Why Your Lawn Mower Won’t Start After Seasonal Storage

Small Engine Won’t Start After Sitting? Read This First

Quick Summary
Lawn mower won’t start after seasonal storage — the root cause is almost always fuel-related. Gasoline degrades faster than most people realize, especially in vented small-engine fuel systems. That degradation leads to clogged carburetors, corrosion, stuck needles, and fouled spark plugs. The fix usually isn’t complicated—but understanding why it happens helps you prevent it next season and avoid unnecessary repairs.

Article

Why Your Lawn Mower Won’t Start After Seasonal Storage: A Small-Engine Reality Check

Every spring, I watch the same story play out. Homeowners roll their mower out of the shed, pull the cord, and… nothing. The conclusion is immediate: the mower is junk. But in reality, when a lawn mower won’t start after storage, it’s rarely a catastrophic failure. It’s chemistry, time, and neglecting to do exactly what they’ve always done to small engines.

I’ve rebuilt and repaired hundreds of carburetors over the years, and I can tell you this with confidence: when a small engine won’t start, stale fuel and carburetor contamination are almost always at the top of the list. Lawnmowers, snowblowers, generators, and string trimmers all suffer the same fate when they sit unused for months.

Stale Fuel: The Number One Reason a Lawn Mower Won’t Start

Modern gasoline is a complex cocktail—up to 150 hydrocarbons blended with highly volatile compounds designed to evaporate easily and ignite quickly. That volatility is great when fuel is fresh. It’s a disaster when fuel sits.

Gasoline begins degrading the moment you pour it into a gas can. Volatile components slowly escape, oxidation begins, and ethanol complicates matters by attracting moisture. Long before winter ends, the fuel in your mower has changed chemically. By spring, that altered fuel is often incapable of vaporizing properly, which is exactly why a lawn mower won’t start even though it ran perfectly last fall.

This isn’t a new problem. Stale fuel was killing small engines in the 1960s, long before ethanol entered the picture. Ethanol accelerates the damage, but it didn’t invent it.

In addition to the volatile compounds, refiners add ethanol to the gas to help it burn cleaner.

The exact formula for gasoline varies for each refinery based on:
1) The individual refiner, based on their refining capabilities
2) The market prices of the individual components
3) The season (winter and summer gasoline formulations)
4) The region (altitude and air quality regulations in the region).

One or more refineries may serve a large metropolitan area, and each refinery’s fuel is delivered to a local terminal, where each gasoline brand adds their own
specific gasoline additives package containing:
Deposit Control Additives
Fluidisers / Carrier Oil Additives
Friction Modifier Additives
Corrosion Inhibitor Additives.
Antioxidant Additives
Conductivity Improver Additives
Metal Deactivator Additives
Octane Booster Additives
Demulsifiers / Dehazers / Emulsion Preventatives Additives
Anti-Valve Seat Recession Additives

Why Small Engines Suffer More Than Cars

When a small engine won’t start, people often ask why their car is fine but their mower isn’t. The answer is simple. Cars have sealed fuel systems, pressurized tanks, and computer-controlled fuel delivery. Small engines have vented gas caps, open bowls, and gravity-fed carburetors.

Those vented caps allow fuel vapors to escape every time temperatures rise and fall. Oxygen enters the tank, oxidation begins, and gum and varnish form. Plastic gas cans make things worse because they’re semi-permeable. Volatile compounds literally migrate through the plastic walls, leaving behind heavier, harder-to-ignite fuel.

Add humidity, temperature swings, and months of inactivity, and you’ve created the perfect conditions for fuel breakdown. That’s why a small engine won’t start after storage, even if you “only left gas in it for one season.”

What Happens To Gasoline After You Fill Your Gas Can

Gasoline begins to degrade quickly, losing some of its volatility as soon as you fill your gas can. Everybody blames it on ethanol gas. Ethanol plays a part, but here are the hard facts:

In the 1960’s Stale Gas Was the #1 Gasoline small engine problem
In 2025, Stale Gas is the #1 Gasoline small engine problem

Here’s why:

• Plastic gas cans are semi-permeable. So they lose some volatile components. Most portable gas containers are made from high-density polyethylene (HDPE). The fuel’s volatile components, like HDPE: n-butane, isobutane, pentane, benzene, toluene, xylene (BTX), and Ethanol, can migrate through HDPE, leaving less volatile gasoline in the can.

