Hello,
Some of the Mercury engines have a low speed and high speed ignition circuits in the stator. If you can give me the serial #, I can verify and give testing instructions. Also you can try a timing light (connecting to each plug wire one at a time) with engine running. They should all be firing.
Thank you
I will check tomorrow and get back to you.
Marine Mechanic
Certified Technician Mercruiser Mariner Mercury Johnson Evinrude & business owner