SUBJECT: 1993 Mercury 25 hp, high speed miss

Submitted by Phytoplankton (128.115.27.11) from CALIFORNIA on

I have a 1993 short shaft 25 hp Mercury. The engine idles and runs at low speed just fine, but, about a year ago it developed a high speed miss, didn't bother me too much since the engine is on a 12 foot semi-vee and I rarely ran it flat out (too fast, too scary), however, now the miss has moved into the lower power band. I took the motor to my local shop, and they are pretty good, they tuned it up and checked the carbs, but they couldn't get it to miss because they couldn't run it fast enough in their tank. The miss occurrs under a load and around 3/4 to full throttle. It seems to surge, like one cylinder is dropping in/out. The first time it occurred I pulled the plugs and one was a nice brown color, the other was black oily. This time both plugs look black oily. Putting in new plugs helps for a while, but doesn't totally eliminate the miss. I'm thinking either coils or the power pack. At this point it might be cheaper to just throw those parts at it rather than pay shop rate to have them haul the boat to a lake where they could run it at speed. Anyone have any ideas?