Will Mixed Gas Hurt A Lawn Mower? (5 Mistakes People Make)

Many people need clarification about whether to use mixed gas in a lawnmower or not. Most gardeners use mixed gas in the lawn mower, which can cause different results in the ignition chamber. 

If your lawnmower has a 2 stroke engine, you can use mixed gas without any problem. But if your lawnmower has a 4-stroke engine, mixed gas can choke the engine, causing it to overheat and smoke. Although you can temporarily run a 4-stroke engine with mixed gas, but it is not recommended.

If you have already used mixed gas in your mower or have just purchased it and don’t know if it is suitable, then this article will help you know what you should do, what the outcomes are, and which lawnmowers are good for mixed gas.

We will also discuss which fuel is best for lawnmowers. Now let’s dive into it. 

Lawn mower gas

Does the lawnmower need mixed gas?

Whether your lawnmower needs an engine depends on the stroke it has. 

If you have 4 stroke engine, it will need fresh unleaded gasoline with an octane level of 87 or even higher. 

You can use mixed gas in your 4-stroke engine by mixing gasoline with Ethanol but adding more than 10% is not suitable. 

On the other hand, lawnmowers with 2-stroke engines use the same gas added with high-quality 2-cycle engine oil and work completely fine. 

So yes, the mixed gas is one of the good choices for lawnmowers which has 2 stroke engine but not 4 stroke engine.

What can happen if you use mixed gas in a lawnmower?

Using a mixed gas will not hurt your lawn mower until and unless the octane level is low and the engine is 2-stroke.

By mixing the gas, the engine will start to operate at an average of two octane levels. 

For example- If you mix 87 gasoline with 91 gasoline, it will form an 89-octane blend. 

Before you make a blend by yourself, you should properly know about your lawnmower’s octane requirements and then choose the octane gas rate accordingly. 

If the octane level is too low for the engine, it can overheat or knock the lawnmower and even stop working. 

How does using mixed gas in a lawnmower depend on the different engines?

Lawnmower engines can be either 2-stroke or 4-stroke. 

The mixed gas is best for 2-stroke engines as it doesn’t cause any harm. 

While on the other hand, using mixed gas in the 4-stroke engine is quite difficult. 

A different oil intake is not needed in the case of 2 stroke engine. 

You must purchase one kind of gasoline that is already a mixed oil. 

But in the case of 4 stroke engine, the oil and fuel are poured individually. 

That means you have to purchase oil and fuel separately for the engine.

Lawn mower gas refill

Effect of mixed gas in a 2-stroke engine

Gasoline helps to lubricant the 2-stroke engines. So, it is important to mix the oil and fuel properly.

A 2-stroke engine requires ordinary gasoline to run with a little quantity of oil added to it. 

The fuel is used for ignition, and the oil is used for lubrication. 

Using mixed gas in a 2-stroke engine doesn’t cause any effect internally. 

On the other hand, it’s good for the engine as it helps it to work more efficiently and smoothly. 

Effect of mixed gas in a 4-stroke engine

If you plan to use mixed gas in your 4-stroke engine, immediately change your mind. 

The mixed gas is not suitable for 4-stroke engines as it can overheat, choke, and smoke the engine.

If you have already used mixed gas, then drain the mixed gas and clean the gas tank properly. 

Then change it with the suitable gas for the 4-stroke engine. 

Let’s discuss the effects a mixed gas can have on a 4-stroke engine.

  • Choking and overheating- If the oil is mixed with gasoline, it can choke 4-stroke engines. This leads to overheating because they are usually built for greater endurance. 
  • Smoke from run-through- It is quite difficult to operate a 4-stroke engine on 2-stroke fuel. As time passes, you might notice foul plugs or smoke from the exhaustion in the 4-stroke engine.
  • Ignition and explosion– If you mix fuel and oil in the 4-stroke engine, it will get damaged more quickly because it will cause more wear and tear on the cylinder liner, also causing complete damage to the piston. 

There will be abnormal ignition, explosion, and banging. So, the mixed gas is not for 4-stroke engines like 2-stroke. 

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How to find out if a lawnmower has a 2-stroke or 4-stroke engine?

You can easily differentiate the engine by the following feature:

  • If you notice that the lawnmower has a single port fill for the engine gas and oil, it has a 2-stroke engine. 
  • But if you see that it has two separate fill ports, one for oil and one for gas, it has a 4-stroke engine. That’s why you should never mix gas and oil in this type of engine.
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Fuel for 2 stroke engine

Using fuel where gasoline is not mixed with oil is not recommended for 2 stroke engine lawnmower. 

It is because mowing the lawn with a 2-cycle engine with little oil can damage the unit. 

Oil helps in cooling the cylinder and piston by lubricating them. 

Without proper lubrication, the metals will melt and crush against each other, permanently twisting them. 

You don’t have to mix a huge quantity of oil with gas. Just take 1 part of the oil and 40 parts of gas to make a perfect mixture for the 2-stroke engine.

