80hp outboard with issues

[towser@live.com]

My 80hp outboard runs well until it gets to about 20km/hr and then it starts to miss.
i changed spark plugs but didn’t fix issue.
does anyone know what the issue will be?
it happened all of a sudden whilst I had boat at about 40km/hr.

[99yam40]

hard to say what it might be, but giving a complete model number and date of production would help others know what motor you have.
is this a F80 or a 2 stroke?

a guess from me would be to check the fuel pump for leaking into crankcase causing over rich condition.
or a main jet is restricted with debris causing a lean condition in a cylinder

[towser@live.com]

It’s a F80.