Quick Answer
To replace a spark plug on a lawn mower, disconnect the spark plug wire, use a socket wrench to remove the old spark plug, install the new plug (torqued to 18–21 ft-lbs for most residential mowers), and reconnect the wire. The process typically takes 10–15 minutes. Note: corded electric mowers have no spark plugs and do not require replacement.
Before You Start: Safety and Preparation
Before touching your lawn mower’s spark plug, take these essential precautions:
- Confirm your mower type: Gas-powered mowers use spark plugs; corded electric mowers do not. If you own a cordless electric model, consult your manual—most have no spark plugs to replace.
- Allow the engine to cool: Never work on a hot engine. Wait at least 30 minutes after the last use to prevent burns.
- Disconnect the spark plug wire: This safety step prevents accidental engine startup while you work.
- Locate your mower’s manual: Your manual specifies the correct spark plug model, gap setting, and torque specifications for your particular mower.
- Work on a clean, level surface: Use a workbench or flat ground with good lighting to avoid losing small parts.
What You’ll Need
Gather these tools and materials before starting:
| Item | Purpose |
|---|---|
| Socket wrench set | Removes and installs the spark plug (typically 5/8-inch or 13/16-inch socket) |
| Spark plug socket | Specialized socket with rubber lining to protect the spark plug ceramic insulator |
| Ratchet handle | Turns the socket wrench to loosen and tighten |
| New spark plug | Correct model for your mower (check manual or old plug for part number) |
| Gap gauge or feeler gauge | Sets the correct gap between spark plug electrodes (typically 0.025–0.030 inches) |
| Wire brush or cloth | Cleans debris around the spark plug before removal |
| Dielectric grease (optional) | Protects spark plug connections from moisture and corrosion |
How to Locate the Spark Plug on Your Mower
The spark plug location varies by mower type:
- Push mowers: The spark plug is usually on the side of the engine, near the top. Look for a thick, black wire (the spark plug wire) connected to a ceramic component.
- Riding mowers: Spark plugs are often on the engine block, sometimes hidden behind a shroud or air filter housing. Refer to your manual if you cannot locate it immediately.
- Vertical-shaft mowers: The plug sits on top of the engine cylinder; access is typically straightforward.
Once located, use a wire brush or cloth to clean away grass clippings and dirt around the spark plug area. This prevents debris from entering the engine when you remove the plug.
Step-by-Step Spark Plug Replacement Process
Step 1: Disconnect the Spark Plug Wire
Locate the thick rubber wire connected to the top of the spark plug. Grasp the wire boot (the connector end) and twist it gently while pulling away from the spark plug. Do not yank the wire itself—always pull by the boot to avoid damage to the wire’s internal conductor.
Step 2: Clean the Spark Plug Area
Use a wire brush or cloth to remove grass, dirt, and debris from around the spark plug. This prevents contamination from falling into the engine cylinder when you remove the plug.
Step 3: Remove the Old Spark Plug
Insert the spark plug socket onto the ratchet handle. Lower it over the spark plug and turn counterclockwise until the plug loosens. Once loose enough to turn by hand, carefully unscrew the spark plug and remove it completely. Inspect the old plug for signs of wear, buildup, or damage to understand your mower’s condition.
Step 4: Check the New Spark Plug Gap
The gap—the space between the spark plug’s center and ground electrodes—must match your mower’s specifications, typically 0.025–0.030 inches. Use a gap gauge or feeler gauge to measure the gap on the new plug. If the gap is incorrect, carefully bend the ground electrode (the side arm) until the gauge fits snugly. Consult your manual for the exact gap required for your mower.
Step 5: Install the New Spark Plug
Thread the new spark plug by hand into the engine cylinder. This prevents cross-threading, which can damage the cylinder’s threads. Once hand-tight, use the ratchet and spark plug socket to tighten the plug to the manufacturer’s specification—typically 18–21 foot-pounds for residential lawn mowers. Do not over-tighten, as this can crack the spark plug ceramic or strip the threads.
Step 6: Reconnect the Spark Plug Wire
Push the spark plug wire boot firmly onto the top of the new spark plug until you hear or feel a click. Ensure the connection is secure; a loose wire can cause misfiring or starting problems. If desired, apply a small amount of dielectric grease to the wire boot’s interior to protect against moisture and corrosion.
