This all started when you changed the spark plug?? Was changing out the plug for "routine" maintenance or "just because it was time"? If that's the case, it would seem that you still don't have good contact with the plug wire and it's cap.
Or, were you changing the plug to improve a perceived problem? If that's the case, it's possible that the carburetor needs cleaning.
Or, the fuel isn't fresh and volatile.
These carbs are pretty easy to clean.
As a matter of fact, you might not even need to take it off and disassemble it. If you think it needs cleaning, do this first:
Find the "screw, plug" (item #8 in the link below) on the top of the carb and remove it. DON'T LOSE IT!
They don't sell them anymore!
Use the little plastic straw on a spray can of brake kleen or carb cleaner and spray into the hole. First try to point the fluid toward the front of the carb and then try to point it towards the back. There is a channel in there you're trying to flush out.
Do this with the spark plug removed and do it several times. Replace the scew plug.
Then, let everything sit for 20 minutes.
Then pull the engine over a couple of times with the spark plug out.
Reinstall the spark plug and try running it and see if there's any improvement.
https://www.boats.net/catalog/honda/outboard-by-hp-serial-range/5hp/bf5ah-la-pre-1997/carburetor
Good luck.