Intro
Hey, welcome back to Backyard Engine Pro. Old gas is one of the most common and most preventable causes of lawn mower problems. If your mower has been sitting for a season or you’re getting it ready for spring, draining the old fuel is one of the first and most important steps you can take. It takes about ten minutes, requires almost no tools, and can save you from a carburetor cleaning or a frustrating no-start situation down the road.
Let’s walk through it the right way.
Quick Overview
- Turn off the mower and let it cool
- Disconnect the spark plug wire
- Drain the fuel tank safely
- Run the engine to clear the carburetor
- Dispose of old gas properly
- Refill with fresh fuel
Why You Need to Drain Old Gas
Gasoline starts breaking down in as little as 30 days and is significantly degraded by 60 days, especially fuel containing ethanol. When gasoline goes bad it loses its ability to combust reliably and leaves behind a sticky, varnish-like residue that coats every surface it contacts inside the fuel system. That residue accumulates in the carburetor’s tiny jets and passages and restricts or blocks fuel flow entirely.
The result is exactly the kind of hard starting, rough running, and no-start situations that send people looking for answers. Draining the old fuel before it has a chance to degrade in the system prevents all of that from happening in the first place.
Tools You’ll Need
- Fuel-safe container with a lid for collecting old gas
- Siphon pump (optional but makes the job cleaner)
- Clean rag or towel for any drips
- Safety gloves
Work outdoors or in a well-ventilated area away from any ignition sources. Gasoline vapor is flammable and accumulates quickly in enclosed spaces.
Step 1: Turn Off the Mower and Let It Cool
Before you do anything, make sure the engine is completely off and has had time to cool down. A hot engine near open fuel is a situation you don’t want to be in.
- Shut the engine off if it’s running and let it sit for at least 15 to 20 minutes before working with the fuel system
- If the mower has a fuel shutoff valve, close it now to prevent fuel from flowing while you work
Step 2: Disconnect the Spark Plug Wire
This is a non-negotiable safety step every time you work on a mower. Disconnecting the spark plug wire ensures the engine cannot accidentally fire while you’re working around fuel.
- Pull the rubber boot off the spark plug and tuck the wire away from the plug so there’s no chance of accidental contact
- Leave it disconnected for the entire fuel draining process
Step 3: Drain the Fuel Tank
There are two practical methods for draining the tank, and which one works best depends on your mower’s design and what tools you have on hand.
Option 1: Use a Siphon Pump (Recommended)
A hand siphon pump is the cleanest and most controlled way to remove fuel from the tank and the least likely to result in spills.
- Insert the siphon tube into the fuel tank
- Pump the fuel out into your fuel-safe container
- Continue until the tank is empty or as close to empty as the siphon can reach
- Wipe up any drips with your rag immediately
Option 2: Tilt the Mower
If you don’t have a siphon pump, you can drain the tank by carefully tilting the mower.
- Always tip the mower with the carburetor side facing up. This is important because tipping with the carburetor down or the air filter down allows oil to migrate into the cylinder and cause a hydro lock situation
- Position your fuel-safe container under the fuel tank opening or fuel line
- Tilt the mower slowly and allow the fuel to drain out
- This method is less precise and more prone to spills, so have your rag ready and work slowly
On some mowers you can also drain the tank by disconnecting the fuel line at the carburetor end and directing it into your container. This is often the cleanest method if the line is accessible.
Step 4: Run the Engine to Clear the Carburetor (Recommended)
Draining the tank removes fuel from the reservoir, but old gas can still be sitting in the carburetor bowl and fuel lines. Running the engine until it dies from fuel starvation clears those remaining passages completely.
What to do:
- Reconnect the spark plug wire
- Start the mower and let it run at normal operating speed
- It will run for a minute or two and then sputter and die as it exhausts the remaining fuel in the carburetor and lines
- This is exactly what you want. The engine dying from starvation confirms the carburetor is now dry
- Disconnect the spark plug wire again after the engine stops
This step is optional but highly recommended, especially before long-term storage. A dry carburetor can’t develop varnish deposits during storage.
Step 5: Dispose of Old Gas Properly
Old gasoline is a hazardous material and needs to be handled responsibly. Never pour it on the ground, down a drain, or into the trash.
Proper disposal options:
- Take it to a local hazardous waste collection facility or recycling center. Most municipalities have free drop-off programs for old fuel and other hazardous household materials
- Some auto parts stores and service stations accept small quantities of old fuel for recycling
- Check your local government’s website for hazardous waste collection events and locations in your area
Keep the old fuel in a clearly labeled, sealed, fuel-safe container until you can dispose of it properly.
Step 6: Refill with Fresh Fuel
With the old fuel gone and the system clear, you’re ready to put in fresh gasoline.
- Use clean, fresh gasoline that’s less than 30 days old
- Ethanol-free fuel is the best choice for small engines if it’s available in your area since it doesn’t absorb moisture and leaves far fewer deposits over time
- Add a quality fuel stabilizer if the mower will be sitting for more than 30 days before its next use. Stabilizer extends fuel life to 12 to 24 months and prevents varnish formation during storage
- Fill to the appropriate level and replace the fuel cap securely
Common Mistakes to Avoid
- Skipping the spark plug wire disconnect before working near fuel
- Tipping the mower the wrong way and getting oil in the cylinder, which causes a hydro lock situation on the next start attempt
- Spilling fuel on hot engine components or near any ignition source
- Pouring old gas into a container that isn’t fuel-safe or leaving it in an unsealed container
- Putting the old fuel back in after draining because “it doesn’t look that bad.” If it’s been sitting more than 30 days, drain it and start fresh
Pro Tip
The best time to drain old gas is before you store the mower for the season, not after it’s been sitting all winter. Run the engine dry at the end of the mowing season and you’ll come back to a carburetor that’s clean and ready to go in the spring. It’s a five minute habit that eliminates one of the most common small engine headaches entirely.
Final Thoughts
Draining old gas is one of the simplest and most effective things you can do to keep your lawn mower running reliably. It takes ten minutes, costs nothing, and prevents the carburetor clogs and fuel system problems that cause most starting failures. Make it part of your seasonal routine and your mower will thank you every spring.
Now go get that old fuel out and put some fresh gas in. You’ve got this.