The mower starts on the first pull in April, then sits through a busy month, and the next time you need it the engine coughs, sputters, or refuses to start. In many cases, the engine itself isn't the problem—the fuel is.
Quick Answer
Most small four-cycle engines, including lawn mowers, generators, and pressure washers, run best on clean, fresh gasoline with a minimum octane rating of 87 and no more than 10% ethanol (E10). Two-cycle equipment needs that same gas pre-mixed with the oil ratio the manufacturer prints on the engine. Fresh matters more than grade.
Table of Contents
Small engines and powersports equipment are much more sensitive to fuel quality than the average passenger car. They often sit unused for weeks or months, many rely on carburetors with extremely small fuel passages, and most are exposed to today's ethanol-blended gasoline. Those conditions make stale fuel, moisture, and deposits far more likely to cause starting and drivability problems.
This guide explains what gasoline to use in lawn mowers, generators, chainsaws, motorcycles, ATVs, and other gasoline-powered equipment, why fuel deteriorates during storage, how to recognize stale or contaminated fuel, and what you can do to keep fuel fresh between uses.
Use clean, fresh unleaded gasoline with a minimum octane rating of 87, and keep the ethanol content at 10% or below. Briggs & Stratton, one of the largest small-engine makers, recommends the same 87-octane fuel you would put in your car, provided it is fresh. Ethanol-free gas is better still for equipment that sits between uses.
Octane is a measure of a fuel's resistance to knock, not a measure of power or cleanliness. A higher number does not make a mower run better, and paying for premium gains you nothing unless the engine's manual specifically calls for it. What a small engine cares about is freshness and water content, both of which get worse the longer fuel sits and the more ethanol it carries.
Ethanol matters because it is the single biggest variable at the pump. Most finished motor gasoline sold in the United States is E10, roughly 10% ethanol by volume, according to the U.S. Energy Information Administration. E10 is approved for small engines. The trouble starts above that: higher blends like E15 are not approved for this equipment, which we'll get to below.
Bell Performance has worked on gasoline and ethanol problems since 1927, and the pattern we see in failed small engines is almost always the same story of stale fuel and absorbed water rather than a mechanical fault.
Gasoline can begin to deteriorate surprisingly quickly during storage. Briggs & Stratton notes that fuel may begin degrading in as little as 30 days under some conditions. As gasoline oxidizes, its lighter components gradually evaporate and sticky gums and varnishes begin to form. At the same time, ethanol-blended gasoline can absorb moisture from the surrounding air, making long-term storage even more challenging.
In equipment that's used every day, this process usually isn't noticeable. In a mower, generator, motorcycle, or chainsaw that may sit for weeks or months, however, those changes can create hard starting, rough running, clogged carburetor passages, and other fuel-related problems.
Bad fuel shows up as hard starting, rough or surging idle, stalling under load, or an engine that won't start at all after sitting. A varnish-like smell from the tank and a darker, cloudy color are giveaways. On two-cycle gear, bad gas often looks like a fouled plug or a bog when you hit the throttle. Much of this is the same way stale gas affects performance in a car, only faster.
The clearest tell is timing. If the equipment ran fine when you put it away and refuses to cooperate after weeks or months of storage, suspect the fuel before you suspect the engine. Pull the spark plug and check whether it's wet with fuel, which tells you gas is reaching the cylinder. If it is, the problem is usually quality, not delivery.
Water in the tank has its own signature. An engine that starts, runs for a few seconds, then dies as it pulls the bottom layer of the tank is often drinking the water-ethanol mix from phase separation, one of the classic water in the tank symptoms. This is exactly the situation people run into when they wonder whether they can use last year's gas in a generator that sat untreated through a humid summer.
Bell publishes a fuel health checklist and a set of symptom guides for exactly these situations, because catching bad fuel early is far cheaper than pulling a carburetor apart.
Ethanol itself isn't the problem—storage is. Fresh E10 gasoline is approved by most manufacturers of lawn mowers, generators, chainsaws, and similar outdoor power equipment. The difficulties arise because ethanol naturally absorbs atmospheric moisture, making fuel that sits unused more susceptible to water contamination and, under the right conditions, phase separation.
Higher ethanol blends are a different matter. E15 is approved for many passenger vehicles but generally is not approved for lawn mowers, generators, motorcycles, boats, chainsaws, or similar equipment. Using fuel with more ethanol than the manufacturer specifies can lead to drivability problems and may affect warranty coverage.
The confusion is understandable, because E15 pumps sit right next to E10 at many stations and the price is often a few cents lower. For a car built after 2001, that trade can make sense. For the mower in your garage, it doesn't, since the higher alcohol content carries more of the water-absorbing, seal-swelling behavior that small engines tolerate poorly. The same caution applies to two-wheelers, which is why the difference between ethanol gas for motorcycles and straight gasoline matters so much.
Even standard E10 asks for a little care. The ethanol that helps a modern car's emissions is the same ethanol that pulls moisture into a tank that sits for weeks. That's why finding ethanol-free gas, where you can, is the easier choice for stored equipment, and why a treatment matters when ethanol-free isn't available.
Ethanol Defense was formulated specifically for this: it packs professional-strength water absorbers to control the moisture ethanol pulls in, and it does the job without adding more alcohol to the fuel.
Buy fuel in quantities you'll use within about a month, store it in a sealed can out of temperature swings, and treat every tank with a stabilizing fuel additive if the equipment will sit. A treatment like Mix-I-Go or Ethanol Defense keeps the gasoline stable, disperses absorbed water, and cleans the deposits that stale fuel leaves behind, so the engine starts when you need it.
