If your lawn mower is hard to start or running rough, learning how to change the spark plug in a lawn mower is one of the easiest and most effective fixes you can do. Replacing a worn spark plug is a simple fix that can restore your mower’s starting power and improve its overall performance.
This guide will walk you through the entire process, from gathering tools to the final test start. You don’t need to be a mechanic to complete this basic maintenance task.
With just a few common tools and about 15 minutes, you can have your mower running like new again.
How To Change The Spark Plug In A Lawn Mower
This section provides the complete, step-by-step procedure. Before you begin, always prioritize safety. Ensure the mower is on a flat, stable surface and that the engine is completely cool to the touch. Disconnect the spark plug wire as your first step to prevent any accidental starts.
Tools And Materials You Will Need
Gathering the right items before you start makes the job go smoothly. You likely have most of these in your garage already.
- A new spark plug (correct type for your mower)
- A spark plug socket wrench (usually 13/16″ or 5/8″) 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 but helpful)
- A torque wrench (optional for precise tightening)
Step 1: Locate And Disconnect The Spark Plug Wire
The spark plug is a small, porcelain-coated component typically found on the side of the engine. It has a thick, rubber-insulated wire attached to its top. Firmly grasp the rubber boot (not the wire itself) and pull it straight off the plug. This isolates the ignition system.
Step 2: Remove The Old Spark Plug
Use your spark plug socket and ratchet to loosen the old plug. Turn it counter-clockwise. Once it’s loose, you can finish unscrewing it by hand. Be careful to keep the socket straight on the plug to avoid damaging the porcelain or the threads in the engine.
Once removed, take a moment to inspect the old plug. Its condition can tell you a lot about your engine’s health, which we’ll cover in a later section.
Step 3: Check And Adjust The Gap On The New Plug
Most new spark plugs come pre-gapped, but it’s a good habit to check. The “gap” is the space between the center and side electrode. Find the correct gap specification in your mower’s manual; it’s often between .028″ and .035″.
Insert your gap tool between the electrodes. If the gap is too wide, gently tap the side electrode on a hard surface to close it. If it’s too narrow, carefully pry the side electrode open with the tool’s adjuster. Avoid touching the center electrode as you can damage it.
Step 4: Install The New Spark Plug
Begin by threading the new spark plug into the hole by hand. This is crucial to prevent cross-threading, which can damage the engine head. If it doesn’t turn easily, stop and realign it.
Once it’s hand-tight, use your socket wrench to tighten it further. A good rule of thumb is to tighten it about 1/8 to 1/4 of a turn after it seats. If you have a torque wrench, refer to your manual for the exact specification, usually around 15-20 ft-lbs. Overtightening can crack the porcelain or strip the threads.
Step 5: 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 the connection is secure so the wire won’t vibrate off during operation.
Step 6: Test Your Work
With the wire securely connected, you can now test the mower. If you had fuel or other issues, adress those first. Then, try starting the engine. A quick, smooth start and even run are signs of a successful spark plug change.
If the mower doesn’t start, double-check that the wire is fully seated on the plug. Also, verify you have fresh fuel and that the engine stop lever or key is in the “on” position.
Choosing The Correct Replacement Spark Plug
Using the right spark plug is essential. The wrong type can cause poor performance or even engine damage.
Where To Find Your Spark Plug Model Number
The easiest way to find the correct plug is to check your lawn mower’s owner’s manual. The manual will list the exact spark plug type and gap. If you’ve lost the manual, you can often find the information on the engine manufacturer’s website using your engine model number.
This model number is usually stamped on a metal tag or sticker directly on the engine. As a last resort, take the old spark plug to a hardware store or small engine shop; they can match it for you.
Understanding Spark Plug Heat Range And Types
Spark plugs are designed with different “heat ranges.” A plug that runs too hot can cause pre-ignition; one that runs too cold can foul with carbon. Always use the heat range specified by your engine manufacturer.
Common brands for small engines include Champion, NGK, and Autolite. For most standard lawn mower engines, a resistor-type plug is common, which helps reduce radio frequency interference.
