Here’s something almost nobody tells you when you buy a mower: the gas sitting in the tank right now has a shelf life of roughly 30 days. After that, the ethanol in it starts pulling moisture out of the air and separating out inside your fuel tank. By March, what’s left in there isn’t really gasoline anymore — it’s a varnish-like residue that dries into the carburetor’s tiny fuel passages like glue.
That’s the actual reason so many mowers refuse to start after winter. It usually isn’t the engine, the spark plug, or the battery. It’s five months of stale fuel quietly gumming up a part that costs $60 to $150 to have cleaned or replaced at a small-engine shop.
The good news: learning how to winterize a lawn mower properly takes about 20 minutes on one dry fall afternoon, costs less than $15, and it’s the difference between pulling the cord twice in April and loading the machine into your car for a repair appointment.
Do This Before the Last Mow, Not After
The single easiest version of this whole job is a timing trick: stop filling the tank in early fall.
Once nighttime temperatures start dropping and the grass slows down, quit topping off the tank. Just run whatever fuel is in there down toward empty over your last two or three cuts of the season. An empty tank has nothing left in it to go bad, which means there’s no varnish forming and no water separating out.
If you’re reading this with a full tank already, you have two options and both work fine:
- Run it dry. Add fuel stabilizer, run the engine 5–10 minutes so the treated fuel reaches the carburetor, then let it run until it stalls out on its own.
- Store it full and treated. Fill the tank nearly to the top, add stabilizer at the dose on the bottle, and run the engine 5–10 minutes. A nearly full tank leaves less air space for condensation to form in.
What you should not do is leave the mower sitting with a half tank of untreated fuel all winter. That’s the exact combination that causes the problem — plenty of gas to degrade, plenty of air space to sweat moisture into it.
Change the Oil Now, Not in Spring
Used engine oil isn’t just dirty; it’s slightly acidic. Leaving it in the crankcase for five idle months means those acids and combustion byproducts sit against the engine’s internals the whole time.
Draining warm oil is also just easier. Run the mower for a few minutes so the oil thins out, shut it off, then drain it — either through the drain plug or by tipping the mower on its side with the air filter and spark plug facing up (tip it the wrong way and you’ll flood the air filter with oil, which is a genuinely annoying mess to fix).
Refill with the oil weight in your manual — usually SAE 30 or 10W-30 for a walk-behind — and check the level on flat ground, not on your sloped driveway.
Real talk: I ignored this step for three years running because it felt optional. The fourth spring, my mower started, ran rough, and smoked. The shop’s diagnosis was basically “you never changed the oil.” A $6 bottle would have prevented a $90 conversation. Lesson learned the expensive way.
Pull the Spark Plug — Two Reasons
First, safety. With the plug wire disconnected, the engine physically cannot fire while your hands are near the blade. Do this before you touch anything underneath.
Second, condition. Pull the plug out and look at the tip:
- Light tan or gray: healthy. Clean it up and reinstall.
- Black and sooty: running rich, often a clogged air filter.
- Oily or wet: possible internal wear worth having looked at.
- White and blistered: running hot or lean.
Plugs cost a few dollars. If yours is more than two seasons old, just replace it while it’s already out — spring you will appreciate it.
Scrape the Deck (Yes, Really)
The packed layer of grass under your mower deck holds moisture against bare metal for months. That’s how decks rust through from the inside, and a rusted-out deck effectively ends the mower’s life — nobody repairs those economically.
With the spark plug wire off, tip the mower up and scrape the underside with a putty knife or a stiff plastic scraper. Then wipe it down so it’s dry. If you want to go one step further, a light spray of silicone lubricant on the clean deck makes next year’s clippings release much easier.
Sharpen or Swap the Blade
A dull blade tears grass rather than cutting it, which leaves ragged brown tips across the whole lawn and makes it more vulnerable to disease. Fall is the smart time to deal with it because the blade is already off while you’re scraping the deck.
You can sharpen it yourself with a file or bench grinder — keep the original angle, remove the same amount from each end so it stays balanced — or drop it at a hardware store for $10 to $20. A new blade typically runs $15 to $40 and takes ten minutes to install.
Clean or Replace the Air Filter
Paper filters get replaced; foam filters can be washed in soapy water, dried fully, and lightly re-oiled. Either way, do it now. A clogged filter starves the engine of air, which makes it run rich, which fouls the spark plug you just cleaned. Everything on a small engine connects to everything else.
Where You Store It Matters More Than People Think
An unheated garage or shed is fine. What causes trouble is a spot with big daily temperature swings, because that’s what drives condensation inside the engine and tank.
A few storage rules worth following:
- Keep it off bare concrete if you can — a scrap of plywood or an old rug underneath helps.
- Cover it loosely with a breathable cover, not a sealed plastic tarp that traps moisture.
- If it’s a battery-start model, pull the battery and store it somewhere above freezing, charging it once mid-winter.
- Never store a mower in a space with a pilot light — water heaters and furnaces don’t mix with fuel vapor.
The Fifteen-Minute Version
If you only have time for the essentials, do these three in order and you’ll avoid the vast majority of spring no-start calls:
- Deal with the fuel — stabilize it or run the tank dry.
- Change the oil while the engine is still warm.
- Disconnect the spark plug wire, scrape the deck, and store it dry.
The Tip Most People Never Hear
Here’s the one worth remembering: write the date on a piece of masking tape and stick it to the mower handle when you put it away. Note what you did — oil changed, fuel stabilized, blade sharpened.
It sounds almost too simple, but it solves the real problem. Nobody remembers in April whether they changed the oil last October. That uncertainty is exactly why people skip steps, double up on some, and end up guessing when something runs rough. One strip of tape turns your mower into a machine that tells you its own history.
Twenty minutes this fall. A mower that starts on the second pull next spring. That’s a trade worth making.
