How to Mix Fuel for a Weed Eater

Weed Eater is a brand of gas-powered string trimmers for homeowner use. The trimmers are powered by small two-cycle engines that are lubricated through the fuel system. This lubricating method requires users to add lubricating oil to the gasoline. The gas-oil mixture provides continuous lubrication of the engine's moving parts. For best performance and longest trimmer life, it's important to get the fuel-oil mix correct.

Fuel Jug
credit: jdwfoto/iStock/Getty Images
Use a safety-approved can for storing your mixed trimmer gas.

40:1 Mixture

All Weed Eater brand gas trimmers require that oil be added to the gas in a ratio of 40:1, which equals a mixture of 3.2 ounces of two-cycle air-cooled engine oil to 1 U.S. gallon of 87-octane regular-grade unleaded gasoline. Other trimmer manufacturers specify different gas-oil ratios such as 50:1, or 2.6 ounces of two-cycle oil per gallon of gas, and 32:1, or 4 ounces of oil per gallon of gas. Be careful to mix fuel correctly. Too much oil in the mix causes excessive exhaust smoke and exhaust carbon buildup. Too little oil in the mix can cause lack of power, overheating and eventual internal engine damage.

How to Mix

Start with a clean, empty, 1-gallon gasoline can. Pour the correct amount of oil into the empty can. Slowly add 1 gallon of gasoline, screw on the cap and swish the container around to ensure complete mixing of the gas and oil. The container will have room to accommodate the small amount of oil combined with the gas. Agitate the can again before each refueling. Never try mixing the gas and oil together in the trimmer's gas tank. You're likely to get the ratio wrong and you won't get a thorough blending. Don't mix more fuel than you will use within 30 days. Modern gasolines contain additives that can break down in long-term storage.

Fuel Warnings

Weed Eater warns against using gasoline that has more than 10 percent alcohol. Too much alcohol in the gas can damage fuel lines and other fuel system parts. Excessive alcohol also will cause engines to run hotter than normal, which can lead to vapor locking that stalls the engine. Weed Eater also warns against using straight unmixed gasoline in its trimmers; the lack of lubricating oil in the gas will destroy the engine within minutes.

Oils to Avoid

Never use marine oil designed for liquid-cooled two-cycle outboard motors or motor oil for automobiles in your trimmer. Air-cooled trimmer engines operate at much higher internal temperatures than these liquid-cooled engines. As a result, oils formulated for liquid-cooled engines can break down in a trimmer engine and leave harmful, power-robbing carbon deposits in the cylinder and muffler.