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How To Get Old Gas Out Of A Lawn Mower (Safely And Completely)

Old gasoline in a lawn mower is one of those sneaky problems that causes all kinds of headaches — hard starting, rough running, or a mower that just refuses to fire up at all. I’ve lost count of how many neighbors have dragged a “dead” mower to my driveway, only for us to discover the real culprit: stale fuel. If your mower has been sitting for months (or since last season) and you suspect the gas is old, you’re absolutely right to deal with it before trying to mow. In this guide, I’ll walk you through how to get old gas out of a lawn mower properly, step by step, and what to do after the tank is empty so your mower runs smoothly again.

Why You Need To Remove Old Gas From Your Lawn Mower

Gasoline does not age gracefully. Even treated fuel doesn’t last forever. When it sits too long in a small engine like a lawn mower, it starts to break down and cause problems. Here’s what old gas can do to your mower:

Lose volatility, making the engine hard to start
Form varnish and gum that clog the carburetor and jets
Attract moisture, leading to rust in the fuel system
Cause rough running, surging, or stalling under load
Leave deposits in the fuel lines and on internal parts

From my own experience, fuel that’s more than 3 months old in a lawn mower is asking for trouble, especially if it’s ethanol-blended gas and the mower has been sitting in a hot shed. If it’s been in there since last season or longer, don’t even try to “burn it off.” Remove it.

Safety First Before You Touch The Fuel

Before you start pulling fuel lines or tipping the mower on its side, take a minute to set things up safely. Gas is flammable, obviously, but the fumes are also no joke. Here are a few safety steps I always follow:

Work outdoors or in a very well-ventilated garage with the door fully open
Keep away from open flames, pilot lights, and anything that sparks
Let a hot engine cool completely before working on it
Wear gloves and safety glasses to avoid splashes in your eyes or on your skin
Have a proper fuel-safe container ready for the old gas

And one firm rule: Never dump old gas on the ground, down a drain, or into the trash. We’ll talk about disposal a bit later.

How To Tell If The Gas In Your Lawn Mower Is “Too Old”

If you’re not sure how long the gas has been in your mower, a quick look and sniff can tell you a lot.

Signs Your Lawn Mower Gas Is Stale

The mower has been sitting unused for a full season or over the winter
Fuel smells sour, like varnish or paint thinner instead of fresh gas
The color of the gas is dark yellow, orange, or brown instead of clear or very light
The mower starts but runs rough, surges, or dies under load
The primer bulb (if you have one) feels sticky or doesn’t work well

In my shop, if the gas smells “off” at all, I don’t mess around — out it goes. Trying to “make do” with old fuel is how carburetors end up needing an expensive cleaning.

Tools And Supplies You’ll Need

You don’t need a full mechanic’s tool chest to get old gas out, but a few simple tools make the job cleaner and safer.

Basic Tools

Fuel-safe container (approved gas can is best)
Hand siphon pump or turkey baster–style fuel siphon
Rags or shop towels
Flathead screwdriver or pliers (for hose clamps)
Socket or wrench set (if you’ll remove the carburetor bowl)

Optional But Very Helpful

Fuel stabilizer (for the fresh fuel you’ll add later)
Carburetor cleaner spray
New fuel filter (for mowers that have one)
Disposable tray or container for catching drips

I like to keep a cheap hand siphon pump in my shed. It pays for itself every time I have to empty a tank — no need to tip the mower and risk oil spilling into places it shouldn’t go.

Method One: Siphon Old Gas Out Through The Tank

For most walk-behind mowers, the easiest way to remove old gas is by siphoning directly from the tank. This minimizes mess and avoids disturbing hoses if you’re not comfortable with small engine parts.

Step-By-Step Siphoning

Move the mower outside on level ground and let the engine cool completely if it was running
Remove the gas cap and set it somewhere clean
Insert the siphon hose into the tank, making sure it reaches the lowest point in the tank
Place the other end of the hose into your approved fuel container
Operate the pump (or squeeze bulb) until fuel starts flowing steadily
Continue until no more gas comes out and the tank is as empty as you can get it
Wiggle the hose around gently to grab any small “puddles” in corners of the tank

At this stage, you’ve removed the bulk of the old gas, but there’s still some left in the carburetor and lines. That fuel needs to come out too, or you may still have trouble.

Method Two: Drain Gas By Disconnecting The Fuel Line

If you’re comfortable working a bit closer to the engine, disconnecting the fuel line gives you a more complete drain. This is how I usually tackle stubborn “sat-all-winter” mowers.

Finding The Fuel Line

On a typical walk-behind mower:

Look under the gas tank and follow the small black rubber hose going from the tank to the carburetor
The carburetor is usually bolted to the side of the engine, behind the air filter housing
There may be a small clamp on the hose where it attaches to the carburetor

Draining Through The Fuel Line

Use your siphon first if there is a lot of fuel in the tank — this prevents a big gush when you pull the hose
Place your fuel container under the carburetor area
Use pliers or a screwdriver to loosen the hose clamp at the carburetor end
Gently twist and pull the fuel line off the carburetor nipple (a bit of fuel will start to flow)
Direct the open end of the fuel line into your container
Allow all the fuel from the tank to drain out completely
Once the flow stops, you can gently tilt the mower (with the air filter and carburetor facing up, not down) to help any remaining fuel reach the line

When everything has drained, reconnect the fuel line securely and slide the clamp back over the fitting. Double-check that the hose isn’t cracked or brittle. If it is, replacing it now is cheap insurance against leaks later.

