While most gas-powered string trimmers available today use 2-cycle engines, you will find some string trimmers that use 4-cycle engines as well. It’s important to know what type of engine you have on your string trimmer so you don’t accidentally use the wrong fuel and ruin it.

String trimmers use unleaded gasoline with a minimum octane rating of 89 and a maximum ethanol content of 10%. String trimmers with 2-cycle engines require oil to be mixed with gasoline. String trimmers with 4-cycle engines will have separate fill ports, one for fuel and one for oil.

If you are unsure what type of engine you are running on your string trimmer, refer to your operators manual.

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Follow all safety instructions provided in your equipment operator’s manual prior to diagnosing, repairing, or operating.Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

Old Gas in a String Trimmer

Old gas left in a string trimmer will not only cause fuel restrictions, but it can also damage the carburetor and engine. Gasoline can begin to break down as quickly as 30 days after purchase.

The ethanol found in most gasolines attracts moisture from the air. This moisture and ethanol mixture gums up the fuel system and can corrode fuel components.

Because gasoline begins to break down quickly, it’s best to use a stabilizer in the fuel. Some oils for 2-cycle engines will include a stabilizer. Add a stabilizer like Sea Foam to your fuel if you are running a 4-cycle engine or if the oil added to your 2-cycle fuel doesn’t already include a stabilizer.

String trimmers require unleaded gasoline with a minimum octane rating of 89 (mid-grade) and maximum ethanol content of 10% (E10). Never use E15, E30, or E85 in the engine as this will damage the engine and most likely void manufacturer warranties.

Many manufacturers will not warranty replace carburetors after 30-90 days of purchase because of the effects fuel can have on a string trimmer. Using the wrong type of fuel or running bad fuel through your string trimmer is not a manufacturer defect.

Ethanol-free fuels are always best to run in your string trimmer, but it is a more expensive fuel choice.

Ethanol Free Fuels for String Trimmers

• 2-Cycle 50:1 Premix
• 2-Cycle 40:1 Premix
• 4-Cycle Fuel

Gas and Oil to Use in a 2-Cycle String Trimmer

2-Cycle Gas and Oil Mix in a String Trimmer

Don’t make the mistake and add straight gas to a 2-cycle string trimmer. This will damage the engine and cause it to seize up. Without oil added to the gas, the engine will not get the lubrication it needs for its internal parts to move freely.

A 2-cycle engine in a string trimmer uses gas and oil mixed at a ratio of 50:1, 40:1, or 32:1. For example, 50:1 mix equals 50 parts gasoline to 1 part oil.

The mix your string trimmer requires varies by manufacturer. You can find the correct ratio in your operator’s manual. You may also find it located on the fuel cap.

When creating this mix, use unleaded gasoline with a minimum octane rating of 89 (mid-grade) and maximum ethanol content of 10%. Add a 2-cycle premium oil that is ISO-L-EGD and JASO M345 FD certified.

Here are common string trimmer manufacturers along with their current required fuel to oil mix ratio:

How to mix gas and oil for a 2-cycle string trimmer:

• Use the chart below to determine how much fuel mix to prepare.
• Remove the cap from an approved gas can. Add unleaded fuel (minimum 89 octane rating & maximum 10% ethanol content) to a gas can.
• Using the chart for ounces of oil required, add the 2-cycle oil to the gas can.
• Replace the cap.
• Gently shake the fuel and oil until they are mixed.
• Add to the string trimmer’s fuel tank.

You can use the manufacturer’s 2-cycle oil. Another alternative is this 2-cycle mix by Kawasaki. It comes in 5.2 oz. and 6.4 oz. bottles that can be mixed with 2 gal. or 2.5 gal. of gas respectively for a 50:1 mix. Use 2 gal. of gas and a 6.4 oz bottle of oil to achieve a 40:1 mix.

2-Cycle Gas to Oil Mix

2-Cycle Premixed Fuel

A great option to reduce fuel problems and extend engine life is using an ethanol-free fuel mix. Many manufacturers offer their own brand of premixed fuels. This is an ethanol-free blend of oil and fuel that is ready to pour into your string trimmer’s fuel tank.

You won’t have to deal with the bad effects of ethanol as discussed in the fuel section. Also, it’s convenient to have fuel available on your shelf when you need it. TruFuel also makes a good 40:1 premixed fuel or 50:1 premixed fuel.

Gas and Oil to Use in a 4-Cycle String Trimmer

Gas for 4-Cycle String Trimmer

With so many fuel options available at fuel stations, it can get a little confusing knowing what type of gas to purchase.

4-cycle string trimmers require unleaded gasoline with a minimum octane rating of 89 and a maximum ethanol content of 10%. Add a fuel stabilizer like Sea Foam Motor Treatment to keep it from breaking down quickly.

The exception to this is the Stihl 4-MIX engine. Stihl designs their 4-cycle engine to use a gas and oil mix. Do not run straight gas through a Stihl 4-MIX engine.

Never use types of gasoline with ethanol contents higher than 10% like E15, E30, and E85 fuels. Because of the problems that can develop from using fuels with ethanol, ethanol-free fuel is best to run through your string trimmer.

This is a more costly product. It can be purchased in small fuel cans online or at your local hardware store. You can also purchase ethanol-free fuel at some fuel stations. This is typically sold as REC-90 or recreational fuel.

Engine Oil in a 4-Cycle String Trimmer

4-cycle engines have separate fill ports for the engine oil and the fuel. DO NOT mix the oil and fuel together in a 4-cycle engine. Never use 2-cycle oil in a 4-cycle string trimmer.

Oil is required to keep the engine components lubricated. When the wrong type or not enough oil is used, friction can build in the engine and overheat causing your string trimmer not to start and possibly ruin the engine.

The engine oil viscosity may need to be changed to one better suited for very hot or very cold ambient temperatures when you are using your string trimmer in these types of conditions.

4-Cycle Manufacture and Engine Oil Viscosity