This Sunday I got together with my son and a couple of his friends to troubleshoot why his 4.3L inboard outboard merc-cruizer wouldn't run. They used a flashlight (I don't know which one) to trace some color-coded wires. We found that it didn't have spark, and no power to the ignition coil. When we jumpered 12 volts to it, she started up but didn't run well.
After checking for fuel, we pulled the carb apart, (which had been rebuilt the previous year), and re-adjusted the floats. Finally it was decided to run fuel to it with a funnel and a hose, rather than from the gas tank, and she started and ran well. A piece of paper was found to be clogging the fuel line.
He had it to a marine mechanic last year who gave up on it.
I have been so happy that we got it running, that I can't stop thinking about it.