Your lawn mower won’t start after sitting for a while? One of the most common and easily overlooked culprits is old gasoline in the tank. Bad or stale gas causes more mower no-start issues than almost anything else. Here’s why old gas is a problem, how to spot the signs, and what you should do to fix it fast and prevent future trouble.
Quick Answer: Yes, Old Gas Can Definitely Keep a Lawn Mower from Starting
When gasoline sits for a month or longer—especially in hot or humid conditions—it starts to break down, losing its ability to ignite easily. Modern fuel (especially with ethanol) absorbs moisture and forms varnish and sticky residue that can clog fuel lines and carburetor jets, stopping your mower cold.
Why Old Gas Causes Lawn Mower Starting Problems
- Evaporation: The most flammable components evaporate first, leaving behind fuel that’s harder to ignite.
- Absorbs Moisture: Ethanol-blended gas attracts water, which won’t burn and can cause corrosion or blockages.
- Forms Varnish & Gum: Sticky residues clog up carburetor jets, fuel lines, and filters, preventing fuel from reaching the engine.
- Poor Combustion: Stale gas burns poorly, causing rough running, misfires, or no-start situations.
Symptoms of Old Gas in Your Lawn Mower
- Mower cranks but won’t start or only runs for a few seconds
- Engine sputters, surges, or stalls repeatedly
- Strong, sour, or varnish-like smell from the gas tank
- Visible discoloration—old gas may look darker or cloudy
- Sticky residue in carburetor or on fuel filter
How to Fix a Lawn Mower With Old Gas
- Turn off the mower and let it cool completely.
- Use a siphon or turkey baster to remove all old fuel from the tank and carburetor bowl (if accessible).
- Dispose of old gas safely—never pour it on the ground.
- Refill with fresh, clean gasoline. For best results, use non-ethanol fuel or add fuel stabilizer to extend shelf life.
- If the mower still won’t start, clean or replace the fuel filter and spray carburetor cleaner into the carburetor intake. You may need to remove and clean the carburetor if the mower sat for a long time.
Comparison Table: Old Gas vs. Fresh Gas in Lawn Mowers
| Fuel Condition | Symptoms | Starting Reliability | Fix |
|---|---|---|---|
| Old/Stale Gas | Won’t start, sputters, smells bad, sticky residue | Poor/None | Drain & replace with fresh gas, clean carburetor |
| Fresh Gas | Easy starting, smooth running | Good | Use gas less than 30 days old |
Frequently Asked Questions
How long does gasoline stay good in a lawn mower?
Most gasoline starts to go bad in as little as 30 days, especially if stored in a warm or humid place. Use a fuel stabilizer if storing for longer.
Can I mix old gas with new gas to use it up?
It’s best to drain old gas completely. Even a small amount of bad gas can cause hard starting or running issues.
How can I prevent this in the future?
Run the mower dry or drain the tank before winter or long storage. Use fuel stabilizer or buy only as much gas as you’ll use in a month.
What if my mower still won’t start after fresh gas?
Check the spark plug, air filter, fuel filter, and carburetor for additional issues caused by stale fuel. Deep carburetor cleaning may be needed if residue remains.
Conclusion
If your lawn mower won’t start and the gas is old, replace it with fresh fuel right away. Stale gas is one of the quickest and cheapest fixes—helping you get your mower started fast and avoid costly repairs down the line!