Don’t Forget The Carburetor Bowl

Even after the tank and fuel line are empty, there’s usually a small amount of old gas sitting in the carburetor bowl. That little reservoir can be just enough to gum things up.

How To Drain The Carburetor Bowl

On many mowers, the carburetor has a small metal bowl at the bottom with a bolt in the center. That bolt usually doubles as a drain.

Place a small container or tray directly under the carburetor bowl
Use the appropriate socket or wrench to carefully loosen the center bolt
Fuel will start to dribble (or sometimes gush) out of the bowl as you remove the bolt
Let it drain completely until no more fuel comes out
Inspect the drained fuel — if it looks dirty, rusty, or full of debris, you may need a deeper carburetor cleaning
Reinstall the bolt and snug it up firmly, but don’t overtighten

Whenever I drain a carb bowl, I like to give the outside a quick spray with carburetor cleaner and wipe it down. It’s not a full rebuild, but it does help keep grime from making its way into your fresh fuel.

What To Do With The Old Gas

Now you’ve got a container full of stale fuel. The next question is: what do you do with it?

Safe Disposal Options

Depending on where you live, your best options may include:

Household hazardous waste collection sites run by your city or county
Local recycling centers that accept fuels
Auto shops or small engine shops that take waste gasoline
Community hazardous waste events held seasonally

Check your local regulations. Some areas are stricter than others, but in general, pouring it out anywhere is both harmful and often illegal.

Can You Reuse Old Gas?

Personally, I never reuse gas that’s been sitting in a mower for a year or more. If it’s only slightly old (a few months) and doesn’t smell terrible, some folks will mix a small amount — maybe 10 to 20 percent — into a fresh tank in a car or truck, but this is a judgment call and not my first recommendation. For most home gardeners, disposal is the cleaner and safer route.

Refilling With Fresh Gas (The Right Way)

Once the old gas is out, your mower basically has a fresh start. This is the perfect time to be a bit picky about what you put in.

Choosing Good Fuel For Your Lawn Mower

Use fresh, name-brand gasoline from a busy station
Stick with regular unleaded (87 octane is usually fine unless your manual says otherwise)
Whenever possible, choose ethanol-free fuel for small engines
If you must use ethanol fuel, avoid anything higher than E10

I keep a dedicated gas can just for mower fuel and label it clearly. That way I know exactly how old it is and what’s in it.

Adding Fuel Stabilizer

This is one of those small habits that makes a big difference.

Add the recommended amount of fuel stabilizer to your gas can, not directly into the mower tank

Use this treated fuel for your mower, trimmer, and other small engines

With stabilizer, I find fuel stays “mower-friendly” for the whole season, as long as the can is sealed and stored in a cool, shaded place.

Start-Up Routine After Removing Old Gas

With everything drained and refilled, your mower might not start perfectly on the first pull — the carburetor needs to refill with fresh fuel. Here’s what I usually do:

Double-check the fuel cap is on and the fuel valve (if present) is open
Prime the engine according to the manual, usually a few presses of the primer bulb
Set the choke or start lever to the “start” or “choke” position
Pull the starter cord several times until the engine fires
Once it starts, let it run for a few minutes before engaging the blade

If it still won’t start after the old gas removal and fresh fuel, you may have a deeper carburetor clog or spark plug issue, but most of the time, cleaning out old fuel solves the problem.

How To Avoid Old Gas Problems In The Future

Once you’ve wrestled with stale fuel a couple of times, you get motivated to prevent it. Over the years I’ve settled on a few simple habits that virtually eliminated fuel-related mower issues for me.

End-Of-Season Fuel Routine

At the end of mowing season, I usually:

Add stabilizer to the last tank of fuel I put into the mower
Run the mower for 10–15 minutes to get stabilized gas into the carburetor
Either run the mower until it runs out of gas, or shut off the fuel valve and let it die as the carb runs dry
Store the mower in a dry, covered location

Some people prefer to store the mower with a full tank of stabilized fuel to prevent condensation. Both approaches can work, but running the carburetor dry before storage has saved me many cleaning jobs.

General Fuel Storage Tips

Buy only as much gas as you’ll use in 1–2 months
Keep your gas can sealed tightly when not in use
Store fuel in a cool, shaded place away from direct sun and heat
Label your gas can with the purchase date so you know how old it is

When To Call In A Pro

Most homeowners can handle removing old gas from a lawn mower with a little patience and care. But there are times when it’s smart to call a small engine tech:

The fuel in the carburetor bowl looks like sticky syrup or has a lot of rust
The mower still won’t start after draining, refilling, and checking the plug and air filter
You’re not comfortable removing carburetor parts or dealing with fuel lines

A good technician can disassemble and clean the carburetor, replace gaskets and needle valves, and get your mower back into reliable shape. Sometimes that’s cheaper than fighting a badly gummed-up carb all season.

Final Thoughts: Old Gas Out, Smooth Mowing In

Getting old gas out of a lawn mower is not glamorous, but it’s one of the simplest ways to bring a “dead” mower back to life and prevent future headaches. To recap the basics:

Always remove gas if it’s been sitting in the mower for a season or more
Siphon from the tank, then drain the fuel line and carburetor bowl for a thorough cleanout
Dispose of old gas properly — don’t dump it
Refill with fresh, preferably ethanol-free gas and add stabilizer
Adopt a simple end-of-season routine so you’re not fighting stale fuel next spring

From one gardener to another, a few minutes spent dealing with old gas now can save you a lot of frustrating pull-starts when the grass is growing fast and you’d rather be admiring your lawn than fixing your mower.