When a lawn mower hasn’t been run in a while, it may fail to start or run sluggishly like it’s going to die. Storing a lawn mower with gas for long periods can develop issues in the fuel system that have to be dealt with.

A lawn mower with old gas must be drained and filled with fresh gas. The fuel filter and fuel lines must be checked for fuel restrictions. The carburetor is likely to be the cause of a lawn mower not running due to old gas. It must be cleaned, rebuilt, or replaced to get your mower running after storing it with old gas.

Protect yourself when working on your lawn mower. This includes wearing safety glasses and working in a well-ventilated area.

This post may include affiliate links. Purchases made through these links may provide a commission for us, at no extra cost to you. As an Amazon Associate, we earn from qualifying purchases.

Follow all safety instructions provided in your equipment operator’s manual prior to diagnosing, repairing, or operating.Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

How Long Can Gas Sit in a Lawn Mower?

Gas can sit in a lawn mower for up to 30 days before it begins to break down and negatively affect your lawn mower. Old gas leaves behind a varnish that will gum up the fuel system resulting in fuel restrictions and a mower that bogs down and runs sluggish.

Purchase and consume your gas within 30 days. If you have more gas than you can consume, add a fuel additive to stabilize it to make it last a little longer. Read about the right kind of gas to use in a lawn mower and how to properly store and care for it in “This is the Type of Gas Lawn Mowers Use“.

How to Start a Lawn Mower with Old or Bad Gas

Step 1: Remove the Old Gas from the Lawn Mower

Old gas isn’t good for your fuel system. Most gasoline includes ethanol, an alternative fuel often made of corn, that attracts moisture to the fuel system. This ethanol and moisture mixture can leave behind gummy deposits causing fuel restrictions.

It can also separate from gasoline over time and sink to the bottom of the tank. This mixture runs hot when it burns in the cylinder potentially causing engine damage.

When you find you are running old gas in your gas tank, follow these steps to remove the old gas and fill it with fresh gas.

• Remove the gas from the tank using a fuel siphon pump. This is a handy tool that helps extract fuel out of the tank allowing it to flow through a tube and into a fuel container.
• Once the fuel tank is empty, fill it with fresh gasoline with a fuel additive mixed in to clean and remove moisture from the fuel system. I use Sea Foam Motor Treatment in my lawn mower. Mix according to the directions on the bottle.
• Start and allow your mower to run until it doesn’t run sluggishly. If your mower still doesn’t start or keeps running sluggishly, continue troubleshooting and fixing the fuel components that may be have been affected by the old fuel creating fuel restrictions.

Step 2: Check the Lawn Mower for a Plugged Fuel Filter

Your fuel filter can become clogged with dirt and old fuel that has been sitting in your mower. When dirty and sticky deposits flow out of the fuel tank, the fuel filter will not allow them to pass through the filter.

The filter will become plugged which restricts the amount of fuel flowing through the fuel lines keeping your lawn mower from starting.

Replace a clogged fuel filter with a new filter. Pay attention to the arrow on the side of the filter when installing it. For correct installation, place the inline fuel filter between the fuel lines with the arrow pointed in the direction of the lawn mower’s fuel flow.

Step 3: Check the Lawn Mower for a Clogged Fuel Line

Next, move on to check for a clogged fuel line. Again, old fuel can leave sticky deposits that will restrict flow in the fuel lines on your mower. To check for good flow in the fuel lines, you’ll have to stop and start the fuel flow while checking for flow coming out of the lines using these steps:

