When your mower won’t start, checking the condition of the spark plug should be one of your first troubleshooting steps. Learning how to check a spark plug on a lawn mower is a simple skill that can save you time and money. A faulty spark plug is a common culprit behind hard starting, rough running, and poor performance.
This guide will walk you through the entire process, from safe removal to reading the plug’s condition. You’ll need just a few basic tools and about 15 minutes of your time.
How To Check A Spark Plug On A Lawn Mower
This section provides the complete, step-by-step procedure. Always ensure the engine is completely cool and the ignition key or wire is disconnected before you begin any work.
Tools And Safety Equipment You Will Need
Gathering the right tools before you start makes the job smoother and safer. Here is what you’ll need:
- Spark plug socket wrench (usually 13/16″ or 5/8″) with a ratchet and extension
- A new spark plug (check your mower’s manual for the correct type and gap)
- A spark plug gap tool (feeler gauge or coin-style gapper)
- A small wire brush or piece of emery cloth
- Safety glasses
- Work gloves (optional but recommended)
Step 1: Locate And Disconnect The Spark Plug Wire
First, find the spark plug. It is typically located on the side of the engine cylinder, covered by a thick, rubber boot. This boot is the spark plug wire. Firmly grasp the boot (not the wire itself) and twist it slightly while pulling straight back to disconnect it. This prevents damage to the wire connection.
Important Safety Note
Never attempt to remove the spark plug while the engine is running or hot. Always confirm the wire is disconnected to prevent any accidental starting.
Step 2: Remove The Spark Plug
Place your spark plug socket over the plug and attach the ratchet. Turn the ratchet counter-clockwise to loosen the plug. Once it’s loose, you can often finish unscrewing it by hand. Be careful to keep the socket straight to avoid cross-threading or damaging the ceramic insulator.
If the plug is very tight, do not force it. Apply a small amount of penetrating oil around the base and let it sit for a few minutes before trying again. Once removed, inspect the threads on the plug and in the engine for any damage.
Step 3: Inspect The Spark Plug’s Physical Condition
Now you can examine the plug. Look closely at the electrode end (the small metal L-shaped hook and the center electrode) and the ceramic insulator. Here are the key things to check:
- Electrode Wear: The electrodes should have sharp, square edges. If they are rounded, eroded, or the gap looks excessively wide, the plug needs replacement.
- Carbon Deposits: Light tan or gray deposits are normal. Heavy, dry, sooty black carbon indicates a rich fuel mixture or a dirty air filter.
- Oil Fouling: Wet, black, oily deposits suggest oil is leaking into the combustion chamber, which is a more serious engine issue.
- Damage: Check for cracks in the ceramic insulator or a badly bent or melted electrode. These are clear signs the plug must be replaced immediately.
Step 4: Check For Spark (If The Engine Won’t Start)
If your primary issue is a non-starting engine, a simple spark test is crucial. Reconnect the spark plug wire to the removed plug. With insulated pliers, hold the plug’s metal body against a clean, unpainted part of the engine block, ensuring the electrode is not touching anything.
Have someone pull the starter cord (or turn the key if electric start) while you watch the electrode gap. You should see a bright blue spark jump across the gap. A weak yellow spark or no spark at all confirms a faulty ignition system—either the plug, the ignition coil, or a safety switch.
Be very careful not to touch the metal part of the plug during this test, as it can deliver a painful shock.
Step 5: Clean And Gap The Old Plug (Or Prepare The New One)
If the old plug looks reasonably clean and the electrodes are in good shape, you can clean it. Use the wire brush to gently remove deposits from the electrode and threads. Do not scrub the ceramic insulator aggressively.
Next, check the gap. Insert your gap tool between the center and side electrode. The correct gap is specified in your mower manual, but it’s often between .028″ and .035″. The tool should slide through with a slight drag. If the gap is incorrect, carefully bend the side electrode to adjust it. For a new plug, always check and adjust the gap before installation, as it may not be set correctly from the factory.
