Mercury 25 hp 2 stroke firing one cylinder only

the flywheel has a magnet on it that passes a sensor and that fires the plug at the correct time. it could be that cylinder's sensor is bad or has a bad connection. its super rare a stator ever goes out but those pickup sensors do

they can use different styles of setups so im not sure if your flywheel has 2 magnets or one, but if it has 2 then you could have flung off a magnet for that cylinder.

being the age of your motor it might be like a lawn mower with a separate magneto coil for each cylinder

i know this very generic info is not much help, but thats what you need to look into

try watching this youtube video diagnosing a 1989 45 hp Mercury for no spark LINK

Since you said both coils are good, I'm assuming you verified that it has fire on both cylinders?

I had an early 90s mariner 25 that I ran on 1 cylinder for way longer than I should have as a kid/teen. Turned out to be a power pack after my dad finally decided to take it in to get fixed. Fixed it and it ran like a champ again after. That motor was bullet proof.

I swapped the wires going to the coils and it changed which cylinder is firing. Still one cylinder no fire.

And yes, new plugs.

same as mine (late 80’s merc 25).
changed the power pack and good to go

Wanted to give an update on this. The switch box (CDI) was the problem.