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Weed Eater Fuel Dos and Don’ts: What You Must Know


Dos and Don'ts of String Trimmer Fuel

There are two different types of gasoline-fueled weed eaters: 2-cycle and 4-cycle. Each one has different fuel requirements, so you need to know which one you have and how to handle it properly. 

Here’s some essential information regarding weed eater fuel:

  • 2-cycle weed eaters typically use a 40:1 or 50:1 ratio of fuel to oil.
  • 4-cycle weed eaters keep the gas and oil separate, so you can use regular fuel.
  • Never use fuel that contains more than 10% of ethanol, as many small engines aren’t equipped to handle that ingredient.

Those are the basics, but there are a couple of other factors you should consider, such as how to store your weed eater over winter and whether or not you should use fuel stabilizers. You should also understand why you shouldn’t use fuel that contains a lot of ethanol, so continue reading to learn more.

Never Use Fuel With More Than 10% Ethanol

You can use gasoline that contains 10% ethanol, but do not go over that amount. Gasoline with 10% ethanol is also called E10, which is what’s provided at almost every gas station these days.

Personally, I stopped using fuel with ethanol in my outdoor powered yard tools some time back. If you are going to use it though, stay at 10% or less.

It is illegal to use gasoline with more than 10% ethanol in weed eaters and other outdoor power equipment.

What Is Ethanol?

Ethanol is added to gasoline to reduce exhaust emissions. It mixes with the gasoline and therefore reduces the amount of crude oil, so that way, fuel tanks aren’t burning pure gasoline. It’s usually made with corn or sugarcane and a denaturant. A denaturant makes the mixture unfit to be consumed by humans, and the federal law requires that ethanol contains 2% denaturant.

Why High Ethanol Content Fuel Shouldn’t Be Used In Weed Eaters

Why is it unsafe for weed eaters if it can be used in vehicles? Ethanol draws and locks in moisture. Small engines on outdoor appliances like weed eaters and lawnmowers weren’t made to handle significant amounts of water. Fuel tanks are vented, which means air from the outside can get inside, and the air is typically humid, especially in the summer months, and it’s even worse if you live in a very humid area.

Once the ethanol absorbs moisture, the moisture will separate from the gasoline and settle at the bottom of the fuel tank. This is when it becomes a problem. The fuel tank burns what sits at the bottom, so the engine will have to burn through water before it gets to the gas. Burning water can damage your engine. It can cause it to lose engine power and lose acceleration.

Ethanol is a solvent, which is another issue with weed eaters. It can loosen up debris inside the fuel tank and cause it to float around in the fuel. It will eventually make its way through the entire fuel system and clog everything.

Never use gasoline that contains more than 10% ethanol. For one, it’s illegal, but it will also cause several problems.

For more information, read Ethanol And The Growing Issue Of Small Engine Problems.

Pay Attention When Purchasing Gas At The Pump

The Clean Air Act of 1990 and the Energy Independence and Security Act of 2007 have required that most of the gasoline sold in the United States contain some ethanol, so it might be difficult to find ethanol-free or E10 gasoline in some areas. 

It can be tricky to find E10 or ethanol-free fuel, especially since some gas stations sell E15 gas as “88 octane,” making it unclear how much ethanol is in the gas. Some stations offer several different options like E15, E30, and even E85, which is 85% ethanol.

Be a smart shopper. Don’t get duped by enticing marketing and cheap prices. Cheaper doesn’t mean less ethanol. In fact, E85 might be cheaper than E10. As previously mentioned, ethanol is made from crops like corn and sugarcane, which are affordable crops. More ethanol means more crops and less crude oil, so ethanol gas can be cheaper to produce. 

Read your owner’s manual and the gas pump before you buy fuel. Some weed eaters and other outdoor equipment might not even be able to use E10 gasoline, so make sure you know that beforehand. Be sure you’re certain what kind of gasoline you’re buying at the pump, so you don’t inadvertently damage your equipment or break the law. 

Ensure A Proper Mix Of Oil And Gas In Two-Stroke Engines

4-cycle weed eaters have two separate compartments: one for fuel and one for oil. The gas is burned to power the machine, while the oil is used to lubricate the engine. But 2-cycle weed eaters only have one fuel tank where the oil and fuel are mixed together. As the gas is burned for power, the oil simultaneously lubricates the engine as it’s burned along with the gas.

The Ratio

2-cycle weed eaters require a specific ratio of oil to gasoline. Too much oil can damage the engine and won’t power the weed eater correctly, and too little oil won’t properly lubricate the engine.

Read the owner’s manual for the ratio instructions. Many brands and models use the same ratio, but there are several different ratios out there that are commonly used.

The most common ratios are 40:1 and 50:1. I’ve seen a lot of articles online that state all string trimmers use a 40:1 ratio. That is absolutely not true! The Weed Eater brand uses 40:1 (source) but a ton of them recommend the 50:1 ratio. Check your owner’s manual and make sure you know the proper gas-to-oil ratio.

