With the ignition switch on (circuit under load), I read 12.4v at the solenoid, but only 4-5v at the coil + and 8-9v when starting. The engine easily cranks and starts, but misfires, backfires and runs generally poorly.

Hoping to find the "missing" 3v, I measured voltage drop between solenoid "S" terminal and coil + in millivolts. Even though not indicated, I replaced that wire and got the same results, which seems to eliminate it as the problem, but I don't know where to look next. Maybe the ignition switch? The voltage gauge reads 12v with the switch on and 13+ when running. The headlights drop that to 12v.

I'm going to replace the coil, but that's a "hail Mary" rather than a diagnosis.

I was recently very wrong about needing new battery cables as suggested here, so I'm hoping that someone can offer another obvious thing that I'm missing.

Thanks for any and all help.