When you need to fill up your machine, knowing what gas goes in a lawn mower is the first step to keeping it running smoothly for years. Most modern lawn mower engines are designed to run efficiently on regular unleaded gasoline with an octane rating of 87. Using the wrong fuel can lead to poor performance, starting issues, and even costly damage. This guide will walk you through everything you need to know, from fuel types and ethanol concerns to proper storage and troubleshooting.
What Gas Goes In A Lawn Mower
The standard answer is straightforward. For the vast majority of four-stroke walk-behind and riding lawn mowers, the correct fuel is fresh, regular unleaded gasoline with a minimum octane rating of 87. This is the same fuel you’d typically put in your car. The key considerations are the octane rating, the presence of ethanol, and the freshness of the fuel itself.
Two-stroke engines, commonly found in older mowers, string trimmers, and leaf blowers, require a special mix. You must combine gasoline with a specific ratio of two-stroke engine oil. Never put straight gasoline into a two-stroke engine, as it will not be properly lubricated and will fail quickly. Always consult your owner’s manual for the exact fuel specifications for your model.
Understanding Octane Ratings For Small Engines
Octane rating measures a fuel’s resistance to premature ignition, often called “knocking” or “pinging.” Using a higher octane fuel than recommended provides no benefit for small engines and is simply a waste of money. These engines are not built for high compression like some performance cars.
Stick with 87 octane. Using a lower octane fuel than recommended can cause knocking, which harms engine components over time. If you hear a pinging sound from your mower under load, it could be a sign of using fuel with too low an octane rating, though carbon buildup can also cause this issue.
The Ethanol Problem In Lawn Mower Gas
Most gasoline sold today contains up to 10% ethanol, labeled as E10. While fine for modern cars, ethanol can be problematic for small engines. Ethanol is hygroscopic, meaning it absorbs water from the air. This water can separate from the gasoline inside your mower’s fuel tank, leading to corrosion, clogged carburetor jets, and difficult starting.
For optimal lawn mower health, use ethanol-free gasoline if it’s available in your area. Many hardware and outdoor power equipment stores sell it in pre-mixed cans. If E10 is your only option, you must be diligent about fuel management. Never leave ethanol-blended fuel sitting in your mower’s tank for more than 30 days.
How To Find Ethanol-Free Gasoline
Locating ethanol-free fuel is easier than you might think. Websites and mobile apps like Pure-Gas.org can help you find stations near you that offer it. Many marinas also sell it for boat engines, which face similar issues. While often more expensive per gallon, the protection it offers your equipment can save you significant repair costs.
Gasoline For Two-Stroke Vs. Four-Stroke Engines
It’s crucial to identify your engine type. A four-stroke engine has a separate oil reservoir, usually with a dipstick for checking. A two-stroke engine requires you to mix oil directly into the gasoline.
Fueling A Four-Stroke Lawn Mower
- Use fresh, unleaded 87 octane gasoline.
- Consider ethanol-free fuel for long-term engine health.
- Add a fuel stabilizer if you plan to store the mower for more than a month.
- Always fill in a well-ventilated area and avoid overfilling the tank.
Fueling A Two-Stroke Lawn Mower
- Confirm your equipment’s required gas-to-oil ratio (common ratios are 50:1 or 40:1).
- Use a clean, approved mixing container. Never mix directly in the mower’s tank.
- Add a portion of the gasoline first, then the full amount of two-stroke oil.
- Shake the container thoroughly to ensure the oil is completely emulsified.
- Add the remaining gasoline and shake again before fueling your mower.
Choosing The Right Fuel Additives And Stabilizers
Fuel additives can be a valuable tool for lawn mower maintenance. A quality fuel stabilizer is the most important one you can use. It works by slowing the oxidation process that makes gas go stale and helps prevent the negative effects of ethanol.
When And How To Use A Fuel Stabilizer
You should add a fuel stabilizer to every gallon of gasoline you purchase for your small engines. Add it to the gas can first, then fill it with fuel. This ensures it mixes thoroughly. This practice is essential for seasonal equipment. Before storing your mower for the winter, run the engine for a few minutes after adding treated fuel. This circulates the stabilized gas through the carburetor and fuel lines, protecting the entire system.