Recommended gas-to-oil ratios for common string trimmer brands:

ManufacturerGas-To-Oil Ratio
Stihl50:1
Weed Eater40:1
ECHO50:1
Husqvarna50:1
Ryobi50:1

You can buy special 2-stroke oil that is designed for both 40:1 and 50:1 ratios. You should always write the date opened on the oil container. Two-stroke oil has a limited shelf life and it’s easy for it to get lost on a shelf in the garage for a few years without you realizing how much time has passed.

If you prefer the route of convenience, Home Depot sells a 6-pack of pre-mixed fuel. It’s made with ethanol-free gasoline, and the oil is already mixed in, so you don’t have to do any guessing work about ethanol percentages or oil to gas ratios.

If you aren’t sure about which brand of premixed fuel to get, see my side-by-side comparison of four popular premix products.

And one more thing: you might also see a 20:1 recommended ratio on some products. Check the owner’s manual to make sure you know which ratio your weed eater requires. 

How to Mix the Fuel

Mixing the fuel is a simple process that shouldn’t take too much of your time. The hardest part is knowing what ratio of oil to gas you need and making sure you have no more than 10% ethanol in your fuel. 

I’ll explain the basics below but first here’s an excellent instruction and demonstration video from Remington Power Tools. The video is just 1 minute long but they hit all the key points:

How to Mix Oil and Gas: The 2-Cycle Engine Fuel Recipe

Now, for the short and simple of it, here’s a couple of fundamental principles that I followed before I moved to premixed fuel. (Premix costs more on the surface but it lasts a lot longer than homemade mix does and I never have to worry that my mix is off).

All you need to mix fuel is oil, gas, and a gas can.

Add the oil to the gas can with the fuel. If you fill the gas can so that it contains exactly one gallon of gas, you can pour the proper amount of oil directly into the can.

Pour the mixed fuel into the fuel tank. You should never mix the oil and gas in the fuel tank; always mix it in the gas can. This will ensure that it gets properly mixed, and you won’t risk spilling any of it on your weed eater.

Remember: stirred, not shaken. You want to swirl the can around a little to allow the contents to mix but you don’t have to get too aggressive with it.

Properly Winterize The Engine Before Storing

Adding a fuel stabilizer is a crucial step to proper winterizing of a weed eater.

Since grass doesn’t thrive in the winter months in most areas, many people store their weed eaters and other lawn care equipment away while it’s cold and won’t use them again until the spring. You should store them carefully in a specific manner because leaving leftover gas inside can cause damage to the engine. Exposure to moisture can also cause damage.

Gasoline will begin to expire after 30 days or so, which will result in oxidation. Oxidation will cause several issues such as sticky varnishes that lead to over fueling and under fueling, clogs throughout the fuel system, corroded parts in the fuel system, and reduced fuel economy due to the fuel thickening (source). 

Any of these issues will result in lower quality fuel, which means you won’t get the most out of your money. A lot of sources recommend actually topping off the tank for the winter to prevent condensation in the tank but I always drain the fuel since it’s going to go bad and potentially gum up the inner workings.

Note: Always follow your manufacturer’s recommendations. Below are commonly accepted practices but the manufacturer of your string trimmer is the ultimate authority on proper storing.

Storing 2-Cycle Engines

Remember that 2-cycle engines are the engines that mix oil and gasoline together, so their “hibernation” preparation needs will be slightly different than what you need to do for a 4-cycle weed eater.

For this we are going to need a small amount of fuel. Just enough to get it started and run for a few minutes.

You will need gasoline and oil that’s approved for your model of weed eater, a couple of gas cans, a wrench, and maybe a towel.

Begin by cleaning off debris. Use a towel to wipe off any dirt or grass that was left behind on your weed eater. For hard to clean areas, you can scrub it with a toothbrush. Otherwise, a clean shop towel should be sufficient to remove most of the debris.

Empty the fuel tank of any existing fuel by letting the engine run until the tank is empty or by pouring the gas out into a gas can. You need to handle gas in a safe manner since it’s highly flammable, so you should only use a gas can. Any other container isn’t recommended.

Note: If you do pour the gas into another container, you still need to start the engine and let it burn off any fuel left in the carburetor.

Mix gas and oil together in a gas can to run through the weed eater. You should use appropriate gas and oil, as previously discussed in a ratio that’s suitable for your weed eater.

Use the same ratio as you normally would for your weed eater. Since you only need a small amount of gas and oil for this part, you might need to use a ratio calculator to find out how much oil you need to use.

Add fuel stabilizer to the gas and oil. You only need a small amount. This will be discussed further in another section.

Shake the fuel to mix it and then pour it into the fuel tank. Place the gas cap on securely.

Let the engine run until it runs out of gas. This will allow the fuel stabilizer to run through the entire system. The engine should stop when it runs out of gas.