• Higher storage temperatures accelerate loss of volatile components— Higher temperatures increase the vapor pressure inside the container and speed up molecular motion, accelerating the permeation process, especially in older gas cans that develop microcracks and lose some density. However, it can also happen in new plastic gas cans, made with thin walls.

• The volatile components evaporate right out of the fuel tank’s vented cap— All small engine gas tanks have a vented gas cap to
allow for fuel expansion and contraction. Air moves in and out of the gas tank depending on ambient temperatures, leading to loss of the most volatile components such as Benzene, Toluene, Xylene, Propane, and Pentane. As those components evaporate, the remaining fuel becomes harder to ignite.

• Air in the lawn mower tank causes the fuel to oxidize and break down— When gasoline comes in contact with oxygen, it breaks down, causing gum, varnish, and acids. The sticky gum and varnish clog the main and idle jets and passages, as well as the needle and seat.

• Moisture in the tank condenses on the tank walls, diluting the fuel— The nighttime drop in temperature causes moisture in the tank to condense on the tank’s walls and fall into the gas, diluting it with water and causing acis formation.

• Ethanol Fuel encounters Phase separation in about 30 days— E-10 is 90% gasoline and up to 10% ethanol. Ethanol is hygroscopic (water-loving). Ethanol can absorb 3.8 teaspoons of water per gallon and still remain in suspension. After that, the ethanol and water separate from the mixture, falling
to the bottom of the tank. Hot, humid weather can cause phase separation within 30 days. Water on the bottom of the tank causes hard starts, no starts, carburetor corrosion, and even engine damage due to lower octane.

The same goes if your snowblower won’t start. Fuel left in the tank or carburetor bowl over winter breaks down into a sticky residue that prevents combustion. For two-stroke engines like your string trimmer, that leftover fuel/oil mix can separate and foul both the carburetor and spark plug, which is often why your weed whacker won’t start.

Learn more about gasoline shelf life here

Here are some examples of what gum and varnish look like in a carburetor, as well as corrosion caused by water in the gas.

Digging in: Why Your Lawn Mower Won’t Start

Look at the images below. You’ll see the carburetor bowl, main jet, and the emulsion tube, as well as the idle screws and choke plate. Fuel flows from the bowl, through the main jet to the idle circuit, and the emulsion tube. Any gum, varnish, or corrosion that forms in the main jet will cause starting and running problems in the engine.

What to do about a clogged carburetor

These are the 3 main symptoms of a clogged carburetor

1) No-start — Clogged main jet/emulsion tube. Spray carburetor cleaner through the main jet and up through the emulsion tube.
2) No-start due to no fuel — Needle is stuck in seat. Rap on the carburetor body with a screwdriver handle.
3) Fuel is dripping from the Carburetor — Fuel inlet needle is stuck open. Remove the float bowl and float and clean the tip of the needle and seat.

If those techniques don’t work, it’s time to rebuild or replace the carburetor.

Rebuild a Carburetor Versus Replacement

Most carburetors can be rebuilt. However, to do it properly, you’ll have to invest in a half or full gallon container of carburetor cleaner. If you think you can clean up a carburetor with a spray can, think again. It has to soak in the cleaner for at least one hour.

A gallon of carb cleaner costs about $40, and a rebuilt kit costs $17-$35. To rebuild a carb, disassemble and soak it in carburetor cleaner. Wash it off with water and use compressed air to blow out all the passages. Then reassemble with new gaskets, needle, and seat.

Or, replace it

If the carburetor shows any signs of corrosion, it’s not worth rebuilding. You’ll never get the corrosion out of the small idle passages. So purchase a replacement.

You’ll find lots of cheap aftermarket options for replacement carburetors. In my experience, most are garbage. I’ve used them and had no start issues right out of the box, as well as loping and low power issues. They’re just not high quality units. I know you’ll be tempted, but don’t. Buy an OE carburetor.

Check and Replace the Spark Plug

A fouled, corroded, or cracked spark plug is another reason a lawn mower won’t start after winter. Spark plugs degrade over time—especially if your engine wasn’t maintained adequately before storage. Moisture in the combustion chamber can cause rust or corrosion, and oil deposits can foul the tip.

Fix:
Remove the plug and inspect it. If the plug is blackened, cracked, or soaked, replace it. Gap the new plug correctly before installing.

Get Ahead of Storage Season with Preventative Maintenance

To prevent problems next year:

1) Buy a low permeation gas can

2) Add stabilizer to your gas at the pump as you fill your gas can— Adding fuel stabilizer to old gas will not bring it back to life!

3) Run the engine until it stalls to empty fuel from the carburetor bowl.