Well after my optimistic posts earlier, where I claimed I knew my problem had to be in the fuel return line, either to the tank or to the fuel distributor, you can imagine what happened next.
Yup. I pushed the Saab into my ruined garage and got to work! I pulled the fuel return off the distributor and the warm up regulator, fired up the compressor, and spent the next 30 minutes looking for my air sprayer thingy.
Failing at that task, I settled on a short length of tube and just blowing into the damn thing. (I’m sure there is a joke there, but I can’t seem to find it.) Either way, no problems. I shot some carb cleaner in for good measure, and moved on.
Clearly this means the tank return is clogged then. Weird, because I measured the pressure a while back and it was fine, but it has to be the problem. There is no other explanation.
Long story involving a trip to Home Depot, figuring out the banjo fitting is a non-standard thread, buying a new air gun, finding the old air gun, trying to recruit a helper, and dropping a bunch of washers and nuts into the engine bay, short: I rigged up something that would allow me to shoot air into the return. And, as you would expect, the tank return is working beautifully.
At this point that puts a hard stop on any reasonable assumption that can be made. Luckily, the pessimist in me anticipated this and when a used but working distributor came up for sale a month or so ago I bought it. Next step is to throw the baby out with the bath water and replace the fuel distributor and air plate and see what that does.