Choosing the right fuel is one of the most important things you can do for your lawn mower. If you’ve ever wondered what gas in lawn mower is the correct type, you’re not alone. Using the wrong type of gasoline in your mower can lead to performance issues and damage. This guide will explain everything you need to know, from octane ratings to ethanol content, to keep your machine running smoothly for years.
Putting the wrong fuel in your mower is a common mistake. It can cause starting problems, poor performance, and expensive repairs. We’ll cover the specifics so you can fuel your mower with confidence every time.
What Gas In Lawn Mower
For most modern four-stroke walk-behind mowers, the answer is straightforward. You should use clean, fresh, unleaded gasoline with an octane rating of 87 or higher. The critical factor, however, is the ethanol content. For small engines, gasoline with no more than 10% ethanol (E10) is typically acceptable, but ethanol-free fuel (E0) is highly recommended for optimal performance and longevity.
Two-stroke mowers, which require you to mix oil with the gas, have different requirements. Always consult your owner’s manual for the manufacturer’s specific fuel recommendation, as it is the final authority for your model.
Understanding Octane Ratings
Octane rating measures a fuel’s resistance to premature ignition, often called “knocking” or “pinging.” In simple terms, higher octane fuel is more stable under pressure.
Most small engines found in lawn mowers are designed with a relatively low compression ratio. They do not require high-octane fuel to operate correctly. Using regular 87-octane gasoline is perfectly sufficient and cost-effective. There is no performance or cleanliness benefit to using premium (91-93 octane) fuel in a standard mower engine unless specified by the manufacturer.
When To Consider Higher Octane Fuel
Some high-performance commercial mowers or certain models may specify mid-grade or premium fuel. Always check your manual first. If you experience persistent engine knocking with fresh 87-octane gas, switching to a higher octane may help, but this is rare for basic mowers.
The Ethanol Problem In Small Engines
Ethanol is a biofuel additive commonly found in gasoline, such as E10 (10% ethanol). While it’s fine for most cars, it can be problematic for lawn mower engines.
- Absorbs Water: Ethanol attracts moisture from the air, leading to phase separation where water and ethanol mix at the bottom of your gas tank. This can cause corrosion and engine failure.
- Degrades Fuel System Parts: It can damage rubber seals, gaskets, and plastic components in older fuel systems not designed for it.
- Leaves Varnish: As ethanol-blended fuel evaporates, it leaves behind a sticky residue that can clog the carburetor’s tiny jets and passages.
For these reasons, many experts strongly advocate for using ethanol-free gasoline (often sold as “recreational fuel”) in lawn mowers, especially if the mower will be stored for any length of time.
Gasoline For Two-Stroke Vs. Four-Stroke Engines
It’s crucial to know which type of engine your mower has. The fuel requirements are different.
Four-Stroke Engine Fuel
This is the most common type in modern walk-behind and riding mowers. A four-stroke engine has a separate oil reservoir (crankcase). You pour straight, unleaded gasoline into the fuel tank. Never mix oil with the gas in a four-stroke engine.
Two-Stroke Engine Fuel
Some older or very lightweight mowers use two-stroke engines. These engines require a specific mix of gasoline and two-stroke oil. The typical ratio is 50:1 or 40:1, meaning 50 parts gas to 1 part oil. Using straight gas in a two-stroke engine will quickly destroy it due to a lack of lubrication.
- Always check your mower’s manual for the exact oil-to-gas ratio.
- Use a clean gas can and measure the oil precisely with a dedicated measuring cup.
- Mix the fuel and oil thoroughly in the can before pouring it into the mower’s tank.
Step-By-Step Guide To Fueling Your Lawn Mower
Follow these steps to ensure you add the correct fuel properly and safely.
1. Consult Your Owner’s Manual
Before anything else, locate your mower’s manual. It will have a specific section on fuel requirements, including the recommended octane, ethanol limits, and whether oil mixing is needed. This is the most reliable source of information.
2. Select The Right Gasoline
Based on your manual, purchase the correct fuel. For most, this will be fresh 87-octane unleaded, preferably ethanol-free. Only buy as much as you can use in 30 days, as gasoline begins to degrade after that.
3. Use A Clean, Approved Fuel Container
Always use a plastic fuel can rated for gasoline and marked with the proper safety symbols. A dedicated, clean can prevents contamination from dirt or old fuel residue. A funnel with a built-in filter is also a great investment to catch any debris.
