A fresh spark plug can significantly improve your mower’s starting reliability and overall engine performance. If your mower is hard to start, runs rough, or uses more gas than usual, learning how to change a spark plug on lawn mower is a simple fix you can do yourself. This guide provides clear, step-by-step instructions to get your engine running smoothly again.
How To Change A Spark Plug On Lawn Mower
Changing a spark plug is a straightforward maintenance task. With a few common tools and about 15 minutes, you can complete the job. The process involves locating the plug, removing the old one, checking the gap on the new one, and installing it correctly. Following these steps carefully ensures a safe and effective repair.
Tools And Materials You Will Need
Before you begin, gather the necessary items. Having everything ready makes the job faster and prevents interruptions. You likely have most of these tools already in your garage.
- A new spark plug (correct type for your mower)
- A spark plug socket wrench (usually 5/8″ or 13/16″)
- A socket wrench extension (3-inch is typical)
- A gap gauge (feeler gauge)
- A clean rag or shop towel
- A small bit of wire brush (optional)
- Protective gloves and safety glasses
Safety Precautions Before You Start
Your safety is the most important part of any repair. Small engines can be dangerous if not handled properly. Always follow these basic safety rules to prevent injury or damage to your mower.
Disconnect The Spark Plug Wire
This is the most critical step. Always disconnect the spark plug wire before doing any work. This prevents the engine from accidentally starting. Simply grasp the rubber boot at the end of the wire and pull it straight off the plug.
Allow The Engine To Cool Completely
Work on a cold engine. The spark plug and surrounding components can be extremely hot immediately after use. Touching them can cause serious burns. Let the mower sit for at least 30 minutes to an hour to ensure it’s safe to handle.
Work In A Well Lit And Ventilated Area
Choose a flat, stable surface like a workbench or driveway. Good lighting helps you see small parts clearly. If you must work in a garage, ensure the door is open for fresh air, especially if you recently ran the mower.
Step By Step Replacement Guide
Now, let’s walk through the actual process of changing the spark plug. Take your time with each step to avoid mistakes.
Step 1: Locate And Clean Around The Spark Plug
Find the spark plug on your mower’s engine. It is a short, porcelain-coated component with a thick wire attached to the top. It’s usually near the front or side of the engine cylinder. Use your rag to wipe away any loose grass clippings, dirt, or debris from around the base of the plug. This prevents anything from falling into the engine cylinder when you remove the plug.
Step 2: Remove The Old Spark Plug
Firmly grasp the rubber boot on the spark plug wire and pull it straight off. Do not pull on the wire itself. Next, fit your spark plug socket over the plug. Attach the extension to the socket and then to your ratchet. Turn the ratchet counter-clockwise to loosen the plug. Once it’s loose, you can finish unscrewing it by hand. Carefully lift the old spark plug out of its hole.
Step 3: Identify And Gap The New Spark Plug
Check your mower’s owner’s manual for the correct spark plug type and gap specification. This information is often also found on a decal on the mower itself. Even if the new plug says it’s “pre-gapped,” you should always verify the gap yourself. Insert the gap gauge between the center and side electrode. The correct thickness should slide through with a slight drag. If the gap is too small, gently pry the side electrode open with the gauge tool. If it’s too large, carefully tap the electrode on a hard surface to close the gap slightly. Recheck until it’s correct.
Step 4: Install The New Spark Plug
Begin by threading the new spark plug into the hole by hand. This is crucial to avoid cross-threading, which can damage the engine head. Turn it clockwise until it is finger-tight. If you encounter any resistance, stop and restart to ensure the threads are aligned properly. Once it’s hand-tight, use your socket wrench to tighten it further. For most mowers, a quarter to a half turn past hand-tight is sufficient. Avoid over-tightening, as this can crack the porcelain or damage the threads.
Step 5: Reconnect The Spark Plug Wire
Finally, take the rubber boot on the end of the spark plug wire and push it firmly onto the metal terminal of the new spark plug. You should hear or feel a slight click as it seats properly. Give the boot a gentle tug to ensure it’s secure. A loose connection can prevent the engine from starting.
Choosing The Right Replacement Spark Plug
Using the correct spark plug is essential for optimal performance. The wrong plug can cause poor running or even engine damage. There are a few reliable ways to find the right part for your specific mower model.
