Changing the spark plug on your lawn mower involves a few simple steps: locate the spark plug, replace it with a new one, and then give your mower a quick test.
Spark plugs are a crucial part of your lawn mower’s engine. It provides the spark that ignites the fuel-air mixture, making the engine run. Over time, spark plugs wear out or get dirty, leading to hard starts, rough performance, or loss of power. Replacing it improves how it works and makes your mower easier to start.
In this guide, I’ll explain the best step-by-step process to change your mower’s spark plugs. I suggest changing them after 25 to 50 hours of use or at the start of each season.
| Skill: Beginner to intermediate Estimated time: 15 to 30 minutes Cost: $4 to $50, depending on the spark plug model and tools you need to buy |
Tools you’ll need
- New spark plug: Check your lawn mower’s manual to find the correct type. You can also remove the old plug and take it to the store to get a matching replacement.
- Socket wrench with a spark plug socket: Most lawn mowers use either a 13/16-inch or ⅝-inch spark plug socket.
- Ratchet or adjustable wrench: To turn the socket.
- Spark plug gap gauge: To check and adjust the gap on the new spark plug.
- Wire brush or rag: To clean around the plug.
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First, a word on safety
First, make sure your mower is turned off and the engine is cool. Never work on a hot engine. If you just finished mowing, give the mower at least 15 to 20 minutes to cool down. This prevents burns and makes handling the spark plug easier.
Read more: Lawn Mower Safety Tips
Step 1: Locate the spark plug
For push mowers, the spark plug is typically located on the side or front of the engine, with a thick, rubber-coated wire (the spark plug wire) attached to it.
The spark plug is commonly located on the side of a riding mower’s engine block, often beneath the hood. Depending on the model, you may need to lift the hood or remove a small panel to access it. Some riding mowers have two spark plugs, so check your manual if you’re unsure.
Next, disconnect the spark plug wire to prevent the engine from accidentally starting while you work. It might take a little wiggle, but it should come off easily.
| My tip: Now is a good time to clean the area around the spark plug. Use a rag or small brush to remove dirt, grass, and debris. This step is important to prevent dirt from falling into the engine when you remove the plug. |
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Step 2: Remove the spark plug
To remove the spark plug, start by disconnecting the spark plug wire. Grasp the rubber boot (the thick cap at the end of the spark plug wire) and pull it off. If it’s stuck, gently twist it side to side while pulling.
Once the wire is disconnected, it’s time to loosen the spark plug. Use a spark plug socket attached to a wrench (a socket wrench with an extension works best). Fit the socket onto the spark plug, making sure it’s snug. Turn the wrench counterclockwise to loosen the spark plug. Once loosened, remove it by hand.
| My tip: If the spark plug wire is stubborn, you can use needle-nose pliers; just be sure to grab the boot, not the wire itself, to avoid damaging it. |
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Step 3: Inspect the old spark plug
Before installing a new spark plug, check the old one. The way it looks can give you valuable clues about your mower’s engine health. Here’s what to look for:
- Black and sooty: This means too much fuel is burning, possibly due to a dirty air filter or a carburetor issue.
- White, chalky deposits: The engine may be running too hot, which could indicate a cooling system problem or the use of the wrong type of fuel.
- Oily coating: This could be a sign of an oil leak, possibly from worn piston rings or valve seals.
- Worn or damaged electrode: If the metal tip appears eroded, burnt, or broken, the spark plug is past its prime and needs to be replaced.
If your old spark plug shows any of the above signs, it’s best to replace it. However, if you notice excessive buildup, oil, or unusual wear, you might want to check your mower’s air filter, fuel mixture, or oil levels to prevent future issues.
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Step 4: Prepare the new spark plug
Before installing your new spark plug, check and adjust the gap. The gap is the small space between the center electrode (the small metal rod in the center) and the curved metal piece (the ground electrode) at the tip of the spark plug. If the gap is too wide or too narrow, the spark might not ignite the fuel mixture correctly.
Start by finding the correct gap size. You can find this information in the owner’s manual. Most gaps are around 0.02 to 0.03 inches, but they can vary depending on the model. Once you have determined the correct size, use a gap gauge or feeler gauge to measure it accurately. Slide the correct-sized blade or gauge into the gap. It should fit snugly, neither too loosely nor too tightly.
