If your lawn mower won’t start or is running rough, a faulty spark plug is a common culprit. Knowing where is the spark plug on a lawn mower is the first step to checking or replacing it, and you’ll almost always locate it under the engine cover.
This small but vital part is responsible for igniting the fuel mixture in the engine. Finding it is a simple task once you know what to look for. Let’s get your mower running smoothly again.
Where Is The Spark Plug On A Lawn Mower
On nearly every walk-behind or riding lawn mower, the spark plug is positioned on the side of the engine. It’s covered by a rubber boot and connected to a thick wire. To access it, you must first remove a plastic shroud or metal cover that protects the engine’s top and sides.
This cover is held on by a few screws, clips, or sometimes just friction. Its main job is to keep grass clippings and debris out of the engine’s moving parts. Once that cover is off, finding the spark plug is straightforward.
What Does a Lawn Mower Spark Plug Look Like?
You’re looking for a short, porcelain-white insulator (which can be dirty) attached to a metal base. It’s about the size and shape of a large finger. A thick, rubbery wire called the spark plug lead or boot will be attached to its top.
Here are its key features:
- Porcelain Insulator: The white, ceramic-like top section.
- Hex Nut: A six-sided metal nut used for tightening and loosening with a socket wrench.
- Metal Body: The threaded portion that screws into the engine cylinder.
- Electrode: The small metal hook or post at the very tip inside the engine.
Step-by-Step: How to Locate Your Spark Plug
Always safety first. Before you start, ensure the mower is on a flat, level surface. Disconnect the spark plug wire to prevent any accidental starts. If it’s a ride-on mower, also remove the key.
Step 1: Prepare Your Mower
Turn the engine off and let it cool completely. Hot engines and parts can cause burns. Pull the wire boot off the spark plug by gripping it firmly and twisting it slightly while pulling upwards. This isolates the ignition system.
Step 2: Remove the Engine Cover
Look for the engine cover. On most push mowers, it’s a single plastic piece covering the top. It might be labeled “DO NOT REMOVE” or have a warning symbol—this is normal. Use a screwdriver or socket to remove any fasteners. Sometimes the cover just snaps off.
For riding mowers, you may need to open the hood. Consult your manual if your unsure, as some models have latches or specific release procedures.
Step 3: Identify the Spark Plug
With the cover off, look at the engine block. Follow the thick, rubber ignition wire from where it was dangling. It will lead you directly to the spark plug, which is screwed into a hole in the engine’s cylinder head. You can’t miss it.
How to Remove and Inspect the Spark Plug
Now that you’ve found it, you can check its condition. You’ll need a spark plug socket wrench, which has a rubber insert to grip the plug. A standard deep-well socket might also work.
- Blow away any loose debris from around the spark plug base. You don’t want dirt falling into the engine.
- Fit your socket wrench snugly over the spark plug’s hex nut. Turn it counter-clockwise to loosen. It should unscrew easily once broken free.
- Lift the old spark plug straight out. Examine the electrode tip.
A healthy spark plug will have a light tan or gray coating on the electrode. Here’s what to look for:
- Black, Sooty Deposits: Often means the engine is running too “rich” (too much fuel).
- Oily, Wet Deposits: Indicates oil is leaking into the combustion chamber.
- White, Blistered Electrode: Suggests the engine is running too “hot” or the plug heat range is wrong.
- Heavy Carbon Buildup: Can be from a dirty air filter or too much idling.
If the plug is damaged or heavily worn, it’s time for a new one. Make sure you get the correct replacement by taking the old one to the store or checking your mower’s manual.
Installing a New Spark Plug Correctly
Putting the new plug in is just as important as taking the old one out. A mistake here can cause poor performance or even engine damage.
- Check the gap of the new spark plug using a gap tool. The correct measurement (usually between .028″ and .035″) is in your manual or on the engine sticker. Bend the side electrode gently to adjust.
- Carefully thread the new spark plug into the hole by hand. Turning it clockwise. Do not cross-thread it, as this can strip the engine head.
- Once it’s hand-tight, use the socket wrench to give it a final snug turn—about 1/8 to 1/4 of a rotation. Overtightening can crack the porcelain or damage threads.
- Reattach the rubber spark plug wire boot. Push it on firmly until you feel or hear a click.
- Finally, reinstall the engine cover and secure any screws or clips.
Why Regular Spark Plug Maintenance Matters
A clean, properly gapped spark plug ensures a strong spark for reliable ignition. This leads to easier starts, smoother engine operation, and better fuel efficiency. A worn plug makes the engine work harder.
As a general rule, you should check your lawn mower spark plug at least once a season. Replace it annually or every 25-50 hours of operation. It’s one of the cheapest and easiest forms of preventative maintenance you can do.
If you mow in dusty conditions or have an older mower, checking it twice a year is a good idea. A five-minute check can save you from frustration later.
Troubleshooting: What If I Still Can’t Find It?
While most mowers follow the standard design, some models can be tricky. If you’ve removed the obvious cover and don’t see the plug, don’t panic.
- Check for a Second Cover: Some engines have a smaller, round metal shroud directly over the spark plug, in addition to the main cover.
- Look for the Wire: Trace the thick ignition wire from the ignition coil or kill switch. It will always lead you to the plug.
- Consult the Manual: Your owner’s manual has an engine diagram that pinpoints the spark plug location. You can often find these online if you’ve lost yours.
- Vertical vs. Horizontal Engine: On some mowers, the engine shaft is vertical, and the plug may be angled. On others, it’s horizontal, and the plug points straight out the side.
Safety Reminders You Should’nt Ignore
Working on small engines is generally safe, but you must respect a few critical rules. Always disconnect the spark plug wire before doing any work. This is the most important step to prevent the mower from accidentally starting.
Work in a well-lit, dry area. Keep your hands and tools away from the sharp mower blade, even when the engine is off. If you’re testing for a spark, never hold the plug against the engine with your bare hands—use insulated pliers.
Finally, always use the correct tools. A proper spark plug socket makes the job much easier and reduces the risk of breaking the porcelain insulator.
FAQ: Common Spark Plug Questions
Where is the spark plug on a riding lawn mower?
The process is similar. Usually, you open the hood and locate the engine. The spark plug is on the engine block, under a rubber boot. You might need to remove a decorative engine shroud first, which is typically held by a few bolts.
Can I clean an old spark plug instead of replacing it?
You can for minor deposits. Use a wire brush and some brake cleaner. However, for the cost of a few dollars, replacing it with a new one is almost always the better option for reliable performance. A cleaned plug won’t last as long.
How do I know if my spark plug is bad?
Signs include difficulty starting, rough idling, poor acceleration, increased fuel consumption, and visible damage to the plug itself. If your mower is running poorly, the spark plug is a great first thing to check.
What size socket do I need for a lawn mower spark plug?
Most common lawn mower spark plugs use a 5/8-inch (16mm) or 13/16-inch (21mm) socket. A spark plug socket set from any hardware store will include both sizes. Its a worthwhile investment.
Why does my spark plug keep getting dirty so fast?
Frequent fouling points to other engine issues. Common causes are a dirty air filter, incorrect oil level, a clogged carburetor, or using old gasoline. Address the root cause after replacing the plug.
Finding and servicing your lawn mower’s spark plug is a fundamental skill for any gardener. With the engine cover off, it’s a simple component to identify and maintain. Regular checks and timely replacements will keep your mower starting on the first pull and running strong for seasons to come. Set aside a little time this weekend to give it a look—your lawn will thank you for it.