Intro
Hey, welcome back to Backyard Engine Pro. If you’ve been dealing with repeated carburetor clogs, starting problems, or deteriorating fuel lines on your lawn mower, chainsaw, or generator, ethanol-blended gasoline may be doing more damage than you realize. Most people fill up at the pump without thinking twice about what’s in the fuel, but for small engines, ethanol content matters a lot more than it does for your car.
The good news? Once you understand what ethanol actually does inside a small engine, it’s easy to manage and prevent the damage. Let’s get into it.
Quick Overview
- Ethanol attracts moisture from the surrounding air
- That moisture causes corrosion inside the fuel system
- Ethanol-blended fuel leaves stickier deposits when it degrades
- Ethanol degrades rubber and plastic fuel system components over time
What Is Ethanol Gas?
Ethanol is an alcohol derived from plant materials, most commonly corn in the United States, that is blended into gasoline as a fuel extender and emissions reducer. The most common blend at the pump is E10, which contains 10 percent ethanol and 90 percent gasoline. E15, containing 15 percent ethanol, is increasingly available but is not approved for use in most small engines, outdoor power equipment, or boats regardless of what the pump label says.
Modern automobile engines are designed and calibrated to run on ethanol blended fuel without issue. Small engines in lawn mowers, chainsaws, generators, and pressure washers are a different story entirely.
Why Ethanol Damages Small Engines
1. Ethanol Absorbs Moisture
Ethanol is hygroscopic, which means it actively pulls water vapor out of the surrounding air and into the fuel. In a vehicle that gets driven daily, this isn’t a significant problem because the fuel is consumed quickly. In a lawn mower or generator that might sit for weeks or months between uses, moisture accumulates steadily in the fuel tank and fuel system.
Once enough moisture has been absorbed, a process called phase separation occurs. The ethanol and water mixture separates from the gasoline and sinks to the bottom of the tank as a distinct layer. When the engine draws fuel from the bottom of the tank, it pulls this ethanol-water mixture rather than gasoline. The result is an engine that won’t run, runs extremely rough, or won’t start at all despite having a full tank of fuel.
What this looks like:
- The engine cranks but won’t fire despite adequate fuel in the tank
- The engine runs briefly and then dies as if running out of fuel
- Performance is intermittent and unpredictable
2. Ethanol Causes Corrosion
The water that ethanol introduces into the fuel system doesn’t just affect combustion. It also accelerates corrosion of metal components throughout the fuel system. Aluminum and zinc are commonly used in small engine carburetors, and both are susceptible to corrosion from ethanol and water exposure. Over time, corrosion pits and roughens the smooth surfaces inside the carb that fuel passes through, makes jets harder to clean, and eventually causes components to fail entirely.
What this looks like:
- Carburetor jets and passages that are corroded and pitted rather than smooth
- Fuel system fittings that are difficult to seal properly after cleaning
- Accelerated wear on fuel system components compared to engines running ethanol-free fuel
3. Ethanol Leaves Stickier Deposits
All gasoline leaves some residue when it degrades, but ethanol-blended fuel leaves behind deposits that are significantly stickier and harder to dissolve than those from pure gasoline. When ethanol-blended fuel evaporates in the carburetor during storage, the residue it leaves behind can harden into a varnish that requires overnight soaking in carburetor cleaner to dissolve, and in severe cases can’t be cleaned out at all.
This is the primary reason carburetor clogs are so much more common and more severe than they were before ethanol-blended fuel became the standard at the pump.
What this looks like:
- Carburetor jets and passages clogged with a thick, gummy brown or amber residue
- Performance that degrades rapidly after a cleaning when old ethanol-blended fuel is put back in
- Carburetors that need cleaning multiple times per season despite proper maintenance
4. Ethanol Degrades Rubber and Plastic Components
Ethanol is a solvent, and over time it attacks the rubber and plastic components in the fuel system. Fuel lines, primer bulbs, carburetor gaskets, and float bowl O-rings that were designed before ethanol-blended fuel became widespread are particularly vulnerable. Ethanol causes rubber to swell, harden, and crack, and causes some older plastic components to become brittle.
