By · Updated May 10, 2026 · 7 min read

Pulled the cord on your mower this weekend and got nothing but a tired growl — or worse, absolute silence? You're not alone. Every spring my phone lights up with the same complaint from neighbors, friends, and family: “It ran fine when I put it away.” The four most likely culprits are old gas, a fouled spark plug, a clogged carburetor, and a dirty air filter. The good news is that most of these fixes take under an hour and cost somewhere between $0 and $15 in parts. You don't need a shop or a mechanic. You need a wrench and a little patience.

I fixed three mowers just this past week. Two had the exact same problem — old gas had gummed up the carburetor jet over the winter — and one needed a new spark plug it should've gotten at the end of last summer. These are not complicated repairs. Here's the diagnosis order I run every single time, in plain order, starting with the safest and simplest checks first.

Tools & Parts You'll Need

Hand tools flat lay for lawn mower repair - wrenches, sockets, and pliers used for spring tune-ups
Photo: suntorn somtong / Pexels

See Also

While you're doing spring maintenance around the house, these might come in handy:

The Step-by-Step Fix (Diagnosis Order I Run Every Time)

  1. Step 1: Safety first — disconnect the spark plug wire. Before you touch anything else, pull the rubber boot off the spark plug and set it aside so the wire can't accidentally reconnect. The engine cannot fire without it. Let the engine cool completely if you tried to start it. Work outside or in a well-ventilated garage. No open flames, no cigarettes near the fuel system. This isn't theater — gasoline vapors are heavier than air and collect at ground level.
  2. Step 2: Drain the old gas. This is the single most important step. Old ethanol-blend gasoline is the number one cause of “won't start in spring” complaints — full stop. If the gas in your tank smells like turpentine, varnish, or nail polish remover, it has gone bad. Use a siphon to pull it into an approved fuel container. Take old gas to a recycling center or a local auto-parts store — don't dump it.
  3. Step 3: Pull and inspect the spark plug. Use your socket wrench. Look at the electrode end. A black, oily plug means the engine has been running rich or flooded. A white or cracked porcelain tip points to lean running or overheating. A plain gray-brown deposit is normal wear — clean it with a wire brush or just replace it. A new plug is about $3, and when in doubt, that's the right call. Gap it to manufacturer spec, which is typically 0.030" on most residential walk-behind mowers.
  4. Step 4: Check the air filter. Pop the air filter cover (usually one screw or a simple clip). If the paper filter is gray, brittle, oil-soaked, or you can't see light through it when you hold it up, replace it. Foam filters get the soap-and-water treatment — wash gently, rinse, let dry completely, then apply a light coat of clean motor oil before reinstalling. A choked air filter starves the engine of air and prevents a clean start.
  5. Step 5: Clean the carburetor. For most “won't start after winter” cases, the main jet inside the carburetor bowl is partially or fully blocked by varnish left behind when old fuel evaporated. With the air filter off, spray carburetor cleaner directly into the carburetor throat — two to three good bursts. Pump the primer bulb three or four times if your mower has one. If there's a bowl drain screw at the bottom of the carburetor, crack it open briefly to let old fuel flush out, then close it. While you're at it, pull the in-line fuel filter (a $3 part) and replace it if it looks discolored or you can see sediment inside.
  6. Step 6: Fresh fuel and a pull. Add fresh 87-octane fuel to the tank — ethanol-free is ideal if your area carries it, since ethanol attracts water and accelerates the varnish problem you just cleaned up. Reattach the spark plug wire firmly. Set the choke to the “Start” or closed position. Three controlled pulls. If the engine pops but doesn't catch, use the choke and primer bulb — work them together. If it runs rough for the first 30–60 seconds, that's residual old fuel being burned through. Let it idle until the stumbling clears.
Hands fixing a small engine - close-up of carburetor and spark plug work covered in this guide
Photo: Artem Podrez / Pexels

What to Check If It Still Won't Start

If you've gone through every step above and the mower still refuses to run, here are the second-tier issues worth investigating before you haul it to a shop:

When to Call a Pro

Most mowers respond to the steps above. But here's a clear list of situations where DIY stops making sense:

Small-engine shop labor typically runs $80–120 per hour, but a full tune-up — plug, filter, blade sharpen, oil change — is often offered as a flat $80 package. That's fair value if this process isn't something you enjoy or have time for.

FAQ

Q: How long can gas sit in a lawn mower before it goes bad?
Standard ethanol-blend pump gas (E10) starts degrading in as little as 30 days in a hot garage or shed. By three months, it's unreliable. By six months — which is exactly what winter storage looks like — it has almost certainly left varnish deposits somewhere in your fuel system. Ethanol-free fuel or properly stabilized gas can last up to 24 months without gumming issues.

Q: Should I drain the gas every fall, or use a fuel stabilizer?
Both approaches work. Draining is cleaner but requires more steps — run the engine dry after draining so no residual fuel is left in the carburetor bowl. A fuel stabilizer (STA-BIL or equivalent) added at the last fill of the season is faster and protects the carburetor just as well. I use STA-BIL every fall. Three minutes of work in October saves an hour of carb cleaning in May.

Q: My mower starts but dies right away — what's that?
Nine times out of ten, that's a partially clogged carburetor main jet. There's just enough fuel getting through to fire the engine but not enough to sustain a run. The fix is the same carb-cleaner treatment from Step 5. If cleaning doesn't hold, the carburetor may need a full rebuild kit (usually under $10) or replacement. A sticky choke that isn't releasing fully after startup can cause the same symptom.

Sources

Our Point of View

Here's my honest take: spend the $5 on a bottle of STA-BIL every fall and you will almost never be standing in your driveway frustrated in May. Most mowers that “stop working” aren't broken — they're chemistry experiments gone wrong. The fuel system is small and easily overwhelmed by ethanol-blend gas sitting in a hot shed all winter. The mower industry is perfectly happy to sell you a new machine when your old one just needed a $3 spark plug and a can of carb cleaner. About 80% of “dead” mowers come back to life with nothing more than fresh gas and a clean carburetor. Don't let yours become a statistic.

This article was reviewed by our editorial desk for accuracy. John Fix is verified at LinkedIn. Sources are linked inline and listed above. We update articles when new information becomes available. Last reviewed: May 10, 2026.

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FixItWhy Score: 8.7

Master handyman with two decades of small-engine and home-repair experience. Believes most “broken” things just need fresh fuel, a clean filter, and someone willing to read the manual.

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Disclaimer: The information in this article is provided for general informational and educational purposes only. It is not professional advice. Every lawn mower, engine, and situation is different — always read your equipment's owner's manual and follow manufacturer safety guidance. FixItWhy Media and the author assume no liability for injury, equipment damage, or other loss arising from the use of this information. If you are unsure about any step, are uncomfortable working with fuel or sharp components, or smell or see anything unusual (fuel leaks, smoke, oil pooling), stop work and consult a licensed small-engine technician.

— FixItWhy Media · Practical fixes for the stuff in your house, garage, and yard.