Intro
Hey, welcome back to Backyard Engine Pro. Every spring, the same scenario plays out in garages and sheds across the country. Someone pulls their lawn mower out of winter storage, gives it a few pulls, and it either won’t start or runs terribly. In most of those cases, the culprit is the same thing: gas that was left in the mower last fall without any treatment.
Understanding what actually happens to that fuel over the winter helps explain why spring starting problems are so predictable and so preventable. Let’s walk through it.
Quick Answer
Leaving gas in your lawn mower over winter almost always leads to at least one of these problems when spring comes around:
- A clogged carburetor from varnish deposits
- Hard starting or a complete no-start situation
- Fuel system corrosion from ethanol and moisture
- Degraded fuel lines and rubber components
- Poor engine performance even if it does start
Why Gas Goes Bad Over Winter
Gasoline is a complex mixture of hydrocarbons that begins to degrade as soon as it’s refined. The volatile compounds that make gasoline easy to ignite start evaporating within 30 days, and the heavier compounds left behind don’t burn as cleanly or reliably. Over a winter storage period of four to six months, fuel that was perfectly fine when the mower was put away in October can become a thick, varnish-producing problem by March.
Ethanol-blended fuel, which is most pump gas, makes this significantly worse. Ethanol actively absorbs moisture from the air throughout the storage period, and that moisture eventually causes phase separation, where the ethanol-water mixture separates from the gasoline and sinks to the bottom of the tank as a distinct contaminated layer.
What Happens Inside Your Mower
1. The Carburetor Gets Clogged
This is the most common and most impactful consequence of leaving old fuel in a mower over winter. As the gasoline in the carburetor bowl evaporates slowly during storage, it leaves behind a sticky, gummy residue that coats every surface it contacts inside the carb. The tiny jets and passages that meter fuel flow are particularly vulnerable because even a small amount of varnish deposit is enough to restrict or block them entirely.
By spring, what was a clean carburetor in October may have jets that are completely blocked and passages so coated with varnish that spray-through cleaning won’t touch them. The engine cranks and cranks but can’t get fuel through.
What you’ll see in spring:
- Engine won’t start despite fresh fuel in the tank
- Engine fires briefly and then dies
- Carburetor requires removal and overnight soaking to clean properly
2. Fuel Separates and Corrosion Forms
If the mower was stored with ethanol-blended fuel, moisture absorption during storage leads to phase separation inside the tank. The ethanol-water mixture that settles to the bottom is what the engine draws from first, and it won’t support combustion. Beyond the starting problem, the water introduced by ethanol accelerates corrosion of aluminum and zinc components throughout the carburetor and fuel system.
Over a single winter, corrosion can pit the smooth surfaces inside the carburetor that fuel passes through, making future clogs more likely and thorough cleaning more difficult.
What you’ll see in spring:
- Fuel that looks cloudy or has a distinct layer at the bottom of the tank
- Carburetor components that are corroded or pitted during disassembly
- Starting problems that persist even after draining the old fuel
3. Fuel Lines Degrade
Old gasoline, particularly ethanol-blended fuel, continues to attack rubber fuel line material during storage. Fuel lines that were supple and flexible in October may be noticeably harder, stiffer, or showing early cracks by spring. In more severe cases, the interior of the fuel line degrades and sheds particles that travel directly into the carburetor and contribute to clogging.
What you’ll see in spring:
- Fuel lines that are stiff, hard, or cracked when you go to start the mower
- Fuel leaks at hose connections that were fine when the mower was put away
- Debris visible in drained fuel that came from deteriorated hose material
4. Hard Starting or No Start in Spring
Even if the visible symptoms above aren’t obvious, old winter fuel simply doesn’t ignite as reliably as fresh gasoline. The volatile compounds that make cold starting easy have evaporated over the winter months, and what’s left is harder for the engine to ignite during cranking. The mower may start eventually after many pull attempts, or it may not start at all until the old fuel is replaced.