Try to start the engine a few times. It won’t be able to run, but this will ensure that all of the gas and oil have burned, and the fuel tank is completely empty.

Remove the spark plug with a wrench. You can discard it and replace it in the spring, or you can add motor oil in the hole to lubricate it. If you use motor oil, you only need a few drops, and then you can return the spark plug back to its place.

Store the weed eater in a dry place indoors. You don’t want it to be near any kind of moisture.

Storing 4-Cycle Engines

The steps for preparing 4-cycle weed eaters for storage are different because the oil and gas are kept in separate compartments. Gasoline can expire, so you will have to remove that. It’s not absolutely necessary that you remove the oil, but you should consult the owner’s manual because the manufacturer might recommend removing the oil, as well.

You will need the same materials as before: oil, gas, a wrench, gas cans, and a towel.

Remove debris from the weed eater.

Empty the gas tank into a gas can and add fuel stabilizer. Since the gas is kept separate from the oil, you can add the fuel stabilizer directly to it. Use fresh gas if the gas from the weed eater contains any kind of debris.

Let the engine run for 10-12 minutes. This will allow the stabilizer to completely run through the fuel system.

Turn off the engine and wait 20 minutes. The stabilizer needs time to dissolve ethanol residue that may have been stuck inside the engine.

Empty the fuel bowl. Your 4-cycle weed eater should have a drain plug so you can empty the fuel bowl. If it doesn’t, you will have to remove the bowl completely so you can empty it.

Store the weed eater in a dry place indoors. Again, you don’t want any moisture to damage the engine. 

Things You Should Avoid

Never leave gasoline in the fuel tank for extended periods of time. Ethanol can cause damage to engines over time, so if fuel sits untouched, it can speed up the process. If any debris is left behind in the gasoline, it might grow mold and clog up the system.

Gasoline can also cause issues because of oxidation. After 30 days, the gasoline will begin to go bad and will damage the engine. It can cause corrosion, gunk, and can affect the quality of the fuel you put into it.

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Never store the weed eater near moisture. Moisture can ruin engines, especially if you accidentally leave a little fuel behind in the weed eater. If any moisture is left on the engine or gets into the weed eater, you may find your machine not working in the spring. 

Don’t guess the oil and gas ratio. Even though you are only using a small amount in the 2-cycle weed eater, you still need to measure it out. As mentioned before, too little or too much oil will cause damage to the engine. Take the time to be precise.

Use A Fuel Stabilizer

Fuel stabilizer prevents fuel from going bad. The stabilizer bonds with the gas to prevent oxidation and evaporation. It’s usually made with petroleum, so it will help repel water. 

Fuel begins to expire after about 30 days unless it’s been kept in an airtight container. Fuel left in your weed eater isn’t airtight since the engine and fuel tank are vented. Besides, the gas has been exposed to air so the degradation process has already started.

Running gasoline through your weed eater with a little bit of stabilizer will allow the stabilizer to coat the system and prevent any leftover gasoline from oxidizing.

Fuel stabilizer is meant to be left in with the fuel, but you should not leave any fuel inside your weed eater during the winter. If you have any doubts, consult the owner’s manual for your weed eater. 

Conclusion

Caring for your weed eater is a fairly simple task as long as you’ve read up on the subject beforehand. There are many do’s and don’ts that you need to remember so you don’t make any mistakes or get in trouble with the law.

Remember that E15 (or higher) gasoline is illegal to use in outdoor equipment, including weed eaters, lawnmowers, and even motorboats. 

Don’t wave this off because it can cause serious damage to your equipment. Many small engines simply aren’t made to function with large amounts of ethanol. Yes, it’s illegal, but it will also damage your engine beyond repair in some instances.

Read up about your weed eater before you go out to buy gasoline and oil. 2-cycle engines require specific ratios of gasoline to oil, so you need to make sure you know which ratio you require.

Know which gas station carries the right fuel. It’s becoming more common for gas stations to carry E15 or higher. Pay attention or skip the pump entirely and buy ethanol-free fuel.

Do not estimate ratio measurements. Too little or too much oil can damage the weed eater.

Store your weed eater properly during the winter. Harsh conditions and exposure to moisture can cause damage.

Use a fuel stabilizer when preparing it for winter. This will prolong its lifespan and make it ready for use in the spring.

Articles You May Be Interested In:

Can You Use Ethanol-Free Gas in a Weed Eater? Know The Facts

Adding Fuel Stabilizer To Ethanol-Free Gas: Is It Needed?

Does Ethanol-Free Gas Go Bad? Fuel Shelf Life Comparisons

Paul Brown

Paul has a two-acre yard on red clay soil in Southeast Texas. He knows exactly what the challenges are to nurturing a thriving yard in difficult soil. He takes a practical approach to yard improvement and enjoys putting best practices and “golden rules of lawn care” to the test. Click here for Paul’s author page

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