When your lawn mower engine sputters or loses power, inspecting and replacing the spark plug should be your first step. Learning how to change spark plug on lawn mower is a fundamental maintenance task that can restore performance and improve starting. This guide provides clear, step-by-step instructions to help you complete the job safely and correctly.
How To Change Spark Plug On Lawn Mower
Replacing a spark plug is a straightforward process that requires minimal tools. By following a logical sequence, you can ensure the new plug is installed properly, giving your mower a reliable spark for combustion. The core steps involve locating the plug, removing the old one, checking the gap on the new one, and installing it securely.
Tools And Materials You Will Need
Gathering the right equipment before you start makes the job smoother. You likely have most of these items already in your toolbox.
- Socket wrench or spark plug socket (usually 5/8″ or 13/16″)
- Spark plug gap gauge (feeler gauge)
- Ratchet extension (optional, for hard-to-reach plugs)
- A new, correct spark plug for your mower model
- Clean rag or shop towel
- A small amount of anti-seize compound (optional)
- A torque wrench (recommended for precision)
Safety Precautions Before You Begin
Your safety is paramount. Always take these precautions to prevent injury or damage to your equipment.
- Disconnect the spark plug wire. This is the most critical step to prevent accidental starting.
- Allow the engine to cool completely. A hot engine can cause burns.
- Work in a well-ventilated area, especially if you need to tip the mower.
- Clear your work area of debris and have a clean space to lay out parts.
Locating The Spark Plug On Your Mower
The spark plug is always attached to the engine cylinder. Look for a short, thick rubber wire boot, usually black or orange, coming from the engine. This boot is connected to the top of the spark plug. Trace this wire to find the plug. On most walk-behind mowers, it’s on the top or side of the engine and is easily visible.
Identifying The Correct Replacement Plug
Using the wrong spark plug can lead to poor performance or engine damage. The correct type is listed in your mower’s owner’s manual. If you don’t have the manual, you have two reliable options.
- Take the old spark plug to a hardware or auto parts store and ask for an identical replacement.
- Look up your mower’s model number (found on a decal on the engine or mower deck) online or at a dealership to get the exact part number.
Step-By-Step Removal Of The Old Spark Plug
Now, let’s get to the practical work. Follow these steps carefully to remove the old plug without causing issues.
Step 1: Disconnect The Spark Plug Wire
Firmly grasp the rubber boot at the end of the wire. Do not pull on the wire itself. Twist the boot slightly back and forth while pulling straight up to disconnect it from the plug. Set the wire aside where it cannot accidentally snap back onto the plug terminal.
Step 2: Clean The Area Around The Plug
Use a clean rag or a brush to wipe away any grass clippings, dirt, or debris from the area around the spark plug base. This prevents contaminants from falling into the engine cylinder when you remove the plug, which could cause serious damage.
Step 3: Loosen And Remove The Spark Plug
Fit your spark plug socket onto the plug. Attach the socket wrench or ratchet. Turn the wrench counterclockwise to loosen the plug. Once it’s loose, you can finish unscrewing it by hand. Carefully lift the old spark plug straight out of its hole.
Inspecting The Old Spark Plug
Before you discard the old plug, examine it. Its condition can tell you a lot about your engine’s health. A normal plug will have a light tan or gray coating on the electrode. Look for these common signs.
- Oily deposits: This may indicate too much oil in the combustion chamber.
- Heavy black, sooty carbon: This often suggests a rich fuel mixture or a dirty air filter.
- White, blistered electrode: This can point to an engine running too hot or a plug that is too hot for the engine.
- Worn or eroded electrode: This is simple wear and tear, confirming the need for replacement.
Preparing And Gapping The New Spark Plug
Most new spark plugs come pre-gapped, but it’s essential to verify the gap is correct for your specific engine. An incorrect gap can cause weak spark, hard starting, and poor fuel economy.
Step 1: Check The Required Gap
Find the correct gap specification in your mower’s manual or on the engine’s “spec” decal. It is usually between 0.028 and 0.035 inches. If you cannot find it, a common default is 0.030 inches.