Fuel for 4-stroke engine

A 4-stroke engine needs regular gasoline, which is used to run a car. Here, the oil is filled from a separate reservoir pump. 

Don’t ever use mixed gas for a 4-stroke engine, as it can cause huge damage to the engine. 

How to recognize mixed gas from regular gas?

You can easily recognize mixed gas from regular gas by doing a simple experiment. 

Just bring a piece of white paper and drop the gasoline in it. 

Then wait until it gets evaporated. 

If the gasoline is unmixed, then the paper will get dry completely. 

But if the gasoline contains oil, you will see some oil spots on the paper after evaporation of the oil. 

It is because the gas evaporated much faster than the oil because of the lower boiling point. 

The best gas for the lawnmower

You can purchase gas from different companies, but you must take care of some necessary things. Let’s discuss the following:

  • The gas must be fresh and clean. 
  • The gasoline should have a minimum octane level of 87 and a maximum of 91. The higher octane levels are recommended only for higher altitudes. 
  • Gasoline, which contains up to 10% ethanol or 15% methyl tertiary butyl ether, is suitable. Avoid using gas that contains more than 10% of Ethanol.

Also read: Where To Get Gas For Lawn Mower?

How is more than 10% of Ethanol harmful to lawnmowers?

If you are using E10 fuel, then it is recommended for lawnmowers. 

But gas with a much higher percentage of Ethanol is not approved for lawnmowers.

Ethanol is harmful to the engine because it absorbs water from the air gradually, leading to overheating, corrosion, and rusting of the engine, due to which you will notice the engine’s poor performance.

Regular gasoline absorbs 50 times less water than E10 gas, so it is best to use non-ethanol fuel. 

Most non-ethanol fuels are rated 89 octane, which is excellent for small engines like lawnmowers. 

Mistakes you should avoid while using mixed gas in a lawnmower

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Let us discuss what you should do if you use mixed gas in your lawnmower:

1. Not examining the lawnmower’s gas octane level demand

Before using the gas, you should know the octane level suitable for your lawnmower engine. 

You should choose the octane rate wisely; otherwise, the gas combination can be too low for the lawnmower’s octane requirements, leading to overheating and knocking the engine. 

2. Not removing the gas from the lawnmower’s tank

If you fail to mix the gas in the correct octane levels, it is best to empty the gas tank by draining the fuel. 

Otherwise, it can be too late for you to fix the engine from damage.

After removing the gas, ensure the fuel filter is completely cleaned or changed. 

You can also leave the tank clear for a long time. 

3. Not mowing the lawn until the lawnmower’s gas tank is completely empty

Mixed gas usually doesn’t hurt the lawnmower but is a little less powerful than other fuels. 

It is best to mow the lawn until there’s no fuel left in the gas tank rather than keep it for the next mowing session. 

After the gas is empty from the tank, fill it with one type of correct octane-level gasoline.

4. Not caring for the engine of the lawnmower

Generally, mixed gas doesn’t cause harm until the octane level is low and the engine is 2-stroke. 

Still, it is better to look after the signs such as overheating, knocking, and other unusual issues to avoid stressing the engine. 

If you notice the engine is hot and some irregular sounds are coming, it’s better to turn it off. 

5. Not removing water from the gas tank

Sometimes, the gas tank is filled with water because of the rain. 

You should check and empty the gas tank before using it. 

Water can get boiled, which can overheat the lawnmower and cause smoke, misfire, and steam. 

Mixed gas can sometimes be beneficial, but it can also cause engine failure if used in a 4-stroke engine. So you don’t have to worry about it. 

It just depends on the octane level and the lawnmower’s type. 

How to check the quality of the mixed gas in your lawnmower?

Lawn mower 1

You can check the gas quality by smelling the gasoline. 

If the gas is oxidized, you will get a strong and suffocating smell, unlike clean and fresh gas. 

Another method is to drain the oil from the machine’s gas tank and pour it into the see-through container. 

If the gas has turned dark, then it means the gas is spoiled. 

But if you use mixed gas in the 4-stroke engine, it will show smoke because of oil burning.

How long gas remains good in a lawnmower?

It just depends on where you are storing your gas. 

If the gas is stored in the lawnmower gas tank, it can degrade in 30 days.

But if you have stored the gas properly and are treated with gasoline stabilizers frequently, it can last up to 1 to 3 years.

Final words

So, now you know the effect of mixed gas depends on the type of engine the lawnmower has. 

You should always select the correct engine; otherwise, it can suffer permanent damage. 

The mixed gas is only recommended for 2-stroke engines. 

While in a 4-stroke engine, you will have to add fuel and oil separately in a separate fuel tank.

Reference: Lawn Mower Technology, Lawn Mower SafetyLawn MowingLawn CareMowing your lawn.


Hello everyone, My name is Richa and I am here to make you a better gardener by creating an in-depth and helpful resource for all the fellow gardeners out there. If I could help even a few people understand their plants better then I call it a success for my efforts.

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