Intro
Hey, welcome back to Backyard Engine Pro. Bad gas is one of the most common causes of lawn mower problems, and it’s also one of the most frequently overlooked. Because fuel degrades gradually, people often don’t connect starting problems or rough running to the gas sitting in the tank. By the time symptoms appear, the fuel has been causing damage for weeks.
The good news? Bad gas is easy to identify and even easier to fix. Let’s walk through the signs.
Common Signs of Bad Gas
- Lawn mower won’t start or is very hard to start
- Engine starts briefly then dies
- Rough running or sputtering during operation
- Surging RPMs instead of steady engine speed
- Fuel smells sour, stale, or like varnish
- Fuel looks dark, cloudy, or discolored
How Long Does Gas Last in a Lawn Mower?
Gasoline starts degrading faster than most people expect. Without treatment, the volatile compounds that make fuel easy to ignite begin evaporating within 30 days. What remains behind burns poorly and leaves sticky residue throughout the fuel system.
Typical fuel lifespan:
- Untreated gasoline: approximately 30 days
- Gasoline with fuel stabilizer: several months to over a year
- Ethanol-blended fuel (E10): degrades faster than pure gasoline in both cases
Because most lawn mowers sit between uses for days or weeks at a time, fuel quality is always worth considering when problems develop.
1. Lawn Mower Won’t Start
Old gas is one of the first things to suspect when a mower won’t start, especially after sitting for any period of time. Because degraded fuel loses its volatility, it can’t ignite reliably during cranking. As a result, the engine may crank over normally but never fire. In addition, extremely old fuel can leave enough varnish in the carburetor to block fuel delivery entirely.
What happens:
- The engine cranks at normal speed with the starter
- The fuel simply won’t combust reliably enough to start
- Spraying starter fluid into the intake and getting a brief fire confirms the engine is fine but fuel delivery is the problem
2. Engine Runs Rough or Surges
Degraded fuel doesn’t burn consistently from one combustion cycle to the next. Because the energy content and volatility vary across a tank of old gas, the engine fires unevenly. As a result, you get sputtering, rough running, and RPMs that rise and fall instead of holding steady.
Common symptoms:
- Sputtering or popping during operation
- Engine speed fluctuates even at a fixed throttle position
- Poor throttle response when you try to increase power
- Rough idle that smooths out somewhat at higher RPM
3. Fuel Smells Sour or Like Varnish
This is one of the fastest and most reliable ways to identify bad gas without running any tests. Fresh gasoline has a relatively light, clean petroleum smell. As it degrades, the smell changes noticeably.
Bad gas often smells:
- Sour or fermented, similar to old vinegar
- Stale and heavy compared to fresh fuel
- Like varnish, paint thinner, or lacquer in advanced degradation
If you pull the fuel cap and get any of these odors, the gas is past its useful life regardless of how it looks. Because smell is often the first sign of degradation, catching it early allows you to drain and refuel before carburetor damage occurs.
4. Gas Looks Dark or Cloudy
Visual inspection is quick and often revealing. Because fresh gasoline is typically clear to lightly straw-colored, any significant color change indicates degradation.
Bad fuel may appear:
- Dark yellow or amber compared to fresh fuel
- Brown or brownish-orange in advanced degradation
- Cloudy or hazy, which often indicates water contamination from phase separation in ethanol-blended fuel
- Separated into two distinct layers, which is a clear sign of ethanol phase separation
To check, remove the fuel cap and look directly into the tank with a flashlight. Alternatively, disconnect the fuel line briefly and observe the fuel that runs out into a clear container.
5. Carburetor Problems Develop
Even when old fuel doesn’t prevent starting immediately, it causes damage over time. As gasoline degrades, it leaves behind sticky varnish and gum deposits throughout the fuel system. Because the carburetor contains the smallest passages and orifices in the fuel delivery chain, it’s the first place those deposits cause problems.
This can cause:
- Clogged jets that restrict or block fuel flow
- Sticky needle valves that don’t open and close correctly
- Gummed passages that require soaking to clean
If you’re dealing with a carburetor that keeps clogging despite cleaning, stale fuel is often the recurring cause. Addressing fuel quality alongside carburetor cleaning prevents the problem from returning.
6. Mower Sat for a Long Time
Storage significantly increases the likelihood of fuel-related problems. Because fuel degradation is time-dependent, a mower that sat all winter with untreated gas almost certainly has bad fuel. In addition, ethanol-blended fuel absorbs moisture from the air during storage, which accelerates degradation and can cause phase separation inside the tank.
Signs that storage-related fuel degradation is the cause:
- The mower ran fine before storage but won’t start or runs poorly now
- There’s a strong stale smell when you open the fuel cap
- The fuel looks darker than it did when you stored the machine
- The mower was stored without fuel stabilizer
How to Fix Bad Gas
The fix is straightforward, and it works in almost every case.
Step 1: Drain all old fuel
- Remove the fuel cap and use a hand siphon to empty the tank completely
- Alternatively, run the fuel line into a container and let gravity drain the tank
- Don’t add fresh gas on top of old gas. The old fuel is still the problem regardless of dilution
Step 2: Drain the carburetor bowl
- Remove the bowl bolt at the bottom of the carburetor
- Let the old fuel drain out completely
- Wipe the bowl clean before reinstalling
Step 3: Refill with fresh gasoline
- Use gas that’s less than 30 days old
- Ethanol-free fuel is the better choice since it leaves fewer deposits and stays viable longer
- Add a quality fuel stabilizer if the mower will sit for more than 30 days between uses
Step 4: Clean the carburetor if needed
- If symptoms persist after fresh fuel, varnish deposits may already be present
- Spray carb cleaner through the jets and passages as a first step
- A full removal and overnight soak resolves most deposit-related issues
How to Prevent Bad Gas Going Forward
Prevention is far easier than dealing with the consequences. These habits eliminate most fuel-related problems:
- Use fuel stabilizer every time you fill up if the mower sits between uses for more than two weeks. Sta-Bil and similar products extend fuel life to several months
- Avoid long-term storage with untreated fuel. Either drain the tank before storage or treat the fuel with stabilizer and run the engine to circulate it through the carburetor
- Run the mower dry before winter storage as an alternative to stabilizer. An empty carburetor can’t develop varnish deposits during storage
- Use ethanol-free fuel when available. Because it degrades more slowly and leaves fewer deposits, it’s the better choice for equipment that sits between uses
Common Mistakes to Avoid
- Adding fresh gas on top of old gas and assuming the problem will resolve. Because old fuel is still present throughout the system, dilution doesn’t fix the underlying issue
- Leaving fuel in the mower over winter without stabilizer. Because several months of untreated storage is one of the most reliable ways to cause carburetor problems, this single habit causes a disproportionate number of spring starting issues
- Cleaning the carburetor but leaving old fuel in the tank. Because new deposits form immediately from degraded fuel, both need to be addressed together for lasting results
Pro Tip
If your mower starts running poorly after sitting, check the gas before touching anything else. Remove the fuel cap and smell the fuel. If it smells sour, stale, or like varnish, drain it completely and refuel with fresh gas. In many cases, that single step resolves starting problems, rough running, and surging without any additional repairs needed. Because fresh fuel is the cheapest fix available, it’s always the right first step.
Final Thoughts
Bad gas causes more lawn mower problems than most people realize. Fortunately, identifying it is fast, fixing it is inexpensive, and preventing it is simple. Check for the signs above, drain and refuel when fuel quality is in doubt, and use stabilizer to protect stored fuel going forward.
Now go check that tank. You’ve got this.