Testing Your Work
After replacing the spark plug:
- Visual inspection: Confirm the wire boot is seated fully and the spark plug is tight.
- Start the mower: Prime the engine according to your manual, then pull the starter cord. The mower should start smoothly without hesitation or sputtering.
- Run briefly: Let the mower idle for 30 seconds, then run at full throttle for a few seconds to confirm smooth operation.
- Listen for problems: Unusual knocking, pinging, or misfiring sounds indicate a problem. Stop the mower and recheck the spark plug installation and wire connection.
If the mower does not start or runs poorly, the spark plug wire may be loose, the gap may be incorrect, or there may be other engine issues unrelated to the spark plug.
Troubleshooting Common Issues
Mower Won’t Start After Spark Plug Replacement
Check these items in order:
- Is the spark plug wire fully seated on the spark plug?
- Is the spark plug gap set to the correct specification?
- Did you accidentally damage the spark plug wire while removing it?
- Is the engine flooded with too much fuel? Prime it correctly per your manual.
Engine Misfires or Sputters
Misfiring usually means the spark plug wire is loose or the gap is incorrect. Remove the wire, inspect it for cracks, and reseat it firmly. If misfiring persists, remove the spark plug and check the gap again with your gauge.
Black Residue on the Spark Plug
If the old spark plug had heavy carbon buildup, the engine may be burning fuel too rich. This is normal on older mowers but could indicate a clogged air filter or carburetor issue. Replace the air filter and clean the carburetor according to your manual.
Spark Plug Maintenance and Replacement Schedule
Most residential lawn mower manufacturers recommend replacing the spark plug annually or every 100 operating hours, whichever comes first. If you use your mower intensively (more than twice weekly), replace the plug every 50 hours for optimal performance.
Keep these maintenance practices in mind:
- Inspect annually: At the start of each mowing season, remove the spark plug and inspect it for wear. Replace if the electrodes are heavily corroded, pitted, or if the ceramic insulator is cracked.
- Check the air filter: A clogged air filter causes heavy carbon buildup on spark plugs, reducing their lifespan. Clean or replace the air filter every season.
- Use fresh fuel: Old, stale fuel leaves varnish deposits on spark plugs. Use fresh gasoline and consider a fuel stabilizer if storing your mower for more than 30 days.
- Store correctly: Over winter, drain the fuel tank and run the engine dry, or use a fuel stabilizer. This prevents corrosion of the spark plug and carburetor.
Frequently Asked Questions
How do I know what spark plug my lawn mower needs?
Check your mower’s owner manual for the exact spark plug part number and specifications. You can also remove the old spark plug and find the part number printed on its ceramic body. Bring this number to a lawn mower dealer or hardware store to purchase the correct replacement. Common sizes are Champion, Briggs & Stratton, and MTD brand plugs, depending on your mower’s engine manufacturer.
Can I replace a spark plug without disconnecting the wire first?
Technically, yes, but disconnecting the wire first is safer. It prevents accidental engine startup if someone pulls the starter cord while you’re working. Always follow your mower’s manual for safety procedures.
What if my spark plug gap is too wide or too narrow?
Use a feeler gauge or gap gauge to measure the gap. If it is too narrow, use a spark plug gap tool or carefully bend the ground electrode outward. If it is too wide, tap the electrode inward slightly. Recheck the gap after adjusting until it matches your manual’s specification (typically 0.025–0.030 inches).
How often should I replace my lawn mower’s spark plug?
Most manufacturers recommend annual replacement or every 100 operating hours, whichever comes first. Inspect the plug at the start of each mowing season and replace it if the electrodes are worn, corroded, or if the ceramic insulator is damaged. High-use mowers may need replacement every 50 hours.
Summary
Replacing a lawn mower’s spark plug is an essential maintenance task that extends engine life and ensures reliable starting. By following these steps—gathering the right tools, safely disconnecting the wire, removing the old plug, checking the gap, and installing the new plug with proper torque—you can complete the job in under 15 minutes. Regular spark plug replacement, combined with air filter cleaning and fresh fuel, keeps your mower running smoothly for years to come. If you encounter problems or your mower still does not start after replacement, consult your owner’s manual or contact a qualified lawn mower technician.
