Bad Gas In Lawn Mower

If your lawn mower is sputtering, refusing to start, or running rough, you might be dealing with bad gas in lawn mower. Old or contaminated fuel is one of the most common causes of small engine problems, and it’s often the first thing you should check. This guide will help you diagnose the issue, fix it, and prevent it from happening again, so you can get back to a clean cut.

Gasoline doesn’t last forever. In fact, it begins to degrade in as little as 30 days. When fuel sits in your mower’s tank over the winter or even for a few months, it can cause a chain of problems. The lighter components evaporate, leaving behind a thicker, gummier substance. This bad fuel can clog the delicate passages in your carburetor and fuel system, leading to poor performance or a complete failure to start.

How Bad Gas Causes Problems

Modern ethanol-blended fuels attract moisture from the air. This water separates from the gasoline and sinks to the bottom of your tank, which is where your fuel line picks it up. Running water through your engine prevents proper combustion. Meanwhile, the degrading fuel leaves behind sticky varnish and hard deposits that act like glue in small orifices.

Clogged Carburetor: Jets and passages become blocked by varnish.
Corroded Fuel Lines: Ethanol can degrade rubber and plastic parts over time.
Fouled Spark Plug: Improper combustion leaves carbon deposits on the plug.
Engine Sputtering: The engine gets an inconsistent mix of fuel, water, and air.

Signs Your Mower Has Bad Gas

How can you tell if bad fuel is the culprit? Watch for these telltale symptoms. They often appear after your mower has been sitting for a while.

The engine won’t start at all, even after multiple pulls.
It starts but then dies immediately or after a few seconds.
You notice a loss of power when cutting grass, especially under load.
The engine runs unevenly, surging up and down in speed.
There’s a rough idle or excessive vibration.
You smell a strong, sour gasoline odor from the tank.

Step-by-Step: Diagnosing and Fixing Bad Gas

Before you take anything apart, perform this simple check. Remove the gas cap and look inside the tank. Smell the fuel. Fresh gas has a characteristic aromatic smell, while old gas smells sour and unpleasant. You might also see discoloration or separation in the fuel.

How to Remove Bad Gas from Your Lawn Mower

Once you’ve identified bad fuel, you need to get it out of the system. Here’s how to do it safely. You’ll need a siphon pump, a safe fuel container, and some rags.

Safety First: Work in a well-ventilated area, outdoors is best. Let the engine cool completely if it was recently run. Have a fire extinguisher nearby just in case.
Drain the Fuel Tank: Use a hand siphon pump to transfer all the old gas from the mower’s tank into an approved disposal container. Do not pour it down a drain.
Dispose of Old Fuel Properly: Take the old gasoline to a local hazardous waste disposal facility or a service station that accepts it. Never mix it with new gas.

Cleaning the Carburetor and Fuel Lines

Simply removing the old gas might not be enough if the system is already gunked up. You’ll likely need to clean the carburetor. This sounds harder than it is, but it’s a very common repair.

Disconnect the Spark Plug Wire: Always do this before any maintenance to prevent accidental starting.
Locate the Carburetor: It’s usually attached to the engine, connected to the air filter and the fuel line.
Remove the Fuel Line: Use a clamp to pinch it off or be ready for a small spill. Drain any remaining fuel from the line into a container.
Remove the Carburetor Bowl: There’s typically a single bolt on the bottom of the carburetor. Unscrew it to let the bowl and it’s contents fall away.
Clean Jets and Passages: Use a can of carburetor cleaner and a small wire (like a guitar string) to gently clear any clogged jets. Spray all parts thoroughly.
Reassemble and Refill: Put everything back together, reconnect the fuel line, and add fresh, high-quality gasoline to the tank.

Preventing Bad Gas Problems in the Future

An ounce of prevention is worth a pound of cure, especially with small engines. Follow these simple habbits to avoid bad gas issues next season.

Use Fuel Stabilizer: This is the single most important step. Add a stabilizer like STA-BIL to every gallon of fresh gas you put in your can. It prevents degradation for up to 24 months.
Consider Ethanol-Free Fuel: If it’s available in your area, using recreational fuel (often labeled Ethanol-Free) is much better for small engines and lasts longer.
Run the Engine Dry for Storage: At the end of the mowing season, either run the mower until it stalls from an empty tank, or use the fuel shut-off valve if your mower has one.
Store Gasoline Properly: Keep fuel in a clean, sealed, approved container. Store it in a cool, dry place away from direct sunlight.
Use Last Season’s Fuel First: Mark your gas cans with the purchase date. Always use the oldest fuel first to keep your supply rotating.

When to Call a Professional

If you’ve gone through all these steps and your mower still won’t run properly, the issue might be more complex. It could be a faulty ignition coil, a damaged flywheel key, or severe internal engine damage. If you’re not comfortable with further disassembly, taking it to a small engine repair shop is a wise investment. They have the tools and expertise to diagnose deeper problems.

FAQ: Common Lawn Mower Fuel Questions

How long does gas last in a lawn mower?

Without a stabilizer, gasoline can begin to cause problems in 30-60 days. After 90 days, it’s often too degraded to use reliably. With a quality stabilizer, it can last over a year.

Can I mix old gas with new gas?

It’s not recommended. The old gas will contaminate the fresh fuel, reducing its effectiveness. It’s best to dispose of old gas properly and start with a full tank of new fuel.

What does bad lawn mower gas look like?

Fresh gas is clear with a slight yellow or blue tint. Bad gas often darkens to an amber or brown color. You may also see separation, with layers of different colors or water at the bottom.

Will starting fluid help if I have bad gas?

Starting fluid might get the engine to fire briefly, but it won’t fix the underlying problem. If the carburetor is clogged, the engine will die as soon as the starting fluid is gone. It’s better to address the fuel issue directly.

Is it okay to use premium gas in my lawn mower?

Using a higher octane fuel than your engine requires offers no benefit. What matters more is using fuel with no or low ethanol content and adding a stabilizer. Check your owner’s manual for the manufacturers recommendation.

Dealing with bad gas is a rite of passage for many gardeners. By learning to recognize the signs, safely remove the old fuel, and clean the affected parts, you can solve this problem yourself most of the time. Remember, the key to avoiding it all together is proper fuel management. A little care at the end of the mowing season ensures a quick, easy start when spring arrives, letting you focus on keeping your lawn looking its best.