Starting a lawn mower with old gas can be tricky. Over time, gasoline breaks down and loses its ability to ignite properly. This can cause your mower to run poorly or not start at all. If you’ve left fuel sitting in your mower over the winter or for several months, you may be dealing with degraded gasoline. Fortunately, there are steps you can take to troubleshoot and fix the problem.
This article covers the effects of old gas, symptoms of fuel-related issues, and a detailed step-by-step guide to get your mower running again. Whether you’re a weekend warrior or a seasoned landscaper, this guide uses simple language and practical advice to help you get the job done.
Effects of Old Gas on Lawn Mowers
Gasoline starts to degrade after about 30 days. Ethanol-blended fuels, which are common in the U.S., absorb moisture from the air. Over time, this leads to phase separation, where water and fuel separate in the tank. The volatile compounds in gas that ignite easily evaporate, leaving behind a sticky substance that can clog your fuel system.
Here are some ways old gas can affect your lawn mower:
Varnish buildup: As gasoline breaks down, it leaves behind varnish and gum deposits. These can coat the inside of the carburetor and fuel lines.
Clogged jets and filters: Sticky residue can block tiny passages in the carburetor or fuel filter.
Engine hesitation or stalling: Even if your mower starts, it may not run smoothly if fuel flow is restricted.
Signs of Old Gas Issues
If your lawn mower isn’t running right, old gas might be to blame. Look for these common signs:
Difficulty starting: The engine cranks but won’t fire up.
Rough idling: The mower runs unevenly or surges up and down.
Sputtering or stalling: The mower starts and then dies shortly after.
Foul smell: Old gas has a sour, varnish-like odor.
Step-by-Step Troubleshooting Guide
Drain Old Gas
Start by removing the old fuel. Here’s how to do it safely:
- Turn off the mower and make sure it’s on a flat surface.
- Disconnect the spark plug to prevent accidental starting.
- Use a fuel siphon or turkey baster to remove the gas from the tank.
- Dispose of the old gas properly. Many auto parts stores and recycling centers accept it.
Clean the Fuel System
After draining, clean out the rest of the fuel system.
Check the fuel filter: Remove and inspect it. If it’s dirty or clogged, replace it.
Inspect fuel lines: Look for cracks, blockage, or gummy residue. Replace if needed.
Clean the carburetor
- Remove the carburetor from the engine.
- Take off the bowl and clean all parts with carb cleaner.
- Use a small wire or carb jet cleaning tool to clear any blocked passages.
Use a Fuel Additive
Once the tank is empty and clean:
- Add fresh gas mixed with a fuel stabilizer or cleaner like Sea Foam or STA-BIL.
- This helps clean out any remaining residue in the system.
- Shake the mower gently to mix the fuel and additive.
Check the Spark Plug
- A bad or dirty spark plug can also prevent starting.
- Remove the spark plug using a wrench.
- Inspect the tip for carbon buildup or wear.
- Clean the plug with a wire brush or replace it with a new one.
- Make sure the gap is set correctly (usually 0.02–0.03 inches).
- Reconnect the spark plug wire securely.
Prime the Engine
- Priming helps get fuel into the carburetor.
- Locate the primer bulb (usually a rubber button).
- Press it 3–5 times to push fuel into the system.
If your mower doesn’t have a primer bulb, pull the starter cord a few times to move fuel manually.
Start the Mower
Now, you’re ready to start the engine:
- Set the choke to “on” (if applicable).
- Pull the starter cord firmly.
- If it doesn’t start after a few pulls, wait a minute and try again.
- If the mower fires up, let it run for a few minutes to clear out any remaining bad fuel.
Preventative Measures
To avoid issues with old gas in the future, follow these tips:
- Use a fuel stabilizer any time you store your mower for more than 30 days.
- Run the mower dry at the end of the season or drain the tank.
- Store fuel in a sealed container, away from sunlight and moisture.
- Use ethanol-free fuel if available in your area.
- Start the mower every few weeks during storage to keep the fuel moving.
Alternative Solutions
If your mower still won’t start after trying the steps above, consider these options:
Replace the carburetor: Sometimes, cleaning isn’t enough if internal parts are too gummed up.
Take it to a repair shop: A small engine technician can diagnose deeper issues like valve problems, compression loss, or ignition system failure.
Replace the mower: If the cost of repairs is more than a new unit, upgrading might be the smarter choice.
User Reviews & Experiences
Homeowner Feedback
Many homeowners say a fuel stabilizer saved their mower after a rough winter. One user shared, “My mower wouldn’t start until I drained the tank and added STA-BIL. It fired up after two pulls.”
Another wrote, “I had to take apart the carb, but once I cleaned the jets and added Sea Foam, it ran like new.”
Professional Insight
Landscapers recommend using ethanol-free fuel year-round. “We always drain the tanks before winter,” says Jack L., a lawn care business owner. “A little prevention saves hours of downtime in spring.”
Conclusion
Starting a lawn mower with old gas isn’t impossible, but it does take some work. The key is to remove the degraded fuel, clean the system, and use fresh gas with additives to restore performance. Keep your mower in top shape by taking a few simple preventative steps—your future self will thank you.
By understanding the effects of old gas and following this guide, you can bring your mower back to life and keep your yard looking sharp all season long.
Related topics:
- How to Fix a Carburetor on a Lawn Mower
- Can You Cut Wet Grass with a Petrol Mower?
- 4 Best Corded Lawn Mowers: A Comprehensive Guide