The best time to protect gasoline is before storage begins. Adding a quality fuel treatment to fresh gasoline slows oxidation, helps manage the moisture challenges associated with ethanol-blended fuel, and reduces the deposits that can form during storage. For seasonal equipment, it's also a good idea to run the treated fuel through the engine for several minutes before putting it away so the carburetor and fuel system are protected as well.
Fuel treatments work best as a preventive measure. While they can significantly extend the useful life of fresh gasoline, they cannot fully restore fuel that has already severely deteriorated or undergone phase separation. Treating gasoline when it's fresh is far more effective than trying to rescue it after months of storage.
Here is how Bell's two main gasoline products map to small-engine and powersports use.
| Application | Product | Treat rate | Notes |
|---|---|---|---|
| Gasoline mowers, generators, 4-cycle equipment | Mix-I-Go | 1 oz treats up to 10 gal | All-purpose gasoline treatment; deposit control and stabilization |
| Ethanol-blend or stored gas, phase-separation risk | Ethanol Defense | 1 oz treats up to 10 gal | Professional-strength water absorbers; alcohol-free |
| 2-cycle handheld and small engines | Mix-I-Go Small Engine Formula | 1 oz treats up to 2 gal | Made for 2-cycle mix and small-engine use |
| Marine and 2-cycle watercraft | Marine MXO | 16 oz treats up to 125 gal | Marine gasoline treatment; ethanol and water protection |
Both Ethanol Defense and Mix-I-Go stabilize ethanol blends and help extend the fuel's useful life, which is the whole point of treating gas that has to sit.
Two-cycle engines run on gasoline pre-mixed with two-cycle oil at the exact ratio stamped on the engine or in the manual, commonly 40:1 or 50:1. Motorcycles, ATVs, and boats take fresh 87-octane or higher per the owner's manual, and all of them are more sensitive to ethanol and stale fuel than a daily-driven car.
For string trimmers, chainsaws, leaf blowers, and older outboards, the mix ratio is not optional. Too little oil starves the engine of lubrication and can seize it; too much oils the plug and smokes. Mix only what you'll burn in a few weeks, because pre-mixed two-cycle fuel goes stale on the same clock as everything else, and the oil separates over time.
Powersports engines add their own wrinkle. Motorcycles and personal watercraft often sit for months between rides, and marine tanks are vented to humid air, which makes phase separation more likely on the water than on land. This is why boaters see fuel trouble so often, and why checking whether your boat's fuel is ready before the season matters, and why marine fuel gets its own treatment. A fresh tank, the right octane, and a stabilizer built for ethanol handle the vast majority of it.
Bell makes Mix-I-Go Small Engine Formula for two-cycle and small-engine gas and Marine MXO for watercraft, both built around the same ethanol and water protection that gasoline equipment needs.
For most lawn mowers, generators, pressure washers, and similar equipment, premium gasoline provides no meaningful performance benefit. These engines are designed to operate on regular 87-octane gasoline, and using higher-octane fuel will not increase power, improve fuel economy, or make the engine cleaner.
The exception is equipment whose manufacturer specifically requires higher octane fuel. Certain high-performance motorcycles, powersports vehicles, and specialty engines are designed for premium gasoline and should always be fueled according to the owner's manual.
For the average homeowner, spending money on fresh fuel and proper fuel stabilization will do far more to improve reliability than buying premium gasoline.
Fresh unleaded gasoline with a minimum octane rating of 87 and no more than 10% ethanol (E10). That's the same regular-grade gas most people use in their cars, as long as it's fresh. Ethanol-free gas is a good upgrade for a mower that sits between uses. There's no benefit to premium unless the engine's manual specifically requires it.
Untreated gasoline can start to degrade in as little as 30 days, according to Briggs & Stratton, and ethanol blends spoil faster because they absorb water from the air. A stabilizing fuel treatment such as Mix-I-Go or Ethanol Defense extends that window considerably, keeping stored fuel usable through an off-season.
No. The EPA approves E15 only for model-year 2001 and newer cars and light trucks. It is not approved for small engines, generators, motorcycles, or boats, and using it can void the warranty. Stick with E10 or ethanol-free gas for this equipment.
The most likely cause is stale fuel or water in the tank, not a mechanical failure. Oxidized gas leaves gum in the carburetor, and ethanol that has absorbed water can separate and settle in the fuel bowl, clogging the jets. Draining the old fuel, cleaning the carburetor, and refilling with fresh, treated gas usually fixes it.
Standard E10 is fine when the fuel is fresh and treated. The problem is what ethanol does over time in equipment that sits: it pulls moisture into the tank and can trigger phase separation, which Bell Performance data shows can cut a fuel's useful life by up to 75%. A water-controlling treatment and fresh fuel keep E10 from becoming a problem.
If you burn through a tank every week or two, fresh fuel alone will usually keep you running. Treatment becomes important the moment equipment starts sitting: before winter storage, between mowing seasons, or for a generator that waits for the next outage. Treating fresh fuel is far easier than reviving a gummed-up engine.
Fresh gasoline, proper storage, and preventive fuel treatment are the three biggest factors in keeping small engines and powersports equipment reliable. Most manufacturers recommend fresh 87-octane gasoline containing no more than 10% ethanol (E10), and treating fuel before storage can help reduce oxidation, minimize deposit formation, and address the moisture-related challenges associated with ethanol-blended fuel.
Mix-I-Go is designed to help stabilize gasoline, reduce deposit formation, and keep fuel fresh during storage, while Ethanol Defense is formulated specifically to help manage the moisture issues associated with ethanol-blended gasoline. Using treated fuel before equipment is stored helps improve the chances that your mower, generator, chainsaw, motorcycle, or other gasoline-powered equipment will be ready to start when you need it.