Learning how to change a spark plug on a lawn mower is a simple DIY task that every mower owner should know. Changing the spark plug on your mower is a routine maintenance task that can significantly improve starting and performance. A fresh plug ensures a strong spark for reliable ignition, leading to smoother operation and better fuel efficiency. This guide will walk you through the entire process, from gathering tools to the final test start.
How To Change A Spark Plug On A Lawn Mower
This section provides the complete, step-by-step procedure. Before you begin, always prioritize safety. Ensure the engine is completely cool to the touch and disconnect the spark plug wire to prevent any accidental starting. Let’s get started with what you’ll need.
Tools And Materials You Will Need
Having the right tools makes the job quick and easy. You likely already have most of these items in your garage. Here is your checklist:
- A new, correct spark plug (check your owner’s manual for the exact type)
- A spark plug socket wrench (usually 13/16″ or 5/8″ size) with a ratchet and extension
- A gap tool or feeler gauge
- A small bit of clean rag or paper towel
- A small wire brush (optional, for cleaning the area)
- A torque wrench (optional, but recommended for proper tightening)
Step 1: Locate And Disconnect The Spark Plug Wire
First, find the spark plug. It is typically located on the front or side of the engine cylinder, covered by a thick, rubber boot. Firmly grasp the boot (not the wire itself) and pull it straight off the plug’s terminal. This disconnects the ignition system. It’s a good idea to visually inspect the wire and boot for any cracks or damage while you have it off.
Safety Reminder
Always disconnect the spark plug wire before performing any work. This is the most critical saftey step, as it ensures the engine cannot start accidentally while your hands are near the blade or plug.
Step 2: Remove The Old Spark Plug
Now, use your spark plug socket and ratchet. Place the socket squarely over the old spark plug. Turn the ratchet counter-clockwise to loosen and remove the plug. You may need a short extension if the plug is recessed. Once loose, you can finish unscrewing it by hand. Be careful not to tilt it; you want to keep the threads aligned as it comes out.
Step 3: Inspect The Old Spark Plug
Before you discard the old plug, take a moment to examine it. Its condition can tell you a lot about your engine’s health. Look at the electrode (the small metal tip) and the ceramic insulator. A normal plug will have light tan or gray deposits. Here are some common conditions and what they might indicate:
- Oily, Black Deposits: This can suggest too much oil in the combustion chamber, possibly from overfilling or engine wear.
- Dry, Sooty Black Deposits: Often points to a rich fuel mixture (too much fuel, not enough air).
- White, Blistered Electrode: This indicates the engine is running too hot, which could be due to a lean fuel mixture or incorrect ignition timing.
- Heavy Carbon Buildup: Can be a sign of infrequent use or prolonged idling.
Step 4: Check And Set The Gap On The New Spark Plug
Most new spark plugs come pre-gapped, but it’s always wise to double-check. The “gap” is the space between the center and ground electrode. Find the correct gap specification in your mower’s manual; a common gap is 0.030 inches. Insert your gap tool between the electrodes and adjust gently by bending the ground electrode only until you feel a slight drag on the tool. Avoid touching the center electrode or ceramic insulator.
Step 5: Install The New Spark Plug
Begin by threading the new spark plug into the hole by hand. This is crucial to avoid cross-threading, which can damage the engine head. Turn it clockwise until it is finger-tight. If it doesn’t thread in smoothly, stop and realign it. Do not force it. Once hand-tight, use your socket wrench to finish tightening.
Proper Tightening Torque
Overtightening can crack the plug or the engine head, while under-tightening can lead to poor compression and overheating. If you have a torque wrench, tighten to the specification in your manual (often 15-20 ft-lbs). If not, a good rule is to tighten the plug an additional 1/8 to 1/4 turn after it is snug. Avoid using excessive force.
Step 6: Reconnect The Spark Plug Wire
Finally, take the rubber boot and push it firmly onto the terminal of the new spark plug until you feel or hear it click into place. Ensure it is seated properly to prevent it from coming off during operation. Give the boot a gentle tug to confirm it’s secure. Your spark plug change is now complete.
