Direct Answer: How to Remove Old Gas from a Lawn Mower

To remove old gas from a lawn mower, locate and open the fuel tank drain plug (usually at the tank’s lowest point) and let the fuel drain into a container. If your mower lacks a drain plug, use a siphon or hand pump to extract the fuel. After draining, replace the fuel filter, add fresh gasoline mixed with fuel stabilizer, and run the mower for 10-15 minutes to flush new fuel through the carburetor. If the engine still won’t start, you may need to clean or rebuild the carburetor to remove varnish deposits left by old fuel.

Why Old Gasoline Damages Lawn Mower Engines

Fresh gasoline compared to degraded old fuel showing oxidation and gum buildup

Gasoline doesn’t last forever. When fuel sits unused in a tank for extended periods—typically 3 to 6 months or longer—it begins to oxidize and break down. This oxidation process creates gum, varnish, and sticky residue that accumulates in the fuel tank, fuel lines, fuel filter, and carburetor.

The consequences are significant:

  • Engine won’t start: Varnish clogs the carburetor jets, preventing fuel from atomizing and igniting.
  • Poor performance: The engine sputters, stalls, or runs rough due to incomplete fuel delivery.
  • Fuel system corrosion: Old fuel contains water and corrosive compounds that damage metal fuel components.
  • Clogged filters: Degraded fuel particles block fuel filters, starving the engine.

The severity depends on how old the fuel is. Season-old fuel (3-6 months) may still work after draining and adding fresh fuel. Fuel that has sat for a year or more often requires carburetor cleaning or replacement to remove hardened varnish.

What You’ll Need Before Starting

Gather these tools and materials before beginning the fuel-draining process:

  • Drain container: A metal or plastic container large enough to hold the mower’s fuel capacity (typically 0.5 to 2 gallons for push mowers, more for riding mowers).
  • Wrench or socket set: To access and remove the drain plug.
  • Siphon pump or hand-operated fuel pump: If your mower lacks a drain plug.
  • Fresh gasoline: Use fuel rated for small engines (typically 87-octane regular unleaded); avoid E85 or fuel blends higher than E10 unless your mower is designed for them.
  • Fuel stabilizer: A product like Sta-Bil or Sea Foam to prevent future fuel degradation.
  • Replacement fuel filter: If your mower has a replaceable filter.
  • Safety gear: Gloves, eye protection, and work in a well-ventilated area. Never work near ignition sources.
  • Carburetor cleaning kit (optional): Carb cleaner spray and small brushes for detailed cleaning if varnish buildup is severe.

Safety note: Always dispose of old gasoline at a hazardous waste facility or auto parts store that accepts fuel. Do not pour it down drains, into soil, or burn it. Check your local EPA guidelines for proper disposal.

Method 1: Drain the Fuel Tank Using the Drain Plug

Most modern lawn mowers—both push and riding models—include a fuel tank drain plug at the lowest point of the tank. This is the simplest and safest removal method.

Step-by-step:

  1. Locate the drain plug: On push mowers, this is usually on the underside or side of the fuel tank. On riding mowers, check below the fuel tank (you may need to remove a side panel or belly pan). Consult your mower’s manual if unsure.
  2. Position a drain container: Place your drain container directly under the plug to catch all fuel.
  3. Open the drain plug: Turn the plug counterclockwise using the appropriate wrench or socket. Some plugs have a twist design and require only hand pressure. Fuel will begin draining immediately.
  4. Let it drain completely: Allow at least 5-10 minutes for all fuel to drain. Tilt the mower slightly if needed to ensure complete drainage (but avoid tipping fuel toward electrical components or the air filter).
  5. Reinstall the drain plug: Once empty, clean the plug of any debris, apply a thin layer of new gasoline to the threads, and reinstall it by turning clockwise. Do not overtighten.
  6. Dispose of old fuel properly: Take the container to a hazardous waste facility, auto parts store, or local waste collection event.

Method 2: Siphon Old Fuel If No Drain Plug Exists

Older mowers or models without drain plugs require siphoning. This method works but demands care to avoid ingesting fuel fumes or liquid.

Using a hand-operated fuel pump (recommended):

  1. Purchase a manual siphon pump: These are inexpensive ($10-20) and available at auto parts or hardware stores. They eliminate the risk of mouth-siphoning.
  2. Insert the intake tube: Place the pump’s intake tube into the fuel tank filler opening, lowering it to the tank’s bottom.
  3. Prime and pump: Squeeze the pump handle repeatedly to extract fuel into your drain container. Continue until the tank is empty.
  4. Allow residual fuel to drain: Rock the mower gently to coax the last fuel into the container.

Using a traditional siphon hose (if manual pump unavailable):

  1. Use a clean, food-grade plastic tube (never drink from it afterward). Avoid gasoline-treated rubber hoses.
  2. Do NOT mouth-siphon. Instead, fill the tube with fresh water or siphon starter to initiate flow, then quickly place the intake end into the fuel tank and the output end into your drain container. (Some modern siphons have a one-way valve that starts flow safely.)
  3. Gravity and air pressure differences will pull fuel out. Once flow begins, it continues until the tank is empty.

Cleaning the Fuel System After Draining

Simply draining old fuel is often not enough. Oxidized fuel leaves behind varnish deposits that can still prevent starting or cause poor performance.

Replace the fuel filter:

  • If your mower has a replaceable inline fuel filter (typically a small plastic or metal cylinder between the tank and carburetor), swap it for a new one. This filter catches most degraded fuel particles and degrades over time itself.
  • Refer to your mower’s manual for the filter location and replacement procedure.