Step 6: Reinstall The Spark Plug
Begin threading the plug into the engine by hand to ensure you don’t cross-thread it. Once it’s hand-tight, use the socket wrench to tighten it. Do not overtighten. A good rule is to tighten it about 1/8 to 1/4 turn after it seats. Finally, push the rubber boot firmly onto the end of the spark plug until you feel or hear it click into place.
How To Read Your Spark Plug’s Condition
The color and deposits on your spark plug tell a story about your engine’s health. Here’s a quick guide to diagnosing common conditions.
Normal Condition: Light Tan Or Gray Deposits
A plug with light tan, gray, or brownish deposits on the insulator tip is ideal. This indicates the engine is running efficiently with a good air-fuel mixture and proper combustion. If your plug looks like this, simply clean, gap, and reinstall it.
Carbon Fouled: Dry, Black, Sooty Deposits
This is often caused by a dirty air filter, a clogged fuel jet, or excessive idling. The engine is running too “rich,” meaning too much fuel is entering the cylinder. Clean or replace the air filter, check the carburetor settings, and clean the plug thoroughly before reinstalling.
Oil Fouled: Wet, Shiny Black Deposits
Wet, oily deposits signal that engine oil is leaking into the combustion chamber. This could be due to worn piston rings, valve guides, or a overfilled crankcase. While cleaning the plug might provide a temporary fix, this condition usually requires further engine investigation.
Overheated: White Or Blistered Insulator
A white, blistered, or melted electrode tip means the plug is running too hot. Causes include an incorrect plug heat range, engine overheating, or a lean fuel mixture. Check for cooling fins blockage and ensure you’re using the correct spark plug type specified by the manufacturer.
When To Replace A Lawn Mower Spark Plug
Even with proper maintenance, spark plugs wear out. Here are the signs that indicate it’s time for a new one, not just a cleaning.
- You notice reduced engine power or poor acceleration.
- The mower is difficult to start or frequently stalls.
- Fuel consumption has noticeably increased.
- Visible damage like a cracked insulator or severely eroded electrodes.
- As a general rule of thumb, replace the spark plug at least once per mowing season for reliable performance.
Preventative Maintenance Tips
A little regular care can extend the life of your spark plug and your mower’s engine. Follow these simple tips.
Use Fresh Fuel And Stabilizer
Old gasoline can leave gummy deposits that foul spark plugs. Always use fresh fuel and consider adding a fuel stabilizer if you store your mower for more than 30 days. This helps keep the entire fuel system clean.
Keep The Air Filter Clean
A dirty air filter restricts airflow, causing a rich fuel mixture that leads to carbon fouling. Check and clean your paper or foam air filter regularly, and replace it according to your mower’s maintenance schedule.
Follow A Seasonal Tune-Up Schedule
At the beginning of each mowing season, perform a basic tune-up. This should include replacing the spark plug, changing the oil, and cleaning or replacing the air filter. This proactive approach prevents many common starting and running problems.
Frequently Asked Questions
How Often Should I Check My Lawn Mower Spark Plug?
You should visually inspect and clean your spark plug at least once per mowing season. It’s a good idea to check it if you experience any starting or performance issues. For optimal performance, replace it annually.
What Does A Bad Spark Plug Look Like On A Lawn Mower?
A bad spark plug may have heavily corroded or melted electrodes, a cracked ceramic insulator, or be covered in thick, wet, oily deposits. Any of these conditions mean the plug should be replaced.
Can A Lawn Mower Run With A Bad Spark Plug?
It might run, but poorly. Symptoms include hard starting, rough idle, lack of power, and increased fuel consumption. In many cases, a bad plug will prevent the engine from starting at all.
What Is The Correct Gap For A Lawn Mower Spark Plug?
The correct gap varies by engine model. Always consult your owner’s manual for the precise specification. A common range for many small engines is between 0.028 inches and 0.035 inches.
Why Does My Spark Plug Keep Getting Dirty?
Recurring fouling is usually a symptom of another problem. Common causes are a dirty air filter, a malfunctioning carburetor causing a rich mixture, or, in the case of oil fouling, internal engine wear that allows oil into the combustion chamber.