Step 2: Measure The Gap With A Feeler Gauge
Slide the correct size of the feeler gauge between the center electrode and the curved side electrode. You should feel a slight drag as you pull the gauge through. If the gap is too wide or too narrow, it needs adjustment.
Step 3: Adjust The Spark Plug Gap
To widen the gap, gently pry open the side electrode with the gap tool’s adjuster. To narrow the gap, carefully tap the side electrode on a solid surface. Re-check the gap after each adjustment until it matches the specification. Be very gentle to avoid breaking the ceramic insulator.
Installing The New Spark Plug
With the new plug properly gapped, you are ready for installation. Taking care here ensures a good seal and proper operation.
Step 1: Apply Anti-Seize Compound (Optional)
A small dab of anti-seize compound on the threads of the new spark plug can prevent it from seizing in the aluminum engine head, making future removal easier. Use it sparingly; too much can interfere with the electrical ground or get on the electrodes.
Step 2: Hand-Thread The New Plug
Carefully insert the new spark plug into the hole. Start threading it by hand, turning it clockwise. It should thread in smoothly with little resistance. If it feels gritty or binds, stop immediately. Cross-threading can ruin the engine head, which is a costly repair. Back it out and try again, ensuring it’s straight.
Step 3: Tighten The Spark Plug
Once the plug is hand-tight, use your socket wrench to finish tightening. If you have a torque wrench, tighten to the manufacturer’s specification (often around 15-20 ft-lbs). If not, a good rule is to tighten it 1/8 to 1/4 of a turn after it seats. Avoid over-tightening, as this can damage the threads or crack the plug’s ceramic.
Reconnecting And Testing
The final steps are simple but crucial for completing the job.
Step 1: Reconnect The Spark Plug Wire
Push the rubber boot firmly onto the metal terminal of the new spark plug until you feel or hear it click into place. Ensure it is seated all the way down and is secure.
Step 2: Start The Mower And Check Operation
With the wire connected, you can start the mower. It should start more easily and run smoothly. If the engine still sputters or won’t start, double-check that the wire is fully connected and that you used the correct, properly gapped plug.
When To Change Your Lawn Mower Spark Plug
Regular replacement prevents problems before they start. A good maintenance schedule is to change the spark plug at least once per mowing season, typically in the spring. Also consider changing it if you notice any of these symptoms.
- Difficulty starting the engine
- Rough idling or engine misfiring
- Reduced power or acceleration
- Increased fuel consumption
- Visible damage or heavy deposits on the old plug
Common Mistakes To Avoid
Being aware of these common errors will help you achieve a successful repair.
- Forgetting to disconnect the spark plug wire first. This is a major safety risk.
- Not checking or adjusting the gap on the new spark plug.
- Over-tightening or cross-threading the new plug during installation.
- Allowing dirt to fall into the open spark plug hole.
- Using a spark plug that is not recommended for your engine model.
Frequently Asked Questions
How Often Should I Change The Spark Plug In My Lawn Mower?
For optimal performance, replace the spark plug at the beginning of each mowing season. If you use your mower extensively, consider changing it mid-season as well. An old plug can make starting difficult and reduces engine efficiency.
What Are The Signs Of A Bad Spark Plug In A Mower?
The main signs include the engine failing to start, hard starting requiring many pulls, rough running or misfiring while in use, and a noticeable lack of power when cutting grass. Visible inspection of a worn or fouled plug also confirms it’s bad.
Can I Clean An Old Spark Plug Instead Of Replacing It?
While you can clean light deposits off an old plug with a wire brush, it is generally not recommended. The electrodes wear down over time, and a cleaned plug will not perform as well as a new one. Replacement is inexpensive and guarantees a strong spark.
What Happens If The Spark Plug Gap Is Wrong?
An incorrect gap affects the spark’s strength and timing. A gap that is too wide may prevent the spark from jumping at all, causing a misfire. A gap that is too narrow produces a weak spark that may not reliably ignite the fuel mixture, leading to poor combustion and power loss.
Is It Safe To Change A Lawn Mower Spark Plug Myself?
Yes, it is one of the safest and simplest DIY repairs you can perform on a small engine. As long as you disconnect the spark plug wire before starting and follow the basic steps, it is a very low-risk maintenance task that saves you time and money.