Carburetor And Fuel System Cleaners
For older mowers or engines that have been sitting, a fuel system cleaner can help dissolve varnish and gum deposits left by old gasoline. These are typically poured directly into the gas tank. They are a good maintenance step but are not a substitute for proper mechanical cleaning if the carburetor is severely clogged.
Step-By-Step Guide To Fueling Your Lawn Mower
Following the correct steps ensures safety and prevents spills that can damage your mower and lawn.
- Let the engine cool completely. Never add fuel to a hot or running engine.
- Move the mower to a flat, well-ventilated area away from any ignition sources.
- Clean any debris from around the fuel cap before opening it.
- Use a funnel with a fine mesh screen to prevent dirt from entering the tank.
- Fill the tank slowly, leaving a little space at the top to allow for expansion. Do not overfill.
- Wipe up any spills immediately with a rag. Dispose of the rag safely.
- Securely tighten the fuel cap before starting the engine.
Proper Gasoline Storage And Management
Bad gas is a leading cause of lawn mower problems. Gasoline begins to degrade in as little as 30 days, losing volatility and forming gums and varnishes.
Best Practices For Storing Gasoline
- Store fuel in an approved, sealed safety container made of metal or high-density plastic.
- Label the container with the date of purchase. Use older fuel first.
- Keep containers in a cool, dry, well-ventilated place out of direct sunlight and away from living spaces.
- Aim to use gasoline within 30 days, especially if it contains ethanol.
Preparing Your Mower For Long-Term Storage
Proper “winterization” is critical. Either run the engine until the tank is completely empty, or fill the tank to the top with fresh, stabilized gasoline. A full tank leaves less room for air and moisture to condense. Run the engine for five minutes to circulate the treated fuel, then shut off the fuel valve if your mower has one. Finally, change the oil and clean the mower before storing it.
Troubleshooting Common Fuel-Related Problems
Many mower issues stem from fuel. Here’s how to diagnose them.
Lawn Mower Won’t Start Or Runs Poorly
Old or contaminated fuel is the most likely culprit. Drain the old gas from the tank and the carburetor bowl. Inspect the fuel line for cracks and check the fuel filter if equipped. Replace with fresh, stabilized gasoline. If the problem persists, the carburetor jets may be clogged and require cleaning.
Engine Surges Or Sputters
This often indicates the engine is getting too much or too little fuel. Check for a dirty air filter first, as a restricted airflow can cause a rich fuel mixture. Next, check for water in the fuel or a partially clogged fuel line. Sometimes, a loose or damaged governor linkage can also cause surging.
Frequently Asked Questions
Can I Use Premium Gas In My Lawn Mower?
You can, but it is unnecessary and more expensive. Premium gas (often 91 or 93 octane) offers no performance or cleanliness benefits for a standard lawn mower engine designed for 87 octane. Save your money and use regular, unless your owner’s manual specifically calls for a higher rating.
What Happens If I Put Diesel In My Lawn Mower?
This is a serious mistake. Diesel fuel will not ignite in a gasoline engine’s combustion chamber. If you realize the error before starting, do not attempt to start the mower. You must completely drain the diesel from the tank, fuel lines, and carburetor. You will likely need to replace the fuel filter and spark plug. If you started the engine, significant internal damage may have occured, requiring professional repair.
How Long Can Gas Sit In A Lawn Mower?
Unstabilized gasoline with ethanol can begin to cause problems in less than a month. Stabilized gasoline can last in the tank for up to a year, but for best results, try not to let any gas sit for more than one season. Always use fresh fuel at the start of a new mowing season.
Is It Ok To Use Old Gas In A Lawn Mower?
No, using old gas is not recommended. Degraded fuel loses its combustibility and leaves sticky residues. It is the leading cause of hard starting, rough running, and carburetor clogs. If you have old gas, dispose of it properly at a hazardous waste facility and start with a fresh tank.
What Type Of Gas Goes In A Riding Lawn Mower?
The same rules apply. Most riding mowers use four-stroke engines that require fresh, unleaded 87 octane gasoline. Always check your manual, as some high-compression models may recommend a higher octane. Due to their larger tanks, using a fuel stabilizer is even more critical for riding mowers to prevent issues from stale fuel over time.