Check fuel flow in a lawn mower fuel line

• Stop your mower’s fuel flow by preventing fuel from flowing out of the fuel tank.
  • Use the fuel shut-off valve to turn off the fuel supply. The fuel shut-off valve is typically located near the bottom of the fuel tank. You may find it under the seat of some zero-turn lawn mowers.
  • If you don’t have a fuel shut-off valve on your model lawn mower or just can’t locate it, use fuel pinch pliers to crimp the fuel line to stop the flow.
• Identify a section of the fuel line to check and remove the end of the line furthest from the fuel tank.
• Place the fuel line in a container to collect fuel as it flows out of the line. Make sure the container is placed lower than the fuel tank. Fuel can’t run uphill without the assistance of a fuel pump.
• Start the fuel flow by turning the fuel valve back on or removing the pinch pliers.
• Check the flow coming out of the fuel line and emptying into the container. If you have good flow, your fuel line is not clogged. If you don’t, you’ll need to unclog the line.
• Stop the fuel flow and reinstall the good fuel line. If you have a clog, proceed with the next step.
• Remove a clogged line from the lawn mower so it is no longer attached to the mower.
• Spray carburetor cleaner into the fuel to loosen up the blockage.
• Blow compressed air through the line to remove the blockage and open up the line.
• Repeat the process of alternating carb cleaner and blowing with air until you no longer have a restricted fuel line. If you can’t remove the clog, purchase a fuel line of the same length and diameter. You will most likely have to cut the fuel line to the correct length.
• Install the cleaned fuel line or new fuel line replacement.
• Start the fuel flow and check your mower to see if it starts and runs. If it doesn’t, proceed with checking the carburetor and cleaning it.

Step 4: Check the Lawn Mower for a Dirty Carburetor

Now that you have confirmed you are getting good flow to the carburetor by first checking to make sure fuel is flowing through the fuel filter and fuel lines, it’s time to check the carburetor.

The carburetor is responsible for regulating the amount of fuel that gets mixed with air to form a combustion.

A little fuel gets stored in the carburetor bowl once it leaves the fuel tank. When the fuel is old, it can cause the fuel jet to become clogged or freeze up the small parts of the carburetor and stop functioning. The carburetor must be cleaned.

If you are somewhat mechanical and don’t mind working with small parts, you should be able to handle removing and cleaning the carburetor. Just follow these instructions I provide here to clean it.

Sometimes cleaning isn’t enough. Internal parts can become damaged and require the carburetor to be rebuilt or replaced.

You also have the option of bringing your mower to a small engine mechanic or to your lawn mower dealership to have the carburetor cleaned, rebuilt, or replaced.

More Fuel Problems that Can Prevent Your Lawn Mower from Starting

Bad Lawn Mower Fuel Pump

If you have a carburetor that sits higher than the fuel tank, you’ll need a fuel pump to work against gravity and push fuel to the carburetor. When the fuel pump fails, your lawn mower won’t start.

Once you confirm you are getting fuel to the pump, check the flow out of the pump by removing the fuel line from the carburetor and placing it in a container to collect fuel.

Start the mower and watch for a steady or pulsating flow of fuel coming out of the line. If you’re not getting good flow, replace the fuel pump.

Broke or Plugged Fuel Cap

The fuel cap can break or plug preventing it from venting sufficiently. When air isn’t able to pass through the vent, the fuel tank forms a vacuum. This vacuum prevents fuel from flowing out of the tank and getting to the engine.

Determine whether or not your cap is bad by starting and stopping the lawn mower with the fuel cap on and off.

If your mower starts with the fuel cap off, once the air is allowed into the fuel tank, but quits running after a show period after it’s installed, you most likely have a bad fuel cap. Go ahead and replace the cap.

Is it Okay to Mix Old Gas with New Gas in a Lawn Mower?

It is not okay to mix old gas with new gas in a lawn mower. Gas begins losing its combustible properties and leaves behind a varnish as soon as 30 days after purchase.

Adding new gas to old gas does not solve the problems you’ll encounter by running old gasoline. These problems include fuel restrictions causing a lawn mower to run sluggishly or not run at all.

Dispose of old gasoline in a lawn mower before filling it with fresh gasoline. Remove moisture from the fuel system by adding a fuel additive like Sea Foam to the gas.

This product not only reduces moisture, it cleans the fuel system and stabilizes the fuel so it lasts a little longer before it begins to break down.