Thanks, that helps. I don't think I'm burning rich: nothing at all is visible coming out the tailpipe shortly after startup in current weather, and never anything except a bit of gray in any weather.
I have been thinking about having sensors gone through. Two out of the three oxygen sensors are probably old; one was replaced a few months ago, and reportedly it was awful. ECU reports the other two sometimes erroring, naught else, though I'll check it more often and make a list, I want to keep this vehicle. I replaced the mass air flow sensor myself before the #4 plug was re-replaced, and that definitely helped for a while. I have been wondering if there is a list somewhere of a good order of sensors to have checked visually. My paying profession is IT, and if I mostly relied on the machines' own readouts I'd get very little done. I'll probably get the other two O2 sensors replaced next time I have her in the shop. I have been wondering about the MAP sensor, I wonder if those are prone to wear..
I read about about design of the injector system on this engine. If I understand it right it's a kind of weird hybrid between EFI and TBI, heavy on the TBI. I'm still not sure whether there's one injector nozzle per cylinder, but the one or eight are located where the carburetor would have been, with a distribution gadget. I will think that the system is probably in fairly good order, just by overall behavior, except possibly on the higher end of output when running hard for an hour or so. Apparently the next year's model replaced part of what I have with something a bit better called a "spider", which is a bit more proactive and consistent in delivery, and apparently one can swap the '99 as a direct replacement for mine, and I may have that done sometime. But anyhow, the model I have isn't known for rich, it is known for a tad lean, so I don't think this is the carbon cause.
I do send various fuel-system cleaners through every once in a while. The reason why I'm still looking at carbon, is the carbon problem first showed up immediately after I used a very commonly recommended fuel system in this area -- Chevron Techron -- for the first time. I think it's a total of three worthwhile local mechanics and four or five auto-parts store people who have recommended that stuff. So my theory (given by two of the mechanics) is that the Techron loosened some gunk which then pounced on my plugs. Thus I am looking to do preventatives
Electrically we seem to be quite good. There is a custom digital voltage meter which a previous benefactor put on the dash, and it registers a consistent 13.6 or higher, usually more like 14.0 or even 14.2. One thing I did early on was to replace every single fuse in both fuse boxes, and it was amazing to see her come more alive afterwards.
The air filter and related is a very interesting thought. I did a bit of research a few years ago and found some likely sources, I'll start a new thread about that!