Hey guys,
Sorry for leaving you all hanging. Unfortunately, it's still not resolved.
1. a bad spot on flywheel teeth.
A - I did a quick inspection through the starter hole (turned the flywheel over by hand), but I didn't pop off the inspection cover. That's easy enough to do, so I could double-check that way.
2. You missed something.
A - maybe? I don't know what I don't know.
3. Two bad batteries.
A - Got the battery tested at the local shop - they say it's good.
4. two bad starters.
A - both new starters are brand-new, so the chance of this is low, but I suppose it's possible. I'm not willing to buy a 3rd brand-new starter to find out though.
5. battery cable broken inside the insulation or at clamp and can't see it.
A - possible... I've already added a negative/ground cable direct from the battery to the starter, so I could try replacing the positive cable as well I guess...
6. Clutch sticking. Whatever that means.
A - Are you suggesting the clutch is possibly sticking/grabbing the flywheel? I guess that's possible... there's no neutral safety switch or anything similar, so I don't bother holding the clutch in when I try to start it (I just check to make sure the transmission is in neutral first). I could try holding the clutch in while starting...
7. Piston sticking, rotating component sticking like bearings
A - everything rotates smoothly by hand
8. Oil or water in cylinder
A - changed the oil (looked used, but otherwise normal); compression-tested all cylinders a couple months ago, but I could do it again. I don't remember the numbers, but all 6 cylinders were all the same, which was good.
9. Sticking valve causing compression/back pressure
A - How would I check for this - I assume just pop the valve covers, disconnect the coil (so it doesn't actually start), and then turn it over on the starter? I could see all the valves moving... I suppose I could check compression at the same time.
10. Timing off causing compression/back pressure
A - I have not checked the timing, so I can do that and report back. It's something I've been meaning to do anyway.
11. Those internal compression/check valves in head are stuck
A - I'm not sure what these are...
12. Fan belts too tight. Naw, can't be that.
A - No squeak/squeal/etc from the belt. No power steering, power brakes or A/C, so it's just the one belt (about as simple as it gets). Tension feels ok...
Thanks again for all the help and suggestions, guys. I'm learning as I go on this one, so I appreciate it. At this point, it looks like I have a couple more things I could check when I find some time, but I'm also at the point where I may just leave it as-is until it breaks more or "fixes itself". /shrug.