Bought this boat last fall, came with a 2002 Yamaha F100 4 stroke carburated engine, when i got it started this spring, it was misfiring on #4 and seemed to have very weak spark, seemed to be running on 2 or 3 cylinders, but specially bad on number 4. But it always starts up fine. Swapped plugs with #3 with no success, did a compression test which came up good 120psi to 150 psi. Pulled plug wire while running on #4 with almost no change, tried with #3 it then stalled. Next I swapped coils, top coil was running #1 and #4, bottom coil was running #2 and #3, unplugged them completely including unplugging the feed wire and swapped them, now #4 and #1 run good and #3 runs poorly as well as #2, am I right into thinking that I have a weak coil? How do I make sure that it is the coil that's the problem? How do I test it before purchasing another one, because once purchased electrical parts cannot be returned.
A Yamaha service manual will give specs and make sure you use the proper test equipment like the manual states.
Normally ohm test should show if it is bad, but voltages in and out would be a sure thing for trouble shooting, not just looks weak.
Look for Sparkieboat's posts, he has some links at the bottom of his posts that will lead you to some good info on trouble shooting ignitions