Small outboards can be reliable and wonderful but when they won't start, they can be an Achilles heel for mariners. Gowen Marine (Portland, Maine) service manager Ray Stoddard recommends mariners use aviation gasoline, also known as avgas, for two and four stroke outboards with carburetors–mostly those smaller than about 20 h.p.
Avgas has no ethanol. Outboard carburetors are open to the atmosphere (larger outboards frequently use a closed system and electronic fuel injection) so moisture can drift into their carburetors. Stoddard says ethanol attracts and holds water causing starting problems. Eliminating the ethanol helps with the problem but Stoddard recommends emptying the carburetor after each use anyway so that moisture does not accumulate in residual fuel there.
Stoddard says mixing avgas and regular fuel is no problem in a pinch or during periods of heavy use. But he recommends avgas for the long haul in these small carbureted engines.
Avgas can be purchased at airports. Northeast Air in Portland, Maine sells it by the gallon. It was $6.87 per gallon in June 2013.
For those experienced with small outboards and avgas, please chime in here with tips and feedback as we are planning an article on the topic in the magazine later this year.