For optimal performance and engine life, the choice of fuel for your mower matters. Knowing what gas to use in lawn mower engines is the first step to a healthy, reliable machine. Using the wrong type can lead to starting problems, poor running, and costly repairs. This guide will explain everything you need to know, from octane ratings to ethanol content, so you can fuel your mower with confidence.
What Gas To Use In Lawn Mower
Most modern four-stroke lawn mower engines are designed to run on regular 87 octane unleaded gasoline. This is the standard fuel you find at most gas stations. The critical factor is not usually the octane, but the ethanol content. You should aim to use gasoline with no more than 10% ethanol (E10), and ideally, use ethanol-free fuel if it’s available in your area.
Always check your owner’s manual first. It is the definitive source for your specific model’s requirements. Some high-performance or commercial mowers might have different recommendations. Following the manual ensures you honor the warranty and provide the best care for your engine.
Understanding Octane Ratings
Octane rating measures a fuel’s resistance to premature ignition, often called “knocking” or “pinging.” Higher octane fuel is more stable under pressure.
- 87 Octane (Regular): Perfectly suitable for almost all lawn mower engines. These engines have lower compression ratios than high-performance car engines, so they do not need higher octane fuel.
- Mid-Grade or Premium (89-93 Octane): Generally unnecessary and offers no benefit for a standard mower engine. It will not make your mower run better or more efficiently. Using it is simply a waste of money unless specified by your manual.
The Ethanol Problem In Small Engines
Ethanol is an alcohol-based fuel additive made from plant materials. While it works in modern cars, it causes significant issues in small engines like those in lawn mowers.
How Ethanol Damages Your Mower
Ethanol is hygroscopic, meaning it absorbs water from the air. This is the root of most problems.
- Phase Separation: The absorbed water causes the ethanol and gas to separate. The water-ethanol mixture sinks to the bottom of your fuel tank, where it gets sucked into the engine, causing it to stall or not start at all.
- Corrosion: Ethanol can corrode metal fuel lines, carburetor components, and fuel tanks, especially in older mowers.
- Degradation: It can break down rubber seals, gaskets, and plastic parts over time, leading to fuel leaks.
- Gum and Varnish: As ethanol-blended fuel evaporates, it leaves behind sticky residues that clog the tiny passages in the carburetor. This is the most common cause of hard starting and rough running.
Why Ethanol-Free Gasoline Is The Best Choice
For long-term engine health and easier maintenance, ethanol-free gasoline (often called “recreational fuel” or “E0”) is the superior choice. It is more stable during storage, does not attract moisture, and won’t leave gummy deposits. Many hardware stores, marinas, and some gas stations sell it. While it costs more per gallon, it can save you money on carburetor cleanings and part replacements.
Fuel For Two-Stroke Lawn Mowers
If you have an older or lightweight mower with a two-stroke engine, the fuel rules are different. These engines require a precise mix of gasoline and two-stroke oil.
- You must mix the oil and gas in a separate fuel can, not in the mower’s tank.
- The mixing ratio is critical (e.g., 50:1 or 40:1). Check your mower’s manual for the exact ratio.
- Use the same fuel quality guidelines: 87 octane and, ideally, ethanol-free gas.
- Pre-mixed fuel from the store is a convenient, though expensive, option that ensures the correct ratio and often uses ethanol-free gas.
Step-By-Step Guide To Fueling Your Lawn Mower
Doing it the right way protects your engine. Here is a simple, safe procedure.
1. Check The Fuel Level And Condition
Before adding new gas, look in the tank. If the old fuel is more than 30 days old or looks cloudy, you should drain it. Stale fuel is a leading cause of starting issues.
2. Use A Clean, Approved Fuel Container
Always use a plastic fuel can rated for gasoline and marked with the type of fuel (e.g., “E10” or “Ethanol-Free”). A dedicated can prevents contamination. A funnel with a fine mesh screen can help keep out debris.
