Why does my 20 hp merc run on only one cylinder?

[<Aaron77>]

1989ish 20 hp merc. It starts when I launch it, but it only runs on the top cylinder. Very slow. If I take the top cylinder plug wire off, it wont start on that cylinder.
If I run it on one cyl for 10 minutes or so, the other cylinder seems to pop once or twice for a second, then all of a sudden it jumps up to full speed on both cylinders. Then it runs fine. Every start after that is fine. Even the next day at the lake it runs fine. But if it stands for a week or so on the trailer and I launch it again, the same thing happens. I changed both plugs and wires. The carb has been rebuilt and cleaned.
I would appreciate any help anyone could give me as to the root of this problem. Thanks alot from Winnipeg Canada

[<SamtheMan>]

Have you tried swapping the coils to see if the problem goes to the other cylinder. Sounds like a week coil that likes to be warm in order to run.

[logger]

85 18XD does the same

does the same as your 20 hp merc. when i switch wires on the coils the other cyl spark plug fires. need to know proper readings for ohm testing trigger & stator leads or can some one just tell me what's wrong? starts on one cyl then the other kicks in after you've motored quite a while.