Step 7: Test Your Lawn Mower
With the wire securely connected, you can now test your work. Replenish fuel and oil if needed. Start the mower as you normally would. A quick, smooth start is a good sign your new plug is working correctly. Listen for a steady engine sound without sputtering or misfiring. If the mower starts easier than before, you’ve successfully completed the maintenance.
Choosing The Right Spark Plug
Not all spark plugs are the same. Using the wrong type can lead to poor performance or even engine damage. The best way to find the correct plug is to check your lawn mower’s owner manual. It will list the exact part number and specifications. If you’ve lost the manual, you can often find the information on the engine manufacturer’s website using your engine model number.
Understanding Spark Plug Codes
Spark plugs have a specific code printed on them, like “RJ19LM” or “CR7HS.” This code indicates its size, heat range, and thread type. The heat range is particularly important; a plug that runs too hot can cause pre-ignition, while one that runs too cold can foul with deposits. Sticking with the manufacturer’s recommended code is the safest bet.
Common Spark Plug Types For Small Engines
- Copper Core Plugs: Standard, affordable, and reliable. They have good conductivity but a shorter service life.
- Iridium or Platinum Plugs: More expensive, but offer longer life, better performance, and more consistent sparks. They are often used in newer mowers.
When To Change Your Lawn Mower Spark Plug
Regular replacement prevents problems before they start. A good general rule is to change your lawn mower spark plug at least once per mowing season, ideally in the spring during your annual tune-up. However, there are specific signs that indicate a plug needs changing sooner.
Signs Of A Failing Spark Plug
- Difficulty Starting: The engine cranks but won’t fire up, or it takes many pulls to start.
- Rough Idling or Misfiring: The engine runs unevenly, sputters, or sounds like it’s struggling at low speed.
- Poor Fuel Economy: You notice your mower is using more gas than usual to cut the same area.
- Loss of Power: The mower lacks its usual cutting power, especially in thick grass.
- Visible Damage: Upon inspection, the plug shows cracks, heavy corrosion, or a damaged electrode.
Frequently Asked Questions
How Often Should I Change My Lawn Mower Spark Plug?
For optimal performance, replace the spark plug once every season or after every 25-50 hours of operation. If you use your mower heavily, consider changing it mid-season. An annual spring replacement is a simple habit that ensures reliable starts.
Can I Clean And Reuse An Old Spark Plug?
While you can clean a lightly fouled plug with a wire brush and solvent, it’s not generally recommended. The electrodes wear down over time, and a cleaned plug won’t perform as well as a new one. Given the low cost of a new spark plug, replacement is the more reliable and effective option.
What Happens If I Don’t Change My Spark Plug?
A worn-out spark plug can lead to hard starting, wasted fuel, reduced engine power, and increased exhaust emissions. In severe cases, a badly misfiring engine can cause unburned fuel to wash down the cylinder walls, leading to premature engine wear.
Why Does My New Spark Plug Keep Getting Dirty?
If a new plug fouls quickly (becomes black and sooty), it points to an underlying engine issue. Common causes include a dirty air filter, a clogged carburetor, an incorrect fuel mixture, or using old gasoline. Address these problems to prevent recurring plug fouling.
Is It Safe To Change A Spark Plug Myself?
Yes, it is one of the safest and easiest DIY maintenance tasks for a lawn mower. As long as you ensure the engine is cool and always disconnect the spark plug wire first, the risk is minimal. Following the steps outlined here will help you complete the job safely and correctly.
Changing your lawn mower’s spark plug is a straightforward task that pays off in better performance and reliability. By following these clear steps and understanding when and why to do it, you can keep your mower running smoothly for seasons to come. Remember to always consult your specific model’s manual for the most accurate part numbers and specifications. With a few basic tools and about 15 minutes, you can complete this essential piece of maintenance and enjoy a mower that starts on the first pull.