4. Add Fuel Stabilizer For Storage
If you are fueling up and know the mower will be stored for more than 30 days, add a fuel stabilizer like STA-BIL to the gas can. Mix it in before pouring the gas into the mower. This step is crucial for preventing fuel degradation and varnish buildup during the off-season.
5. Fill The Tank Correctly
Do not overfill the tank. Leave a little space to allow for expansion. Wipe up any spilled gas immediately to prevent fire hazards and damage to the mower’s paint. Always fuel your mower outdoors, never in a garage or shed, and ensure the engine is completely cool.
Consequences Of Using The Wrong Gas
Using incorrect or old gasoline can lead to a range of frustrating and costly problems.
Starting Difficulties And Stalling
Bad gas is the number one cause of a mower that won’t start or that starts and then dies. Varnish or water in the carburetor prevents the correct air-fuel mixture from reaching the engine.
Engine Knocking And Power Loss
Low-octane fuel can cause pre-ignition, making a pinging sound and reducing engine power. You may notice the mower struggles in thick grass it once handled easily.
Carburetor Damage And Corrosion
Ethanol-related water contamination leads to corroded fuel lines, a rusty fuel tank, and a clogged carburetor. Carburetor repair or replacement is one of the most common fixes for neglected mowers.
Increased Emissions And Residue Buildup
Improper combustion from bad fuel leaves more deposits on spark plugs, valves, and the combustion chamber. This makes the engine run dirtier and less efficiently over time.
Seasonal Fuel Management Tips
Proper fuel handling changes slightly with the seasons, especially when preparing your mower for storage.
Preparing Your Mower For Winter Storage
This is the most important fuel-related task of the year. Do not simply run the mower until it stops and leave old gas in the tank all winter.
- Add fuel stabilizer to a fresh tank of gas and run the engine for 5-10 minutes to circulate the treated fuel throughout the system.
- Option A (Recommended): After running the stabilizer through, turn off the fuel valve if your mower has one, and let the engine run until it stalls, emptying the carburetor bowl.
- Option B: Alternatively, you can use a siphon to remove all gasoline from the tank and then run the engine until it stops.
- Store the mower in a cool, dry place.
Spring Start-Up Fuel Procedure
When it’s time to mow again, your properly stored mower should start easily.
- If you left stabilized fuel in the tank over winter, it should be fine to use. Top it off with fresh gas if needed.
- If you drained the tank completely, simply fill it with your recommended fresh gasoline.
- If the mower was stored with untreated old gas, you must drain both the tank and the carburetor bowl before adding new fuel. Old gas will cause problems.
Frequently Asked Questions
Can I Use Premium Gas In My Lawn Mower?
You can, but it’s usually unnecessary and a waste of money. Unless your owner’s manual specifically recommends a higher octane, regular 87-octane fuel is perfect. Premium gas does not contain special cleaners or additives that benefit small engines in a meaningful way compared to regular fuel with a stabilizer.
What Happens If I Put Diesel In My Lawn Mower?
Putting diesel fuel in a gasoline mower is a serious mistake. Diesel will not ignite from a spark plug. The mower will not start, and the entire fuel system—tank, lines, carburetor—will need to be completely drained and flushed. If you accidentally add diesel, do not try to start the engine. Drain the tank and take the mower to a small engine repair shop for a thorough cleaning.
How Long Is Gas Good For In A Lawn Mower?
Unstabilized gasoline begins to degrade and lose volatility in as little as 30 days. Within 60-90 days, it can become “stale” and cause operational issues. Gasoline treated with a quality fuel stabilizer can remain usable for up to 24 months when stored properly in a sealed container. In a mower’s tank, it’s best to use fuel within one month.
Where Can I Find Ethanol-Free Gas For My Mower?
Ethanol-free gas (E0) is often sold at marinas, some small airports, and an increasing number of gas stations. Websites and mobile apps like “Pure Gas” can help you locate stations near you that sell ethanol-free fuel. It is typically more expensive, but for infrequent use or seasonal equipment, the cost is worth the protection it provides.
Is It Okay To Mix Old Gas With New Gas?
It is generally not recommended. While diluting a small amount of old gas with a large amount of fresh gas might work, you risk introducing moisture, varnish, and instability into your new fuel. For optimal performance and to avoid potential problems, it’s best to dispose of old gasoline properly at a hazardous waste facility and start completely fresh. Your mower’s engine will thank you for it.