It turned out the plastic grommet in the sender assembly is cracked. It must of gotten nudged.
The good news is that even though it split the pickup is still snug enough to be nudged back and then recalibrated (with a few frustrating misadjustments before getting it perfect), so I've been happily running with a happy tank and sender for the remainder of the summer.
In the meantime the headliner was dropped to begin with reupholstry, the console was reinstalled and the prewiring began for all the funky accessories.
The final thing that was performed before summer officially ended was to adjust the mixture screws. The carb rebuild left a lean running condition and we verified that all adjustments are still matching to the Carter spec, so now that we know the engine, exhaust, electrical, intake and the carburetor are in good order we can perform the adjustment and not need to worry about external variables.
The problem is that AMC's own documentation does not describe how to do the adjustment on an electronic feedback carburetor. In fact, it's as if the half a page of mixture screw adjustment procedures are not intended specifically for this car at all...
The internet is awash with different ways to do it (by vacuum, by ear....these people are crazy), but me and my mechanic determined the easiest way was:
-Turn the computer on while telling it the engine is cold. This centers the stepper motor and the metering pins. We don't want them trying to correct the mix while adjusting or you will be stuck rubberbanding your adjustments, so sticking them in the middle means we can make the adjustment and not have to bias too lean or rich to compensate.
-Turn the computer off and unplug the stepper motor
-Attach tachometer
-Start the car and let it come up to temp.
-Set the curb idle to spec
-Perform the mixture adjustment, as per spec. You will need two screwdrivers with marks drawn on them to ensure you are turning both screws at the same time and to the same positions.
-Complete adjustment by turning off engine, removing tachometer and reconnecting the stepper motor. It will re-initialize the next time you turn the car on
-Verify adjustment by installing plastic air cleaner cover (the how-to is in the book but it's basically a piece of plexiglass over the top of the air cleaner), starting the car, letting the car enter closed loop and ensure the metering pins oscillate back and fourth within their range
-Optional: Probe the signal wire for the O2 sensor and attach either a Digital Multimeter or an oscilloscope. If the mixture adjustment is correct the voltage should swing between 0.8 and 0.1v with no prolonged time on either end of the waveform (due to still running still too lean or too rich)
The great thing about the electronic feedback is that precise mixture screw adjustment isn't necessary. Just get it in the ballpark and it will do the rest.
So everything under the hood is now done after a year of work, it's getting 24 highway and 17 city in 2WD and the interior is coming together. Next year is gonna be all about suspension and bushings and it badly needs all of that.