I got the same problem that you do and have gone through every thing, that last thing that I think it could be is the fact the engine is so worn out that there's not enough compression for it to run. I would do a leak down test on the motor to see how well it's holding up.

Well that thought has crossed my mind and I do have a spare engine that has close to 100K miles on it and runs great. Everytime we check compression on a cyl it checks out. I'm going to fart around with this thing until it starts to really become a pain in de chulo and then redo my spare and drop it in. So far it's ok though.