Diagnosing Engine Issues From The Old Spark Plug
The appearance of your old spark plug can be a useful diagnostic tool. Here’s what different conditions might indicate.
Normal Wear
A normally worn plug will have light tan or gray deposits on the firing tip. The electrodes will be worn but not excessively. This suggests the engine is running correctly and the plug was simply at the end of its service life.
Carbon Fouled (Black, Sooty Deposits)
A black, dry, sooty coating indicates a rich fuel mixture or weak ignition. Possible causes include a dirty air filter, excessive choking, or too much oil in the fuel mix for 2-stroke engines. A spark plug that is too cold for the operating conditions can also lead to this.
Oil Fouled (Wet, Black Deposits)
Wet, oily deposits on the plug suggest oil is leaking into the combustion chamber. This could be due to worn piston rings, valve guides, or a overfilled crankcase. This often accompanies blue smoke from the exhaust.
Overheated (White Or Blistered Electrode)
A white insulator tip or blistered electrodes mean the plug is running too hot. Causes include an incorrect (too hot) spark plug, lean fuel mixture, insufficient engine cooling, or incorrect ignition timing. This condition can lead to serious engine damage if not corrected.
Essential Lawn Mower Spark Plug Maintenance Tips
Regular attention can prevent problems and extend the life of your mower.
Recommended Replacement Schedule
For optimal performance, replace your lawn mower spark plug at least once per mowing season. If you use your mower extensively (over 25 hours per season), consider changing it mid-season. An annual replacement is a cheap and effective way to maintain easy starting.
Always install a new plug at the beginning of the spring season after winter storage.
Proper Storage To Prevent Fouling
Before storing your mower for the winter, add a fuel stabilizer to the tank and run the engine for a few minutes to circulate it. This prevents old fuel from varnishing and fouling the plug. Alternatively, you can run the carburetor dry.
As part of your winterization, you can remove the spark plug, add a teaspoon of engine oil into the cylinder, and slowly pull the starter cord to coat the cylinder walls. Then reinstall the old plug or put in a new one for spring.
Common Mistakes To Avoid
- Overtightening the spark plug: This is the most common error and can ruin the engine head.
- Forgetting to disconnect the wire first: Always, always disconnect the spark plug wire before any maintenance to prevent accidental starting.
- Not checking the gap: Assuming a new plug is perfectly gapped can lead to poor performance.
- Using the wrong tool: A standard deep socket can crack the porcelain insulator; always use a proper spark plug socket with a rubber insert.
- Ignoring the root cause: If a new plug fouls quickly, you have another issue that needs fixing, like a dirty air filter or carburetor problem.
Frequently Asked Questions
How Often Should You Change A Lawn Mower Spark Plug?
You should change your lawn mower spark plug at least once per year, ideally at the start of the mowing season. For heavy use, consider replacing it every 25 hours of operation. A fresh plug ensures reliable starts and efficient fuel burn.
What Are The Signs Of A Bad Spark Plug In A Mower?
Common signs include difficulty starting, rough engine idle, misfiring or surging during operation, reduced power and fuel efficiency, and visible damage or heavy deposits on the plug itself. If your mower won’t start, checking the spark plug is a great first step.
Can You Clean And Reuse An Old Spark Plug?
While you can clean a lightly fouled plug with a wire brush and regap it, it’s generally not recommended. The materials degrade with heat and use. For the small cost of a new plug, you get reliable performance and peace of mind. It’s best to just install a new one.
What Size Socket Do I Need For A Lawn Mower Spark Plug?
Most lawn mower spark plugs require either a 13/16-inch (21mm) or 5/8-inch (16mm) deep socket. A dedicated spark plug socket has a rubber insert that grips the plug to prevent damage and makes removal easier. Always check your old plug or manual to be sure.
Why Does My New Spark Plug Not Fix My Mower’s Starting Problem?
If a new plug doesn’t solve a starting issue, other likely culprits include old or contaminated fuel, a clogged air filter, a faulty ignition coil, a closed fuel valve, or a problem with the carburetor. The spark plug is just one component of the ignition and fuel systems that need to work together.