What this looks like:
- Fuel lines that are cracked, hard, or brittle when they should be flexible
- Primer bulbs that crack or become stiff and unresponsive
- Carburetor gaskets that leak or crumble during disassembly
- Increased frequency of fuel system component replacement compared to older equipment running pure gasoline
Signs Ethanol Is Causing Problems
If you’re seeing any combination of these symptoms, ethanol-related damage is worth investigating:
- Hard starting or a no-start situation despite fresh fuel and a good spark plug
- Engine stalls repeatedly, especially under load
- Rough, uneven running or surging
- Carburetor that keeps getting clogged shortly after cleaning
- Visible cracks or deterioration on fuel lines and primer bulbs
- Fuel that smells sour or shows visible phase separation in the tank
How to Prevent Ethanol Damage
1. Use Ethanol-Free Gas
This is the most effective prevention available. Ethanol-free gasoline eliminates all of the moisture absorption, phase separation, corrosion, and aggressive deposit formation that ethanol-blended fuel causes. It also stays fresh significantly longer, which is especially important for equipment that sits between uses.
Ethanol-free fuel is available at marinas, outdoor power equipment dealers, some hardware stores, and an increasing number of gas stations. It costs more per gallon than regular pump gas, typically 30 to 70 cents more, but the reduction in carburetor cleaning, fuel system maintenance, and component replacement more than makes up for the difference over a season of use.
2. Use a Quality Fuel Stabilizer
If ethanol-free fuel isn’t available or practical in your situation, a quality fuel stabilizer added to every tank dramatically reduces ethanol-related problems. Stabilizer slows fuel degradation, reduces moisture absorption, and helps keep fuel system components cleaner during storage.
Products like Sta-Bil, Star Tron, and Sea Foam are popular and effective options. Add stabilizer at the dose recommended on the label every time you fill up, not just when storing, for best results. Run the engine for several minutes after adding stabilizer to circulate treated fuel through the carburetor and fuel lines.
3. Don’t Let Fuel Sit
The longer ethanol-blended fuel sits in a tank, the more moisture it absorbs and the more it degrades. Using fresh fuel within 30 days and never letting ethanol-blended gas sit for more than 60 days without treatment eliminates the majority of ethanol-related problems that occur in stored equipment.
If you know the mower will sit for more than two weeks between uses, treat the fuel with stabilizer when you fill up rather than waiting until the end of the season.
4. Drain Fuel Before Long-Term Storage
For seasonal storage of 60 days or more, draining the fuel system entirely is the most reliable protection against ethanol damage. Run the engine until it dies from fuel starvation to empty the carburetor bowl and fuel lines, then drain the tank completely.
A dry fuel system cannot suffer phase separation, cannot corrode from moisture accumulation, and cannot form varnish deposits during storage. This is particularly important for chainsaws, generators, and pressure washers that may sit for six months or more between uses.
5. Maintain the Fuel System Proactively
Even with good fuel practices, periodic maintenance keeps the fuel system in top condition:
- Replace fuel lines every few years or whenever they show signs of brittleness, cracking, or hardening
- Replace the inline fuel filter at the start of every season
- Inspect the primer bulb for cracking or stiffness and replace it at the first sign of deterioration
- Clean the carburetor at the start of each season if the equipment was stored with ethanol-blended fuel
Is Ethanol Always Bad?
Not necessarily. Millions of mowers run on E10 fuel every season without major problems, and the key difference is maintenance habits. Ethanol-blended fuel that’s kept fresh, treated with stabilizer, and not left sitting for extended periods causes far fewer problems than old, untreated ethanol-blended fuel that’s been sitting in a tank all winter. The problems show up most severely when ethanol-blended fuel is left to sit and degrade.
The difference between an owner who never has carburetor problems and one who cleans the carb every spring often comes down to one habit: treating fuel with stabilizer and draining or replacing it before storage.
Common Mistakes to Avoid
- Leaving ethanol-blended fuel in the tank over winter without treatment, which virtually guarantees carburetor problems the following spring
- Using E15 or higher ethanol blends in equipment designed for E10 or less. E15 is not approved for small engines and accelerates all of the damage described above
- Assuming the mower is broken when it won’t start after sitting with old ethanol-blended fuel, and buying parts before addressing the fuel
- Skipping stabilizer because the equipment will “only be sitting for a month or two”
Pro Tip
If you switch to ethanol-free gasoline and use it consistently, you’ll see a dramatic reduction in carburetor problems, starting issues, and fuel system maintenance over the life of your equipment. The difference is noticeable within a single season for most owners. It’s the single most impactful fuel-related change you can make, and for equipment that sits for any significant period between uses, it’s absolutely worth the extra cost per gallon.
Final Thoughts
Ethanol gas isn’t going away, and for most people it’s the most practical fuel available at the pump. But understanding what it does inside a small engine gives you the knowledge to manage it effectively. Use fresh fuel, add stabilizer, drain before storage, and switch to ethanol-free when you can. Do those things consistently and ethanol-related damage becomes a much smaller part of your maintenance routine.
Now go take care of that fuel system. You’ve got this.