Weed trimmers run on a mixture of gas and oil, and two-cycle weed trimmers require a proper mix to run well. If there's too little oil, the engine will overheat and seize up. If there's too much oil, your engine will smoke, which can get the muffler all gummed up. Because weed trimmers do not have separate compartments for gas and oil, you must get the mix just right in order to ensure a healthy garden tool.
The Right Mixture
Most weed trimmers require oil to be added to the gas. Unlike engines that have separate gas and oil reservoirs, a weed trimmer's engine runs on a mix. This keeps the engine lubricated at all times. You should never use straight gas in a two-cycle engine because that will ruin it within minutes.
The make and model of your weed trimmer will determine the correct fuel-to-oil mixture. Some models, such as Stihl, require a ratio of 50:1, while others, including Weed Eater, require a 40:1 mixture and still others, including Ryobi, run on a 32:1 mixture. Check your owner's manual to make sure you're mixing in the proper ratio. Otherwise, you could harm the engine.
Mix It and Shake It
Once you've figured out the correct fuel-to-oil ratio, it's simple to calculate the amount of gasoline and the amount of oil you will need. If your fuel-to-oil mix ratio is 40:1, for example, you will use 40 parts of gasoline to 1 gallon of oil. There are 128 oz. in a gallon. Divide 128 oz. by the mix ratio of 40, which equals 3.2 ounces of gasoline.
Mix 3.2 oz. of engine oil with 1 gallon of regular gasoline. Because the oil will separate from the gasoline over time, thoroughly shake your oil and gas mixture before refueling your weed trimmer. Be careful when you do this so as not to get the oil and gas all over the place.
Use the Right Fuel
Make sure to buy regular unleaded gasoline and not a diesel fuel. It's all right to use gasoline that is blended with a maximum 10 percent alcohol. But if you want to use alternative fuels, such as ethanol or methanol, just be sure they are made up in a 10:2 ratio. If you want the safest fuel possible, purchase premixed fuel supplied by the company that manufactured the trimmer. Stihl, for example, supplies premixed fuel for all its gas-powered tools.
Do not store the trimmer with this mixture inside, unless you add a fuel stabilizer or you use a premixed fuel, which typically already includes a stabilizer. Alcohol-blended fuels can attract moisture and form acids. That can damage the fuel system and clog the carburetor, and cleaning the carburetor is a job you don't want to do. Also, keep in mind that fuels with more than 10 percent alcohol won't work well with your weed trimmer and can decrease the power of the engine, causing it to run erratically. Manufacturer-supplied pre-mixed fuels are typically alcohol-free.