Evinrude offers several fuel additive products. For general use they have their 2+4 FUEL CONDITIONER. And a product called CARBON GUARD to reduce carbon deposits. See
http://shop.evinrude.com/products/343711/Fuel_AdditivesThere is also FUEL SYSTEM CLEANER, a product to be used occasionally as a maintenance procedure. See
http://shop.evinrude.com/product/356813 ... ms_CleanerYamaha has a product called RING FREE that they recommend for their engines to reduce carbon deposits.
Mercury has similar products. I wrote about them in detail in an article I published in 2011, when all the products were re-branded. See
Mercury Fuel Treatments: A QUICK Look
http://continuouswave.com/ubb/Forum4/HTML/007434.htmlSome sources recommend very aggressive shock treatment of older engines by running them of fuel mixtures with extremely high additive content. I take those recommendations with some skepticism. Older engines with older fuel system components need to be given gentle care in my opinion. I wouldn't pour a quart of naphthalene into a gallon of gasoline and try to run them, as is often suggested.
With today's ethanol-gasoline blended fuels, the fuel itself is quite good at cleaning the fuel system--sometimes too good as older rubber components are often not tolerant of ethanol-gasoline fuel blends.