Clean or rebuild the carburetor:

If the mower won’t start after draining and adding fresh fuel, the carburetor needs cleaning. The carburetor is a precision component where fuel is atomized and mixed with air for combustion. Varnish clogs its small jets and passages.

  • For light varnish: Spray carburetor cleaner into the fuel inlet, around the bowl, and through the jets using the spray can’s thin tube. Let the cleaner soak for 15-30 minutes, then repeat. Run the mower briefly to flush cleaner through.
  • For heavy varnish: Remove the carburetor from the engine (consult your manual for fastener locations). Soak it in carburetor cleaner for 1-2 hours. Use a small brush and compressed air to clear jets and passages. Inspect the float needle and seat for wear and replace if necessary. Reinstall and test.
  • When to replace: If cleaning doesn’t restore function, replace the entire carburetor. Rebuild kits are available but require precision and patience; replacement is often easier for DIY users.

Check and clean fuel lines:

  • Inspect rubber or plastic fuel lines between the tank and carburetor for cracks, splits, or blockages.
  • If a line is cracked or kinked, replace it. If clogged, try blowing compressed air through it or soaking it in carburetor cleaner.
  • Clamps should be tight but not so tight they crack the line.

Refueling and Testing Your Mower

Add fresh fuel:

  • Use gasoline rated for small engines. Standard 87-octane unleaded is fine for most mowers.
  • Check your manual for fuel-blend specifications. Most small engines tolerate E10 (10% ethanol) but not E15 or E85.
  • Fill the tank about three-quarters full for initial testing.

Add fuel stabilizer:

  • Pour fuel stabilizer into the tank according to the product label (typically one ounce per gallon).
  • This protects against oxidation and gum formation during future storage.

Prime the fuel system:

  • Some mowers have a manual fuel primer bulb near the carburetor. Squeeze it 5-10 times to push fresh fuel through the lines before starting.
  • If your mower lacks a primer, proceed to starting.

Start the mower:

  • Set the choke to the full or cold position (consult your manual).
  • Pull the starter cord firmly and repeatedly. Do not flood the engine by pulling excessively.
  • Once the engine catches, gradually move the choke to the run or open position.

Run and flush:

  • Allow the mower to idle for 5 minutes, then run at half and full throttle for another 10 minutes.
  • This flushes new fuel through the entire system and burns off any remaining varnish fumes.

If the mower still won’t start:

  • The carburetor likely needs deeper cleaning or rebuilding (see above section).
  • Check that fuel is reaching the carburetor by disconnecting the fuel line at the carb inlet and observing flow when the tank is tilted.
  • Verify the spark plug fires (remove it and check for corrosion or gap issues).
  • Ensure the air filter is clean.

Prevention: Using Fuel Stabilizer for Winter Storage

Prevent old gas problems before they start by using fuel stabilizer whenever you store your mower for more than 30 days.

How fuel stabilizer works:

  • Stabilizer contains antioxidants and corrosion inhibitors that slow fuel degradation.
  • It extends usable fuel life from 3-6 months to 12-24 months, depending on storage conditions (cool, dark places last longer than hot, humid ones).

Best practice for seasonal storage:

  1. Add fuel stabilizer to a full tank of fresh gasoline before storing the mower.
  2. Run the mower for 10-15 minutes so stabilized fuel circulates through the carburetor and fuel lines.
  3. Store the mower indoors in a cool, dry location away from sunlight.
  4. For winter storage longer than 3 months, drain the fuel tank completely or run the tank empty before storage. This prevents any fuel degradation regardless of stabilizer use.
  5. When resuming use the following season, add fresh fuel (stabilized if storage will be long-term again).

Common fuel stabilizers:

  • Sta-Bil: A widely available, affordable option rated to preserve fuel for 12 months.
  • Sea Foam: A premium option with additional cleaning properties to prevent and partially dissolve varnish.
  • Ethanol-specific stabilizers: Products designed for E10 fuel help prevent ethanol-related corrosion.

Frequently Asked Questions

How long can gasoline sit in a lawn mower before it goes bad?

Untreated gasoline degrades within 3 to 6 months, especially in warm environments. After 6 months, oxidation and gum formation accelerate. Fuel left for a year or longer is almost certainly too degraded to use without significant cleaning. Stabilized fuel can last 12-24 months depending on storage conditions and product quality.

Can I just add fresh fuel on top of old fuel?

No. Mixing fresh fuel with old, degraded fuel contaminates the new fuel and keeps varnish in circulation. Always drain the tank completely before refueling. The exception: if you regularly use your mower and the fuel is less than 3 months old, topping off with fresh fuel is acceptable.

What’s the difference between a clogged carburetor and a clogged fuel filter?

A clogged fuel filter typically results in weak or no fuel flow to the engine; symptoms include hard starting or the engine dying under load. A clogged carburetor affects fuel mixing and atomization; symptoms include hard starting, rough idle, and poor throttle response. Replace the fuel filter first (it’s easier); if problems persist, address the carburetor.

Is it safe to store a mower with a full tank of stabilized fuel?

Yes, storing a mower with a full tank of stabilized fuel is safe and actually preferred by many. A full tank minimizes air in the tank, which slows oxidation. Run the mower briefly so stabilized fuel reaches the carburetor. For storage beyond 3-4 months, however, some manufacturers recommend draining the tank entirely to avoid any potential issues.

Summary

Old gasoline is one of the most common reasons lawn mowers won’t start. Removing stale fuel and cleaning the fuel system—whether by simple draining or deeper carburetor work—restores most mowers to working order. Prevent future problems by using fuel stabilizer before seasonal storage and by maintaining a fuel filter replacement schedule. With these steps, your lawn mower will start reliably year after year.