What you’ll see in spring:
- The mower that used to start on the first or second pull now needs eight or ten
- The engine fires briefly on starter fluid but won’t sustain running on tank fuel
- Starting improves significantly after draining old fuel and refilling with fresh gas
5. Poor Engine Performance Even If It Starts
In some cases the mower does start in spring, but doesn’t run well. Old fuel that partially vaporized over winter doesn’t combust efficiently, and varnish deposits in the carburetor that haven’t fully blocked the passages can still restrict fuel flow enough to cause rough running, surging, and loss of power under load.
What you’ll see in spring:
- The engine runs rough, surges, or hunts instead of running smoothly
- Power drops under load even though the engine stays running
- Performance that gradually improves as old fuel is consumed and replaced
How to Fix It If You Already Left Gas In
If you’re reading this in spring and the damage is already done, here’s the fastest path to a running mower:
Step 1: Drain the Old Fuel Remove all the old gasoline from the tank completely. Don’t add fresh gas on top of old fuel hoping to dilute the problem. Drain it fully and start fresh.
Step 2: Clean the Carburetor Drain the carburetor bowl by removing the bowl bolt and letting the old fuel run out. Spray carb cleaner through all jets and passages. If the varnish buildup is significant, remove the carb and soak the bowl and jets overnight in fresh carb cleaner before clearing passages with a cleaning needle.
Step 3: Refuel with Fresh Gas Once the system is clean and drained, refill with fresh gasoline. Ethanol-free if available in your area for the best results going forward.
Follow our carburetor cleaning guide for help
How to Prevent It Next Fall
A few minutes of prep at the end of the mowing season saves hours of repair the following spring. Here’s what to do before putting the mower away:
Option 1: Use Fuel Stabilizer Add a quality fuel stabilizer like Sta-Bil to the tank before the last use of the season. Run the engine for 10 minutes after adding it to circulate treated fuel through the entire fuel system including the carburetor. Stabilizer extends fuel life to 12 to 24 months and prevents the varnish formation that causes carburetor clogs during storage.
Option 2: Drain the Tank and Run It Dry For maximum protection, drain the fuel tank completely and then run the engine until it dies from fuel starvation. This empties the carburetor bowl and fuel lines of any remaining fuel. A dry fuel system cannot form varnish deposits, cannot suffer phase separation, and cannot corrode from moisture during storage. This is the most reliable approach for long-term storage.
Option 3: Switch to Ethanol-Free Gas Using ethanol-free fuel for the last few tanks of the season before storage eliminates the moisture absorption and phase separation problems that make winter storage so hard on fuel systems. Even if you don’t drain the tank, ethanol-free fuel stored with stabilizer is far less likely to cause spring starting problems than treated ethanol-blended fuel.
Common Mistakes to Avoid
- Putting the mower away in October with a full tank of untreated pump gas and assuming it’ll be fine for six months. It won’t
- Skipping winter prep because the mower always started fine before. Eventually it won’t, and carburetor cleaning in March is less fun than five minutes of prep in October
- Adding fresh gas to the tank in spring without draining the old fuel first, which dilutes but doesn’t eliminate the problem
- Trying to start the mower dozens of times in spring with old fuel and then wondering why it still won’t go after all those attempts
Pro Tip
The best winter prep takes about five minutes and costs about two dollars worth of fuel stabilizer. Add stabilizer, run the engine for ten minutes, and shut it down. That’s genuinely all most people need to do to come back in spring to a mower that starts on the first or second pull. A few minutes of attention in the fall is worth infinitely more than a carburetor cleaning in the spring.
Final Thoughts
Leaving gas in your lawn mower over winter is one of the most common and most preventable causes of spring starting problems. The fuel that seemed fine when you put the mower away in October doesn’t stay that way for six months. Treat it with stabilizer, drain it, or switch to ethanol-free fuel before storage, and you’ll spend a lot less time troubleshooting and a lot more time mowing when the grass starts growing again.
Now go get that mower ready for spring. You’ve got this.