- Consult Your Owner’s Manual: This is the best source. It will list the exact spark plug model number (e.g., Champion RC12YC, NGK BPR5ES).
- Check The Engine Model Decal: Look for a sticker on the mower’s engine shroud. It often includes the recommended spark plug type.
- Take The Old Plug To The Store: Bring your old spark plug to a hardware or lawn equipment store. The staff can usually match it to the correct replacement.
- Online Lookup Tools: Most major spark plug brands have online catalogs where you can enter your mower’s make and model to find the right part.
Testing Your Work And Troubleshooting
After installation, it’s time to test your work. Return the mower to a flat, open area. Prime the engine if your model has a primer bulb, and set the throttle to the start position. Pull the starter cord. The engine should start within a few pulls and run smoothly. If you encounter problems, here are some common issues and their solutions.
- Engine Won’t Start: Double-check that the spark plug wire is fully seated on the plug. Verify the plug is gapped correctly and tightened properly. Ensure you reconnected any other wires you may have disconnected.
- Engine Runs Rough or Misfires: The spark plug gap may be incorrect. Remove the plug and check the gap again. Also, inspect the plug wire for any cracks or damage.
- There Is A Clicking Sound When Starting: This could indicate the plug is too loose or, more seriously, cross-threaded. Turn off the mower immediately, disconnect the wire, and check the plug’s installation.
When To Change Your Lawn Mower Spark Plug
Regular replacement prevents problems before they start. Don’t wait for your mower to fail to begin running poorly. Follow a simple maintenance schedule to keep it in top condition.
- Annual Replacement: The general rule is to install a new spark plug at the start of every mowing season. This ensures reliable starts all year.
- After 25 Hours of Use: If you use your mower frequently, check or replace the plug after every 25 hours of operation. This is a common manufacturer recommendation.
- Performance Symptoms: Change the plug immediately if you notice hard starting, rough idling, poor acceleration, increased fuel consumption, or visible damage to the old plug.
Inspecting The Old Spark Plug
The condition of your old spark plug can tell you a lot about your engine’s health. Before you throw it away, take a moment to examine it. The color and deposits on the electrode can indicate if your engine is running too rich, too lean, or if there are other issues.
- Normal Wear: The insulator tip should be a light tan or gray color. The electrodes will be worn but intact.
- Carbon Fouled (Black, Sooty Deposits): This often indicates a dirty air filter, too much fuel, or excessive idling. Check your air filter and carburetor settings.
- Oil Fouled (Wet, Black Deposits): This suggests oil is leaking into the combustion chamber, possibly from worn piston rings or valve guides. This may require a more significant engine repair.
- Overheated (White, Blistered Insulator): The engine may be running too hot due to a lean fuel mixture, incorrect ignition timing, or insufficient cooling. Have the carburetor adjusted if this is the case.
Frequently Asked Questions
How Often Should I Change The Spark Plug In My Lawn Mower?
You should change your lawn mower spark plug at least once per mowing season, ideally in the spring. For heavy use, consider replacing it after every 25 hours of operation. A fresh plug each year guarantees the best performance and starting reliability.
What Happens If You Don’t Change A Lawn Mower Spark Plug?
If you don’t change a worn spark plug, your mower will become increasingly difficult to start. It will run less efficiently, wasting fuel and losing power. Over time, a severely worn plug can lead to incomplete combustion, causing carbon buildup and potentially more expensive engine damage.
Can I Change A Lawn Mower Spark Plug Myself?
Yes, changing a lawn mower spark plug is one of the easiest DIY maintenance tasks. With basic tools like a spark plug socket and a gap gauge, most people can complete the job safely in under 20 minutes. Just remember to always disconnect the spark plug wire first.
What Size Socket Do I Need To Remove A Lawn Mower Spark Plug?
Most lawn mower spark plugs require either a 5/8-inch (16mm) or a 13/16-inch (21mm) deep-well socket. A dedicated spark plug socket has a rubber insert that grips the plug to prevent damage. It’s best to check your old plug or manual to be certain of the size before you buy a tool.
Is There A Difference Between Car And Lawn Mower Spark Plugs?
While they operate on the same basic principle, lawn mower spark plugs are designed for smaller engines with different heat ranges and thread sizes. You should always use the plug specified for your mower model. Using an automotive plug can lead to poor performance or engine damage.