I could have had a multihull dinghy
But CalebD already mentioned something in the first which might be the cause, a bad oil/fuel mix ratio. That might fit, since it has been a long time since I filled up the outboard fuel tank and remember that when I did so I was in a hurry and didn't actually measure the oil but poured in what I thought was about the right amount. In retrospect, I might have put in quite a bit too much oil. The tank is getting empty so I'll know soon if a new tank fill does the trick.