Learning how to change spark plug in lawn mower is a fundamental skill for any homeowner. A faulty spark plug is a common reason a mower won’t start, but swapping it out is often a simple fix. This guide will walk you through the entire process safely and efficiently.
You can complete this job in about 15 minutes with just a few basic tools. It will save you money on service calls and get your mower running smoothly again.
Let’s get started with what you’ll need.
How To Change Spark Plug In Lawn Mower
This section provides the complete, step-by-step procedure. Always prioritize safety by disconnecting the spark plug wire before beginning any work on your mower.
Tools And Materials You Will Need
Gathering the right items before you start makes the job go faster. You likely have most of these in your garage already.
- A new, correct spark plug for your mower model
- A spark plug socket wrench (usually 13/16″ or 5/8″) and ratchet
- A socket extension bar (3-inch is typical)
- A gap measuring tool (feeler gauge)
- A small bit of rubbing alcohol and a clean rag
- A torque wrench is optional but recommended for proper tightness
Step 1: Locate And Disconnect The Spark Plug
First, ensure the mower is on a flat, stable surface and the engine is completely cool. Locate the spark plug. It’s a short, porcelain-coated component usually on the side or top of the engine, with a thick rubber wire attached to its end.
Firmly grasp the rubber boot (not the wire itself) and pull it straight off the spark plug’s terminal. This disconnects the ignition source, a critical safety step.
Important Safety Precautions
Never work on a warm or running engine. Always disconnect the spark plug wire to prevent any accidental starting. This wire carries a high voltage that can cause a painful shock.
Step 2: Remove The Old Spark Plug
Take your spark plug socket and extension, and fit it over the old plug. Turn the ratchet counter-clockwise to loosen it. Once it’s loose, you can finish unscrewing it by hand.
Carefully lift the old spark plug out of its hole. Try to keep it upright to avoid dropping any debris into the cylinder head.
Step 3: Inspect The Old Spark Plug
Examining the old plug can give you clues about your engine’s health. Look at the electrode end.
- A light tan or gray color indicates good combustion.
- Black, sooty deposits may mean the engine is running too rich or the air filter is dirty.
- Oily deposits could suggest worn piston rings or excessive oil.
- Severe white blisters or melted electrodes point to overheating.
This inspection helps you determine if the spark plug was the only issue or if there might be a deeper problem.
Step 4: Check And Set The Gap On The New Spark Plug
Most new spark plugs come pre-gapped, but it’s wise to double-check. The gap is the space between the center and side electrode. Find the correct gap specification in your mower’s owner’s manual; it’s often between .028 and .035 inches.
Use your feeler gauge to measure the gap. If adjustment is needed, carefully bend the side electrode only, using the tool’s bending notch. Avoid touching the center electrode or the porcelain insulator.
Step 5: Install The New Spark Plug
Before installation, use the rag and alcohol to clean any dirt from around the spark plug hole in the engine. Thread the new spark plug into the hole by hand first. This is crucial to avoid cross-threading, which can damage the engine’s threads.
Once it’s hand-tight, use your socket wrench to finish tightening. If you have a torque wrench, tighten to the spec in your manual (usually 15-20 ft-lbs). If not, tighten it snugly until the washer compresses, then give it about a 1/4 to 1/2 additional turn. Do not overtighten.
Step 6: Reconnect The Spark Plug Wire
Finally, push the rubber boot firmly onto the terminal of the new spark plug until you feel or hear it click into place. Ensure the connection is secure so it won’t vibrate off during operation.
Your spark plug change is now complete. You’re ready to test your work.
Choosing the Right Spark Plug
Using the correct spark plug is essential for performance and engine life. The wrong type can cause poor starting, reduced power, or even engine damage.
Understanding Spark Plug Codes
Spark plugs have a long alphanumeric code printed on the box and the porcelain. This code indicates its size, heat range, and other features. The easiest way to find the right one is to use the old plug as a reference or consult your mower’s manual.
Common Spark Plug Types For Lawn Mowers
Most small engines use a standard resistor spark plug. Common brands include Champion, NGK, and Autolite. The most frequent types are:
- Champion RC12YC: A very common plug for many Briggs & Stratton engines.
- NGK BMR4A: Often used in Honda mower engines.
- Always match the plug number exactly for best results, don’t just guess based on size.
When to Change Your Lawn Mower Spark Plug
Regular replacement prevents starting problems. Don’t wait for your mower to fail before you think about the spark plug.
Recommended Replacement Intervals
As a general rule, replace the spark plug at least once per mowing season. For heavy use, consider changing it twice. Always install a new plug at the beginning of the spring season to ensure reliable starts.
Signs Your Spark Plug Needs Changing
Watch for these symptoms that indicate a plug change is overdue:
- The engine is hard to start or requires many pulls.
- Rough idling or the engine misfires during operation.
- Reduced power or acceleration while mowing.
- Increased fuel consumption compared to normal.
- Visible damage to the plug, like a cracked insulator or eroded electrode.
Troubleshooting After Installation
If your mower still doesn’t start after changing the plug, a few other things could be the culprit. Don’t assume the new plug is faulty right away.
Common Post-Installation Issues
Check these items if the engine won’t start:
- Is the spark plug wire firmly connected? It might not be snapped on all the way.
- Did you accidentally damage the porcelain insulator during installation? A crack can cause a short.
- Is there fuel in the tank, and is the fuel valve (if present) turned on?
- Is the air filter clean? A clogged filter can prevent starting.
- Is the engine kill switch or blade engagement lever in the correct “off” position for starting?
Double-checking these simple items can often solve the problem.
FAQs About Changing Lawn Mower Spark Plugs
How Often Should I Change My Lawn Mower Spark Plug?
You should change your lawn mower spark plug at least once per year, ideally at the start of the mowing season. For mowers used frequently or under dusty conditions, consider replacing it every 25-50 hours of operation.
What Happens If You Don’t Gap A Spark Plug?
If the gap is too wide, the spark may be too weak to reliably ignite the fuel mixture, causing hard starting or misfires. If the gap is too narrow, the spark may be too small and weak, leading to incomplete combustion, power loss, and fouling. Always check the gap.
Can A Bad Spark Plug Ruin A Lawn Mower Engine?
While a single bad plug typically won’t cause catastrophic damage immediately, prolonged use can lead to problems. A severely fouled plug can cause incomplete combustion, leading to carbon buildup, poor performance, and over time, increased wear on engine components. It’s best to replace it promptly.
Why Does My New Spark Plug Get Black So Quickly?
Quickly blackening, or sooty fouling, usually indicates a rich fuel mixture. This can be caused by a dirty air filter, a malfunctioning carburetor (stuck choke or float), or excessive idling. Check and clean your air filter first, as this is the most common and easiest fix.
Is It Okay To Use Any Spark Plug That Fits The Thread?
No, it is not okay. While a plug might physically screw in, using one with the incorrect heat range, reach, or resistor type can cause poor engine performance, overheating, or pre-ignition. Always use the spark plug type specified by your mower’s manufacturer for optimal results and engine safety.