Because they do not have an oil reservoir to lubricate their pistons, outboard engines must have oil mixed in with gasoline. The ratio of fuel to oil must be correct or the engine will either run too rich or too lean.
While the fuel-to-oil ratio varies with the specific year, make and model of the engine, older outboards tend to require more oil than newer ones. If you don't have your engine's manual handy, a fuel-to-oil ratio of around 25:1 is a standard estimate for engines built before 1960, according to Brad's Marine.
Newer engines benefited from the advent of two-stroke oil, which allows for ratios of 50 parts fuel to 1 part oil. Two-stroke oil was developed in 1970 and it protects spark plugs from getting fouled and pistons from carbon buildup.
Two-cycle oil should be added to an empty gas tank before gas is added in order to evenly mix the liquids. If you forget and add the fuel to the tank first, make sure to shake up the gas tank to mix as evenly as possible. Measure the oil additive using a bottle with a visible liquid scale.
- Photo Credit outboard motor boat image by Wimbledon from Fotolia.com
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