If the gap needs adjusting, you can bend the curve electrode outward using the tool’s built-in adjuster or a small flathead screwdriver. If the gap is too wide, push the curve electrode inward using light pressure.
| My tip: Adjust the gap in small increments to avoid bending or damaging the electrode. |
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Step 5: Install the new spark plug
Once your new spark plug has the correct gap, it’s time to install it. First, thread it by hand. Hold the spark plug by the metal base and carefully screw it into the engine by hand. This helps prevent cross-threading, which can damage the engine’s cylinder head, according to the University of Illinois Extension’s mower experts.
After the plug is hand-tight, use your spark plug socket and wrench to complete the tightening process. Turn it about a quarter to half a turn more to secure it.
| My tip: Spark plugs are delicate, and over-tightening can strip the threads or make removal difficult in the future. |
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Step 6: Reconnect the spark plug wire
This step is simple yet essential; without a proper connection, your mower will not start. Grab the spark plug wire boot and push it firmly onto the spark plug. You should feel or hear a slight snap or click. After that, gently pull on the boot to ensure it’s firmly in place. If it comes off easily, push it back on more securely.
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Step 7: Test the lawn mower
Before putting your tools away, make sure everything is working properly. Start the mower as you normally would. It should start smoothly and run without problems.
As it runs, listen for anything unusual. If the mower struggles to start, runs rough, or stalls, double-check that the spark plug is tight and the wire is properly connected. If issues persist, recheck the spark plug gap or ensure you have installed the correct spark plug for your mower model.
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Troubleshooting common spark plug issues
If your mower is running rough, the spark plug appears oily, or it won’t start after a spark plug change, there’s usually a simple fix. Take a look at the chart below to help identify and resolve common spark plug issues in a lawn mower.
| Problem | Solution |
| The mower won’t start | Ensure the wire is securely attached to the spark plug. If not, replace the spark plug with a new one or check and adjust the gap to the correct size (approximately 0.02 to 0.03 inches). |
| The engine runs rough or misfires | Clean or replace the spark plug, tighten it properly, or refer to your manual for the correct spark plug. |
| Black, sooty spark plug | Check and clean the air filter and adjust the carburetor if needed. |
| White deposits on the spark plug | Check the cooling fins for debris and ensure the oil level is correct. |
| Oily spark plug | Check oil levels and inspect for leaks. |
Read more: How Often Should You Sharpen Lawn Mower Blades?
FAQ about spark plugs
Swap out the spark plug once per season or every 50 to 100 hours of use, advises Marcus Garner, extension agent at Alabama A&M University. However, even if you haven’t reached the recommended hours, replace the spark plug at the start of each season.
Yes, you can. However, if the electrodes are worn or the porcelain insulator is cracked, it’s time to throw it out. If the old spark plug appears clean and the electrodes aren’t too worn, you can clean it with a wire brush. Just be sure the plug is completely dry before reinstalling it.
Signs that your spark plug may be worn out include trouble starting the engine, reduced power, rough idling, and increased fuel consumption. If you notice any of these issues, it’s time to check your spark plug and possibly replace it.
No, you can’t use any spark plug in your lawn mower. Each mower requires a specific type and size of spark plug with the correct thread size, reach, and heat range to ensure the best engine performance.
Find a lawn care pro near you
Changing the spark plug is a simple maintenance task, but it’s just one part of proper mower care. Regular maintenance tasks, such as oil changes, blade sharpening, and air filter cleaning, can quickly add up, turning into a time-consuming chore that consumes your weekend.
If you’d rather leave lawn care to the pros, Lawn Love has local experts who can handle everything from mowing and fertilization to seasonal yard cleanup, so you can enjoy your yard without the hassle of maintaining the tools.
Sources:
- “Mower Maintenance.” University of Illinois Extension.
- “The 10 Steps of Lawn Mower Maintenance.” By Marcus Garner, Urban Regional Extension Agent. Alabama A&M University.
Main Photo Credit: Ernesto Andrade | Flickr | CC BY-ND 2.0
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