Outboard Motor Fuel Mix: Oil to Gasoline Ratio

by Will Charpentier
A gas can makes a convenient premixing container.

A gas can makes a convenient premixing container.

Not all outboards have crankcase full of oil, as do car and inboard engines. Some do, but two-stroke outboards are lubricated by the oil that's mixed with the fuel. Some two-stroke outboards have oil injection systems that automatically add oil as required but for those that don't, you are the mixing system.

The Golden Ratio

The ratio of oil to gas for most two-stroke outboards under most conditions is 1 part oil to 50 parts gasoline. Planning is either the hardest part or the easiest part of this because, if you go boating, you not only have to carry along enough gas to get where you're going, you also have to pack enough oil for the motor. If something happens and you don't make it to your refueling stop, you may have to impose on someone you encounter for fuel and they may not have a two-stroke motor.

Fuel Mixing 101

Mixing the two-stroke oil with the gasoline before pouring it into the tank is by far the simplest method of lubrication for two-stroke outboards. However, the method is messy and causes the greatest amount of damage to the environment. Imagine: you're in the middle of the lake and a thunderstorm is on the horizon. Waves are beginning to build -- and the motor begins to sputter. It's time to head for the marina, but first you must mix fuel, in a rocking boat, with high winds. Not only does the wind blow the oil into the lake and the bottom of the boat as you try to pour it from the can, you don't know if enough oil ends up in the fuel tank. The best way to mix the fuel and oil is to start with a premixed container.

Fuel Pre-Mixing

One gallon of gas is 128 oz. To arrive at the 50-to-1 mix your two-stroke motor requires, you can worry yourself with math that tells you measure and add 2.56 oz. of oil to each gallon of fuel, or you can avoid the math altogether by adding the oil for a tankful of fuel to a single gallon of gas, agitating the gas can and adding that gallon to your fuel tank before you fill up the tank. If your time on the water will require two tanks of gas, carry two 1-gallon gas cans with this premix in them. You can avoid the hassle of measuring and the potential environmental and personal harm that can result from trying to mix "on the fly."

Gas Before Oil

Whether you opt for premixing the fuel and the oil or make a good guess, if you add one 6-oz. can of oil for every three gallons of gas, there will be enough oil in the fuel to lubricate the motor without being "excessively oil rich," but remember to add the oil first or add the oil as you add the gas. If you add the gasoline first, you face the problem of agitating the tank after you've added the oil -- a difficult proposition if you have built-in tanks. Even with the premix, you must mix the oil and gas by agitating the premix tank. If the oil doesn't mix properly with the gasoline, the motor won't be properly lubricated.

References

  • "Evinrude Repair Manual - 2.5 to 250 HP Models, 2002-2007"; Seloc Marine; 2007

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

Photo Credits

  • Jupiterimages/Comstock/Getty Images