I've got an '89 Caprice with a 5.7 and TBI (it's a 9c1). Milage is 136k. One day it suddenly started running badly, like a steady "miss" at all rpms. I've tried everything I could think of - checking for vacuum leaks, changing plugs, wires, cap and rotor, adjusting the timing, etc. The filters are clean and fuel seems to be flowing pretty good, but I still can't solve the problem.

Although there was no "check engine" light, I called for trouble codes and it flashed a "42". The Haynes manual says that this is a problem with the EST (electronic spark timing) system and possibly a faulty HEI. The book says nothing about how to test or correct this, and I need some guidance.

Can anyone help me with this?

P.S. when I changed the plugs, two of them were very black with soot, as if those cylinders weren't firing - does this make sense with a faulty EST?