How do You Get Bad Gas Out of a Lawn Mower?
If your lawn mower has been sitting for months or you accidentally filled it with old fuel, you likely have bad gas inside the tank and carburetor. The quickest fix is to drain the old fuel completely, replace it with fresh gas, and clean any residue from the fuel system. Below, you will find the exact steps, tools needed, and common mistakes to avoid so your mower starts reliably again.
What Does Bad Gas Look Like in a Lawn Mower?
Bad gas usually appears darker than fresh fuel, sometimes with a varnish-like odor or visible dirt and water droplets. If you notice your mower won’t start, sputters, runs rough, or leaves a gummy layer inside the carburetor, old gas is likely the culprit. Gasoline begins to degrade after about 30 days, forming sticky deposits that clog jets and fuel lines.
What Tools and Supplies Do You Need to Remove Bad Gas?
Before starting, gather a few common items. Most are available at hardware stores or online. You can search for them using these search-friendly links:
- manual siphon pump – for draining the tank safely
- fuel stabilizer – prevents future gas degradation
- carburetor cleaner spray – for cleaning residue if needed
- fresh unleaded gasoline – ethanol-free recommended
Other supplies include a clean gas can, a funnel, rags, safety glasses, and a small container for the old fuel. Do not pour old gas down drains or onto the ground – dispose of it at a household hazardous waste facility.
How Do You Safely Drain Old Gas From the Lawn Mower Tank?
Working with gasoline requires caution. Make sure the mower is on a level surface, the engine is cool, and you are in a well-ventilated area away from open flames or sparks. Follow these steps:
- Disconnect the spark plug wire – this prevents accidental engine startup during the process.
- Locate the fuel tank – usually under the cap on top of the mower.
- Position a drain container below the mower. For most push mowers, you can siphon gas out through the fill opening.
- Use a manual siphon pump to draw old gas into the drain container. Alternatively, you can remove the fuel line from the carburetor and let it drain into a pan (if your mower has a petcock, turn it off first).
- Remove any remaining gas by tipping the mower carefully toward the cap side (only if the engine is cool and you have a container ready). Some mowers have a drain screw on the carburetor bowl – open it to release fuel from the bowl as well.
Common mistake: leaving the spark plug connected while draining. Always disconnect it first.
Do You Need to Clean the Carburetor After Draining Bad Gas?
Often, yes. Old gas leaves sticky varnish inside the carburetor bowl, jets, and passages. Even after draining the tank, residue can cause starting problems. Here is how to clean it:
- Remove the air filter and cover.
- Unscrew the carburetor bowl (usually a single bolt or nut at the bottom). Catch any fuel drips.
- Use carburetor cleaner spray to clean the bowl, the float needle, and the small jets. Avoid using metal tools that can scratch or damage delicate openings.
- If the gasket is damaged, replace it with a new one.
- Reassemble the carburetor and reconnect the fuel line.
If the mower still does not start after cleaning, the jets may be blocked deeper inside. In that case, consider replacing the carburetor – many push mower carburetors are inexpensive and easy to swap.
Can You Just Add Fresh Gas on Top of Bad Gas?
No. Adding fresh gasoline to old gas dilutes the damage but does not remove the varnish. The old fuel will still clog the carburetor over time. The only reliable fix is to drain the tank completely, clean the system, and fill with fresh fuel.
If the gas is only a few weeks old and you added a fuel stabilizer earlier, you might get away with just topping off. But for any gas older than a month, drain it.
What About Stabilized Gas – Does It Go Bad?
Even with fuel stabilizer added, gasoline eventually breaks down. Stabilizers can slow oxidation for up to 12–24 months, but they cannot prevent it forever. If the gas smells sour or looks dark even after using stabilizer, it should be drained. Stabilizers work best when added to fresh fuel immediately after purchase.
How to Prevent Bad Gas in the Future
Avoid letting fuel sit in your mower for more than 30 days without running it. Use these preventive steps:
| Practice | Why it works |
|---|---|
| Use fuel stabilizer every fill-up | Keeps fuel fresh for months |
| Run the mower dry at end of season | No fuel left to degrade |
| Store mower in a cool, dry place | Heat speeds up fuel breakdown |
| Use ethanol-free gas | Ethanol attracts water and forms gum |
One more tip: keep a note on the mower with the last fuel date. That simple reminder helps you know when to drain.
What If You Don’t Drain the Bad Gas – What Happens?
Ignoring bad gas will eventually ruin the carburetor. The mower may fail to start, run poorly, or stall under load. In severe cases, the fuel system can become so gummed up that you need to replace the carburetor entirely – a repair that costs more than a bottle of stabilizer or a can of fresh gas.
Reusing Old Gas – Is It Safe?
Some people try to filter and reuse old gas in other equipment. It is better to dispose of it properly because the volatile compounds have already evaporated. If you must reuse it, let it sit in a clear container for a few hours to separate water and dirt, then siphon the clean middle layer into another container and use it in a vehicle with a fuel filter. Even then, you risk engine problems. Most experts recommend simply discarding old fuel.
Final Checklist for Getting Bad Gas Out of a Lawn Mower
Use this quick reference before you start:
- Turn off mower and let engine cool completely.
- Disconnect the spark plug wire.
- Drain tank via siphon or fuel line.
- Drain carburetor bowl.
- Clean carburetor with carburetor cleaner if residue is present.
- Dispose of old gas responsibly.
- Add fresh fuel with fuel stabilizer.
- Reconnect spark plug and test start.
Why Getting Bad Gas Out Matters for Your Mower’s Longevity
Removing stale gasoline is one of the most important maintenance tasks you can do. A clean fuel system means easier starting, smoother running, and fewer repairs. Spend 20 minutes draining and cleaning now instead of replacing a carburetor later. Remember to check the fuel at the start of each new season – your lawn mower will thank you with a reliable start every time.