Your air/fuel mixture is too rich. Your spark plugs should be a brown color. If they are coming out black/wet/sticky you have too much fuel per air. There are a lot of videos on youtube showing how to adjust the mixture. Im unfamiliar with outboard setups, but generally you have a main jet (a long brass nut that the needle fits into) and the needle that is attached to the carb slide. The needle usually has a clip at the top with several cutouts to adjust how low the needle sits in the slide. If your clip is at the bottom cutout try moving it to the middle, working your way up to lean the mixture. If you reach the top, and your plugs are still looking the same, you need a smaller main jet. Once you have the mixture dialed in, keep in mind temperature and atmospheric pressure can affect your mixture. If you go to a cold area you may have to adjust your needle again. Since you are always at sea level, I dont think you would experience much variation. Fuel/oil mixture rarely affects two stroke performance unless you are mixed at an extreme (100:1 or 10:1).