How To Start A Lawn Mower With Old Gas: Step-By-Step

by Jack Grover
lawn mower in green lawn
Reading time: 14 min Prefer to listen?

You’ve left your lawn mower sitting in the garage all winter, and now that spring has arrived, it’s time to get it started up again. However, there’s a problem – you didn’t drain the gas before storing it away, and now you’re stuck with old fuel in your tank. Can you really start a lawn mower with old gas?

Below, we’re about to embark on a step-by-step journey that will guide you on how to start a lawn mower with old gas. We’ll explore various elements, including the potential issues of using old gas, ways to drain it, and tips to get your lawn mower up and running again.

So whether you’re a seasoned garden enthusiast or a novice homeowner facing this challenge for the first time, this guide is for you!

Key Takeaways

  • Never leave gas in your lawn mower for more than 30 days. Over time, the elements in gasoline begin to break down and can cause issues with the functionality and performance of your lawn mower.
  • Always siphon out old gas before adding fresh gas. Use a fuel siphon pump to safely remove old gas from the tank.
  • Check if the fuel filter is clogged. If it is, replace it with a new one.
  • Make sure fuel lines aren’t clogged, either. If they are, clean them with carb cleaner and compressed air or replace them entirely if necessary.
  • Ensure the carburetor isn’t dirty, as it can cause the mower to run sluggishly or not at all. If required, remove and clean it thoroughly.
  • Avoid mixing old and new gas. Old gas can lose its combustible properties and leave behind a varnish-like residue which can harm your engine’s performance.
  • Always dispose of old gas responsibly according to local regulations.
  • When in doubt, take your lawn mower to a dedicated small engine mechanic or your dealership for professional help.

Can Gas Sit Indefinitely in Your Lawn Mower?

So, you might be wondering, can gas just sit indefinitely in your lawn mower? Let’s dive into that.

The straightforward answer is no; it’s not advisable to leave gas in your lawn mower for an extended period of time. Gasoline is a complex blend of hydrocarbons, including additives such as fuel stabilizers, corrosion inhibitors, antioxidants, and detergents which all serve specific purposes.

However, over time – typically after about 30 days – these elements begin to break down, which can lead to several issues with the functionality and performance of your lawn mower.

When gasoline decomposes, it leaves behind a sticky residue or varnish that can seriously gum up the works within your mower’s fuel system. This residue can clog up the carburetor and other key components involved in the process of combustion. You may notice this happening if your mower begins to bog down during use or runs sluggishly.

Additionally, old gas tends to attract moisture causing further complications like rusting metal parts and degrading rubber or plastic components.

To avoid these issues altogether, it’s best practice to only keep enough fresh gasoline in your tank for immediate use – generally one month’s worth – and ideally use stabilized fuel, especially if you’re heading towards off-season where usage is less frequent.

It’s also a good habit to run out any excess fuel from the system at the end of the mowing season before storing it away for the winter months. This way, you’re keeping potential damage from old gas at bay while ensuring smooth operation come springtime when you need it most!

Now that you know how bad old gas can be for your lawn mower, let’s discuss how you can actually get it up and running again! 

1. Siphon the Old Gas

First off, you’ll need a fuel siphon pump to extract that stale gasoline from your tank. This isn’t just any tool, it’s designed specifically to safely remove fuel from tanks and containers. You can easily find one at an auto parts store or online if you don’t already have one in your garage.

The pump essentially acts as a vacuum, allowing the old gas to flow out of the tank and into a designated container for disposal.

Ensure you’re doing this in a well-ventilated area, and don’t forget safety gear – gloves and goggles are essential. Also, make sure to always dispose of old gas responsibly according to local regulations.

So how exactly you should use the siphon pump? Well, the process isn’t overly complicated:

  • Insert one end of the tube into your lawn mower’s fuel tank and place the other end into a suitable container where you’ll collect the old gas (a certified gas can works great).
  • Begin pumping. Some pumps require manual action, while others might be automatic or battery-operated. Continue pumping until all traces of gasoline have been removed from your lawn mower’s tank.
  • This process can take time, depending on the amount of old gasoline in your mower’s tank, but patience is key here! Removing every last drop ensures no remnants will interfere with fresh fuel when it comes time for filling.

After successfully emptying out that pesky stale gasoline, remember to inspect your equipment visually, ensuring there isn’t any leftover residue or debris inside because these could cause further damage down the line.

2. Make Sure Fuel Filter Isn’t Clogged

Now, let’s turn our attention to the fuel filter – that unsung hero quietly doing its job in your machine. Imagine it as a sentinel, standing guard against all those nasty dirt particles and sticky residues that would love nothing more than to gum up your engine.

Over time, however, this protector can become clogged with all the grime it has filtered out from the old gas that’s been sitting in your mower. This is when you’ll notice your lawn mower struggling to start or even failing to turn over altogether.