3. Fill The Tank Carefully
Do not overfill. Leave about an inch of space at the top of the tank to allow for fuel expansion, especially on hot days. Wipe up any spills immediately to prevent fire hazards and damage to your mower’s paint.
4. Consider A Fuel Stabilizer
If you don’t use ethanol-free gas, or if you plan to store your mower for more than a month, add a fuel stabilizer. Pour it into the gas can before filling it, then shake gently to mix. This helps prevent the fuel from breaking down and forming gums during storage.
Storing Your Lawn Mower With Fuel In The Tank
Proper storage is crucial, especially over the winter. There are two schools of thought, but one is generally more reliable.
The Fuel Stabilizer Method (Recommended)
This is the most common and effective approach for seasonal storage.
- Add fuel stabilizer to a nearly full tank of fresh gas. Run the engine for 5-10 minutes to circulate the treated fuel through the entire carburetor system.
- Turn off the fuel valve if your mower has one.
- Let the engine cool, then store the mower in a dry place.
The Dry Storage Method
Some prefer to run the engine until all fuel is gone from the tank and carburetor. While this eliminates old fuel problems, it can leave seals dry and prone to cracking, and the empty tank can rust from inside from condensation. If you choose this method, adding a few drops of oil into the spark plug hole after draining can help protect the cylinder.
Troubleshooting Common Fuel-Related Problems
If your mower is having issues, fuel is often the culprit.
Lawn Mower Won’t Start
First, check for fresh fuel. If the gas is old, drain the tank and carburetor bowl. Refill with fresh, correct fuel. A clogged carburetor jet from old, gummy fuel is a very common reason for no-start conditions.
Engine Runs Rough Or Sputters
This often indicates water in the fuel or a partially clogged carburetor. Drain the fuel system and refill. You may need to clean the carburetor if the problem persists after using fresh gas.
Loss Of Power During Mowing
Check your fuel cap. Some caps have a small vent that can become clogged, creating a vacuum in the tank that prevents fuel from flowing properly. Also, ensure you are using the correct fuel type and that it is not contaminated.
Frequently Asked Questions
Can I Use Premium Gas In My Lawn Mower?
You can, but it is not necessary and provides no advantage for a standard engine. Unless your owner’s manual specifically recommends premium gasoline, using regular 87 octane is perfectly fine and will save you money.
What Happens If I Put Diesel In My Lawn Mower?
Do not start the engine. Diesel will not vaporize like gasoline and will severely clog the fuel system. You must completely drain the entire fuel system, including the tank, fuel lines, and carburetor. You will likely need to replace the fuel filter and clean or rebuild the carburetor.
How Long Does Gas Last In A Lawn Mower?
Untreated gasoline with ethanol begins to degrade in as little as 30 days. It can cause starting and performance problems. Fuel treated with a stabilizer can last up to a year. Ethanol-free gas has a much longer shelf life, often remaining usable for over a year without treatment.
Is It Ok To Use Old Gas In A Lawn Mower?
No, it is not recommended. Old gas loses its volatility, meaning it won’t ignite properly. More importantly, as it evaporates and breaks down, it leaves behind sticky varnish that clogs the carburetor. It’s best to dispose of old gas properly and use fresh fuel.
Where Can I Find Ethanol-Free Gas For My Mower?
Websites and apps like “Pure Gas” can help you locate stations that sell it. It is often found at marinas, small airports, and some farm supply or hardware stores. While it costs more, the benifits for your small engine equipment are significant.
Choosing the right fuel for your lawn mower is a simple task that has a major impact. Sticking to fresh, 87-octane gasoline with minimal ethanol, or ideally using ethanol-free fuel, will keep your mower starting easily and running smoothly for seasons to come. Always consolt your owner’s manual for the final word, and remember that proper fueling and storage are the keys to avoiding most common lawn mower problems. Taking these steps ensures your mower is ready to work whenever you need it.