- Jan 16, 2020
New here and hope to help out when I can, but guys I'm stumped on this. We have a 1982 150HP Evinrude in the shop. Fresh rebuild, has been running perfect for weeks. It has come down with a bad misfire half throttle through wide open. We have been back through the carburetors, running UL77V plugs that all are burning good. We have tried a different fuel tank and fuel, swapped all plugs, changed power packs (I remember something about a faulty isolation diode even though these seemed to be okay) It will only turn up to around 4000 rpm on the water and starts misfiring bad from 3000 rpm and up. Hitting the primer doesn't seem to affect it. On the trailer at the ramp under a load at WOT every plug wire when pulled is carrying a good load so it seems to rule out an ignition coil, but the misfire is still there and can't isolate it to any one cylinder, it isn't a steady "bog" like a fuel problem, but a steady misfire. A pack should affect 1 or 2 or 3 cylinders, a coil one cylinder, a faulty isolation diode would cause a misfire but we changed packs. We have checked top dead center, set sync and link. This one has me baffled.