Knowing the proper way to mix fuel is the first step in keeping your chainsaw running well for many years to come. Using the right type of fuel mixture is critical for efficient, problem-free operation.
Read Your Manual
Your manual will tell you the fuel mixture you need. Many gas-powered chainsaws run on a 40:1 mixture of gasoline and 2-cycle engine oil. Stihl chainsaws and others run on a 50:1 mixture of gas and 2-cycle engine oil. Some older models use a 30:1 ratio. Check your chainsaw's manual to be sure.
It's better to use too much oil rather than too little. Excess oil can produce a smoky exhaust, oil leaks from the muffler and loss of power, but too little oil can destroy the engine. Oil cools the piston and cylinder to keep them evenly lubricated.
Before you Fuel
Clean the chainsaw's fuel cap and the area around it to ensure no dirt falls into the tank. Then, get your gas and oil ready to be mixed.
What to Use
Don't use just any gas and oil, as it's important to use the right kind of each. Use mid-grade unleaded gas with a minimum octane rating of 89. Fuel with a lower octane rating may raise engine temperatures, increasing the risk of piston seizure and damage to the engine. For the right oil, use a premium 2-cycle oil.
If you need a 50:1 gas and oil mixture, for one gallon of gas, use 2.6 fluid ounces of oil. For 2.5 gallons of gas, use 6.4 fluid ounces of oil. For 5 gallons of gas, use 12.8 fluid ounces of oil. If you need a 40:1 gas and oil mixture, use three ounces of oil for one gallon of gas, 6.5 ounces of oil for two gallons of gas, and 9.5 ounces of oil for three gallons of gas. You can also buy premixed fuel for 2-cycle engines.
You can't mix the gas and oil directly in the fuel tank. Stihl recommends using a No-Spill fuel container, which is specially designed to reduce spillage, evaporation and permeation. Make sure your container is empty and leak-proof before adding the oil and gas. Also, make sure it's slightly larger than the volume of gas and oil needed.
First, add the premeasured oil. Never add the gasoline first. After you've added the oil, add the gasoline. Then, seal the container and swirl it, don't shake it. Once it's well blended, it will be evenly colored with no dark or light streaks. If you have stored properly mixed oil and gas, swirl it before using. Stihl suggests never using stored oil and gas that's more than 30 days old.
Follow your manual's instructions carefully. In order to reduce the risk of fuel spilling on your skin or inhaling fuel vapor, remove the chainsaw's fuel cap carefully to allow pressure buildup in the tank to release slowly. Make sure you're in a well-ventilated area. Position the chainsaw so the cap points upward and add your fuel mixture. Don't start your chainsaw if you have spilled fuel on yourself, the chainsaw or the ground nearby. After the first 10 hours of refueling, don't use the chainsaw at a very high speed.
Only mix sufficient fuel for a few days' work. Fuel storage should never exceed three months. When using stored fuel, shake the mixture in the canister before fueling your chainsaw. If you still have fuel in a chainsaw that you plan to store for longer than a month, drain the fuel into a properly sealed container and add a fuel stabilizer. Remember, however, that this mixture is best used within 30 days.