To check if your fuel filter is clogged and causing trouble, locate it first. It usually sits between the fuel lines running from the gas tank to the carburetor of your mower. The filter typically looks like a small plastic cylinder with an arrow on one side indicating the direction of fuel flow.

If you notice dirt or dark particles inside or if it seems discolored compared to a new one, chances are high that it’s due for replacement.

To replace a clogged fuel filter, simply unclip or unscrew it from its place between the two lines and insert a new one following the same orientation as before – remember to take note of where that arrow points! It should be directed towards where your lawn mower’s fuel travels through.

3. Make Sure Fuel Lines Aren’t Clogged

Having made sure your fuel filter is in top shape, it’s time to turn your detective skills towards the fuel lines – those vital conduits that carry the lifeblood of your machine, ensuring smooth and uninterrupted operation.

Over time, old fuel can leave sticky deposits that clog these lines, restricting the flow of fuel. It’s crucial to check for proper flow through these tubes to keep your mower running as efficiently as possible; this involves stopping and starting the fuel flow and observing its exit from the line.

Again the whole process isn’t too complicated:

  • Start by halting the fuel flow from the tank using either a shut-off valve or pinch pliers if no shut-off valve is available on your model.
  • Next, identify a section of the line and remove its end furthest from the tank; placing it into a container positioned lower than the tank will allow you to monitor how well (or not) it’s coming out.
  • If the line is clogged, remove that line entirely from your mower so it stands alone for inspection.
  • Now you have to spray carb cleaner into one end of the line ( this will help loosen any obstructions) and then follow up with blasts of compressed air aimed down towards where you sprayed earlier (this helps dislodge whatever grime may have been loosened).
  • Continue this back-and-forth until you’re satisfied that any impurities have been cleared out completely.
  • If the problem persists despite cleaning, make sure to replace the fuel line entirely.

Once done cleaning or replacing, reinstalling everything should be straightforward enough: just reattach where necessary and open up again for flowing gas into them. Your lawn mower should now be ready for action!

person fixing mower

4. Make Sure Carburetor Isn’t Dirty

Let’s shift our focus to the carburetor, an essential component that could be lurking behind your machine’s lackluster performance if it’s choked up with dirt and debris. The carburetor is a nifty little device that controls how much fuel is mixed with air for combustion.

When the gas in your mower gets old, it can lead to a buildup of gunk inside the carburetor. This can clog vital parts, causing them to freeze up and stop working effectively. Hence, it’s crucial you ensure this key player isn’t full in filth.

To do this, you’ll have to remove and clean the carburetor thoroughly. Start by disconnecting the spark plug for safety reasons before removing the air filter cover and air filter itself to expose the carburetor. Detach any connected hoses or linkages carefully and remember their configuration for easier reassembly later on.

Once detached, spray a good amount of carb cleaner into all holes and crevices of the part, making sure no area is left untouched.

However, sometimes a simple cleaning might not cut it, especially if internal components have been damaged over time due to grime accumulation or other factors.

Don’t worry if these tasks seem too hard to perform on your own. You always have options available: take your lawn mower either to a dedicated small engine mechanic or even back to your dealership, where specialists can professionally handle cleaning, rebuilding, or replacing your machine’s carburetor as needed.

Tip: Never Mix Old and New Gas

This may be counter-intuitive, but you shouldn’t mix old and new gas. The reason for this lies in the nature of gasoline; over time, it loses its combustible properties and leaves behind a varnish-like residue. This can cause fuel restrictions that result in your lawn mower running sluggishly or not at all.

It’s essential to dispose of old gasoline before adding fresh gasoline. Old gas can clog up your engine, resulting in poor performance or even failure to start. Disposing of old gas responsibly also helps protect our environment from potential harm caused by improper disposal.

So remember, while it might seem like a good idea to just top off your tank with some fresh gas when trying to start a lawn mower with old gas, doing so could actually worsen any existing issues within your engine.

Instead, take the time to properly dispose of any old gas first. This will ensure that you’re giving your lawn mower the best possible chance at firing up successfully after being left unused for a few months!

person oiling red mower


Starting a lawn mower with old gas may seem like a daunting task, but with a little bit of time, patience, and this handy guide, you can get your trusty machine up and running again in no time.

Remember, the key lies in regular maintenance and attention to detail. Don’t let old gas sit indefinitely in your mower; instead, ensure that it’s properly drained and replaced with fresh fuel to keep your machine performing at its best.

Whether you’re a seasoned gardener or a newbie homeowner, learning how to deal with old gas in a lawn mower is an essential skill that will serve you well. So roll up those sleeves, put on your safety glasses, and get ready to take on that challenge!

After all, there’s nothing quite like the satisfying hum of a well-maintained lawn mower slicing through your yard on a sunny day. Happy mowing!

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