Have you ever left your lawn mower sitting in the garage for a few months, only to find it won’t start when you need it? One of the most common—and easily overlooked—reasons for a lawn mower that won’t start is old gasoline in the tank. In fact, old gas is responsible for more starting problems than just about any other cause. Here’s why, what to look for, and how to fix the issue fast.
Why Does Old Gas Cause Starting Problems?
- Gasoline breaks down over time. After just 30 days, fuel begins to lose volatility and the chemicals start to separate.
- Varnish and deposits form. As gas gets older, it can leave sticky varnish and gum behind, clogging carburetors and jets.
- Moisture gets in. Stored gas can absorb water from the air, leading to sputtering, poor combustion, and even rust inside your fuel system.
- Poor combustion. Old fuel doesn’t ignite as easily, causing hard starting, rough running, or a complete no-start condition.
How to Tell If Old Gas Is the Problem
- Hard or no start: The mower cranks but won’t fire up, or runs only on starting fluid.
- Strong, sour odor: Old gas smells sour or “off” compared to fresh fuel.
- Discolored or cloudy gas: Look for yellow, brown, or cloudy liquid in the tank or carb bowl.
- Repeated stalling: The mower may start briefly but stalls and won’t stay running.
- Carburetor issues: Old fuel often means carburetor jets or passages are clogged with sticky deposits.
What to Do If You Have Old Gas in Your Lawn Mower
- Drain the old gas completely. Use a siphon or carefully tilt the mower to empty the tank into a safe container. Dispose of gas responsibly.
- Refill with fresh gasoline. Use fuel that’s less than 30 days old for best results. Consider adding a fuel stabilizer if you won’t use it all quickly.
- Prime and try to start. Sometimes, fresh gas is all you need. Prime the carburetor as instructed and attempt to start the mower.
- Clean the carburetor if needed. If the mower still won’t start, remove the air filter and spray carburetor cleaner into the carb intake. Let sit and try again. Stubborn cases may require removing and cleaning the carburetor or replacing it.
- Replace the fuel filter. If your mower has an in-line fuel filter, change it—old fuel can clog it and prevent fresh gas from reaching the engine.
How to Prevent Old Gas Problems
- Only buy as much gas as you’ll use in 30 days.
- Add fuel stabilizer to every tank if you mow infrequently or store fuel over winter.
- Run the mower dry before storage, or drain the tank and carburetor to prevent varnish and gum build-up.
- Store gas in a sealed, approved container and keep it in a cool, dry place out of direct sunlight.
Comparison Table: Old Gas vs. Fresh Gas Effects on Lawn Mowers
| Type of Fuel | Effect on Starting | Performance | Long-Term Impact |
|---|---|---|---|
| Fresh Gasoline | Easy starting, strong ignition | Full power, smooth running | Keeps carburetor clean |
| Old Gasoline (30+ days) | Hard/no start, may not ignite | Sputters, stalls, weak power | Leaves varnish, clogs carburetor |
| Gas with Stabilizer | Good starting up to 6–12 months | Normal performance if properly stored | Minimizes carb gumming and moisture issues |
Frequently Asked Questions
How long does gasoline last in a mower?
Most gasoline starts to break down after just 30 days, and can cause problems after 2–3 months—especially in hot or humid conditions.
Can I fix old gas by adding fresh gas on top?
No. Mixing new fuel with old won’t solve the problem—the bad gas will still cause starting issues. Always drain and replace completely.
Is it safe to use starting fluid on a mower with old gas?
It can help diagnose a fuel problem (if the engine fires on starting fluid, old gas is the likely issue), but don’t rely on it as a fix. Replace the gas and clean the carburetor as needed.
Will fuel stabilizer help old gas?
Stabilizer only works on fresh fuel—add it when the gas is new. It can’t “revive” fuel that’s already broken down.
Conclusion
Old gas is one of the most common reasons a lawn mower won’t start. Always use fresh gasoline, add stabilizer if storing your mower, and clean your carburetor and fuel system if you suspect bad fuel. These simple steps will keep your mower starting easy and running strong all season long.