A simple visual inspection of the spark plug in your lawn mower can tell you a lot about its health. Learning how to check spark plug in lawn mower is a fundamental skill that can save you time and money. This guide provides clear, step-by-step instructions to help you perform this essential maintenance task confidently.
A faulty spark plug is a common reason for a mower that won’t start, runs rough, or lacks power. By checking it regularly, you can diagnose problems early and keep your engine running smoothly. The process is straightforward and requires only a few basic tools.
How To Check Spark Plug In Lawn Mower
This section outlines the complete process, from safely removing the plug to interpreting what you see. Always ensure the engine is completely cool and the ignition key or wire is disconnected before you begin any work.
Tools And Safety Precautions You Will Need
Gathering the right tools beforehand makes the job easier and safer. You do not need a professional mechanic’s toolkit for this simple task.
Essential tools include a spark plug socket wrench, which typically comes in sizes like 13/16″ or 5/8″, a socket extension, and a gap gauge tool. A small wire brush and some clean cloth are also helpful. For safety, wear protective gloves and safety glasses to shield your eyes from any debris.
- Spark plug socket wrench
- Socket extension
- Spark plug gap gauge
- Wire brush (non-metallic is best)
- Clean cloth or rag
- Safety glasses and gloves
Step-By-Step Removal Of The Spark Plug
Proper removal prevents damage to the plug threads and the engine itself. Rushing this step can lead to costly repairs.
- Locate the spark plug. It is usually on the side or top of the engine cylinder, connected to a thick, rubber-insulated wire.
- Firmly grasp the rubber boot and pull it straight off the spark plug terminal. Do not pull on the wire itself, as this can damage the connection.
- Use a clean rag to brush away any grass clippings or dirt from around the spark plug base. This prevents debris from falling into the engine cylinder once the plug is removed.
- Insert your spark plug socket onto the plug. Attach the extension and ratchet handle. Turn the wrench counter-clockwise to loosen the plug.
- Once loose, unscrew the plug by hand the rest of the way. Carefully lift it straight out of the engine head.
Performing A Visual Inspection And Reading The Signs
This is the core of the check. The condition of the spark plug’s electrodes and insulator provides clear clues about your engine’s internal condition and fuel mixture.
Normal Wear And Tear
A healthy, functioning plug will have a light tan or grayish color on the insulator tip (the ceramic part) and the ground electrode. The electrodes may show slight, even rounding from use. This indicates good combustion and a proper air-fuel mixture.
Common Problem Signs And What They Mean
Different deposits or damage point to specific issues. Here is how to interpret what you find.
- Carbon Fouled (Dry, Sooty Black Deposits): This suggests a rich fuel mixture, a dirty air filter, weak ignition, or excessive idling. The engine may be getting to much fuel.
- Oil Fouled (Wet, Shiny Black Deposits): Oil is leaking into the combustion chamber, often due to worn piston rings, valve guides, or a overfilled crankcase. This requires further engine investigation.
- Ash Deposits (Crusty White Or Gray Buildup): Typically caused by burning oil or fuel additives. It can lead to pre-ignition if severe.
- Worn Or Eroded Electrodes: The center and ground electrode appear excessively rounded, thin, or have a large gap. This is simply the result of long service and means the plug needs replacement.
- Burned Or Blistered Electrodes/Insulator: A white, blistered look indicates the plug is running too hot. Causes include an incorrect plug heat range, lean air-fuel mixture, or insufficient engine cooling.
- Damaged Threads Or Ceramic: Physical damage, often from incorrect installation or impact. The plug must be replaced.
Checking And Adjusting The Spark Plug Gap
The gap is the distance between the center and ground electrode. An incorrect gap affects spark strength and engine performance. Always check this, even on a new plug labeled as “pre-gapped.”
- Consult your mower’s owner’s manual for the correct gap specification. Common gaps are between 0.028″ and 0.035″.
- Use a wire-style gap gauge for accuracy. Slide the correct wire between the electrodes; it should fit with a slight drag.
- If the gap is too wide, gently tap the ground electrode on a hard surface to close it slightly. If it is to narrow, carefully pry the ground electrode open with the gauge tool’s bending accessory. Never pry against the center electrode or ceramic insulator.
- Re-check the gap after any adjustment until it matches the specification.
Testing For Spark (Optional But Recommended)
If your mower won’t start, this test confirms whether the ignition system is producing a spark. It is a crucial diagnostic step.
Reconnect the spark plug wire to the plug. Hold the plug’s metal body against a clean, unpainted part of the engine cylinder head with insulated pliers. Ensure the electrodes are visible. Have a helper briefly pull the starter cord or turn the ignition key. You should see a strong, bright blue spark jump across the electrode gap. A weak yellow spark or no spark at all indicates an ignition system problem beyond the plug.
Cleaning And Reinstalling The Spark Plug
If the plug is in good condition aside from light deposits, you can clean it for reuse. For any signs of oil, severe wear, or damage, replacement is the better option.
Use a specialized spark plug cleaner or a non-metallic wire brush to gently remove deposits from the insulator tip and electrodes. Avoid abrasive metal brushes that can leave conductive traces. Wipe the plug clean with a dry cloth. Before reinstalling, inspect the threads on both the plug and in the engine. Hand-thread the plug into the engine head clockwise to avoid cross-threading. Once seated, tighten it with the wrench for about a quarter to a half turn further, just until snug. Over-tightening can damage threads or crack the ceramic. Finally, push the rubber boot back onto the terminal until you feel or hear a firm click.
When To Replace Your Lawn Mower Spark Plug
Regular replacement is part of routine maintenance, even if the plug appears okay. A worn plug forces the ignition system to work harder.
- At least once per mowing season, typically in the spring.
- If visual inspection shows any fouling, excessive wear, or damage.
- If the mower is hard to start, runs unevenly, misfires, or has reduced power.
- If you’ve put approximately 25-50 hours of operation on the engine since the last change.
Always replace with the exact type and heat range recommended by your mower manufacturer. Using the wrong plug can cause poor performance or engine damage.
Frequently Asked Questions
How Often Should I Check My Lawn Mower Spark Plug?
You should visually inspect your spark plug at least once per mowing season. A good practice is to check it in the spring during your annual mower tune-up and again if you experience any starting or running problems throughout the season.
What Does A Bad Spark Plug Look Like In A Lawn Mower?
A bad spark plug may have cracked ceramic, heavily eroded or melted electrodes, or be covered in thick, wet oil deposits or dry, black soot. Any of these signs indicate the plug is not functioning correctly and should be replaced.
Can I Clean And Reuse A Spark Plug?
You can clean and reuse a spark plug if it only has light, dry carbon deposits and the electrodes are in good shape with a correct gap. However, for oil-fouled, badly worn, or damaged plugs, replacement is more reliable and cost-effective in the long run.
What Happens If The Spark Plug Gap Is Too Big?
If the spark plug gap is to wide, the ignition coil may not be able to generate a strong enough spark to jump the gap consistently. This leads to engine misfires, hard starting, rough idling, and a loss of power under load.
Why Does My New Spark Plug Get Black So Quickly?
Quick blackening, or carbon fouling, usually points to an overly rich air-fuel mixture. Common causes include a dirty air filter, a clogged fuel vent, a malfunctioning carburetor, or operating the engine at idle for to long. Address these underlying issues to prevent the new plug from fouling again.