, all five injectors, and vacuum connections. No leaks found.

I also confirmed that the in-tank pump is working.

I test drove the car to confirm the engine performance above idle. It did indeed perform flawlessly. Plenty of power at all speeds, no indications of stumbling or any other problems.

Some other observations:

1. When starting cold, the dwell sits firm at 54 deg. During that time, the engine runs much smoother. After a minute or two, there is a sudden change and the dwell starts to fluctuate between 40-48 deg. I take this mean the mixture control has been handed over to Lamda at this time. When that change takes place, the engine starts running much rougher. Funny thing is, if I manually adjust the mixture to 54 deg later, after it is warm, it doesn't run as smoothly as it does when cold at 54 deg.

2. I think the aux air valve is stuck closed. That is to say, when totally cold, the passage looks totally closed. When totally warmed up, it looks the same way. I can manually slide it open easily. It seems to spring closed. Sometimes the car wants to die when starting, which I take to mean the air valve is not properly supplying extra air when cold. I don't think this matters at all when warm.

3. Idle remains high at 1200 RPM. Even when the idle screw (now cleaned) is all the way in the idle won't get lower that 1000 RPM. Even if the engine was running smoothly, I'm not sure how I would get the idle low enough.

I'm wondering if this all could be bad gas. The fuel I'm running on is about 9 months old. Any opinions? Could it cause rough idle, but run OK otherwise?

I'm running out of green book inspired troubleshooting ideas. Any thoughts?