Learning how to change spark plugs lawn mower is a simple maintenance task that can make a big difference. A fresh set of spark plugs can significantly improve your lawn mower’s starting reliability and engine performance. If your mower is hard to start, runs rough, or has lost power, a worn spark plug is often the culprit. This guide will walk you through the entire process safely and efficiently.
How To Change Spark Plugs Lawn Mower
Changing a lawn mower spark plug is a straightforward job. You only need a few basic tools and about 15 minutes of your time. Following the correct steps ensures you do not damage the engine and that your mower runs smoothly after the replacement. Let’s get started with what you’ll need.
Tools And Materials You Will Need
Before you begin, gather these items. Having everything ready makes the job faster and easier.
- A new spark plug (correct type for your mower)
- A spark plug socket wrench (usually 13/16″ or 5/8″)
- A socket extension bar (3-inch is typical)
- A ratchet or wrench
- A gap measuring tool (feeler gauge)
- A small bit of wire brush or rag
- A pair of safety glasses
- Gloves (optional)
Safety Precautions Before You Start
Your safety is the most important part of any repair. Never skip these steps.
- Disconnect the spark plug wire. This is the most critical step to prevent the engine from accidentally starting.
- Allow the engine to cool completely if the mower was recently running. A hot engine can cause burns.
- Work in a well-lit, flat, and stable area, like a garage floor or driveway.
- Wear safety glasses to protect your eyes from any debris that may fall out of the spark plug hole.
Finding The Correct Replacement Spark Plug
Using the wrong spark plug can cause poor performance or even engine damage. Here is how to find the right one.
Check Your Owner’s Manual
The owner’s manual for your lawn mower will list the exact spark plug model number. This is the best and most reliable source of information.
Read The Old Spark Plug
If you don’t have the manual, the old spark plug itself has the answer. Carefully remove it first (following the steps below), and look for the model number stamped on its porcelain insulator or metal body. Take this number to your local hardware store or small engine shop.
Common Spark Plug Types
For most walk-behind mowers, common types include Champion RJ19LM, NGK BPR5ES, or Autolite 3924. Riding mowers may use different plugs. Always verify.
Step-By-Step Removal Of The Old Spark Plug
Now, let’s remove the old, worn-out spark plug. Take your time and follow each step carefully.
- Locate the spark plug. It is usually on the front or side of the engine cylinder, with a thick rubber wire attached to the top.
- Firmly grasp the rubber boot (the end of the spark plug wire) and pull it straight off the plug. Do not pull on the wire itself, just the boot. You may need to twist it slightly to break it loose.
- Blow away any loose grass clippings or dirt from the area around the spark plug base. This prevents debris from falling into the engine cylinder when you remove the plug.
- Attach your spark plug socket to the extension bar and ratchet. Carefully fit the socket over the spark plug’s hex nut.
- Turn the ratchet counter-clockwise to loosen the spark plug. Once it’s loose, you can often finish unscrewing it by hand.
- Lift the old spark plug straight out of the engine. Examine its condition, as it can tell you about your engine’s health.
Inspecting The Old Spark Plug
Looking at the old plug gives you clues about your mower’s engine condition. Here’s what to look for.
- Normal Wear: The electrode will be worn down and the insulator may have light tan or gray deposits.
- Carbon Fouled: Black, sooty deposits indicate a rich fuel mixture, a dirty air filter, or too much idling.
- Oil Fouled: Wet, black deposits mean oil is leaking into the combustion chamber, which is a more serious issue.
- Overheated: A white, blistered insulator or melted electrodes suggests the engine is running too hot.
If you see signs of oil or overheating, the new plug is a temporary fix. You may need to adress a larger engine problem soon.
Gapping The New Spark Plug
Most new spark plugs come pre-gapped, but it’s essential to check. The gap is the distance between the center and ground electrode where the spark jumps.
- Find the correct gap specification. This is in your owner’s manual or often on a sticker on the mower’s engine shroud. A common gap is 0.030 inches.
- Use your feeler gauge to measure the gap. Slide the correct-sized blade between the two electrodes. It should have a slight drag.
- If adjustment is needed, carefully bend the ground electrode (the L-shaped one) using the gapping tool. Do not touch the center electrode or the porcelain insulator, as they are fragile.
- Re-check the gap after bending. It make take a couple tries to get it just right.
Installing The New Spark Plug
You’re almost done. Now, let’s install the new, properly gapped spark plug.
- Begin by threading the new spark plug into the hole by hand. This is crucial. If you use the ratchet immediately, you risk cross-threading the plug, which can damage the engine head.
- Turn it clockwise by hand until it is finger-tight. You should not feel any resistance during this step.
- Once it’s hand-tight, attach your socket and ratchet. For a proper seal, tighten the spark plug about 1/8 to 1/4 of a turn further. Do not overtighten. If the plug has a crush washer, tighten until the washer compresses and the plug snugs up.
- Re-attach the spark plug wire. Push the rubber boot firmly onto the end of the new spark plug until you feel or hear it click into place. Ensure it is seated securely.
Testing Your Work
Before you put the mower away, make sure everything works correctly.
- Reconnect the spark plug wire if you had it disconnected at the boot for safety.
- Move the mower to an open area.
- Start the engine. It should start more easily and run more smoothly than before.
- Listen for a consistent engine sound without sputtering or misfiring. If it runs worse, double-check that the wire is fully connected and that you used the correct, properly gapped plug.
When To Change Your Lawn Mower Spark Plug
Regular replacement prevents problems. Don’t wait for your mower to fail to start.
- General Rule: Replace the spark plug at least once per mowing season, ideally in the spring during annual maintenance.
- Heavy Use: If you have a large lawn, consider changing it mid-season.
- Noticeable Symptoms: Change it immediately if you experience hard starting, rough idling, poor fuel economy, or a loss of engine power.
- As Part of a Tune-Up: Always install a new spark plug when performing other annual maintenance like changing the oil and air filter.
Troubleshooting Common Spark Plug Problems
Sometimes, a new plug doesn’t solve the issue. Here are a few common problems and their likely causes.
Mower Still Won’t Start
If you have a new plug and the engine still won’t crank, the problem is elsewhere. Check for old gasoline, a clogged fuel line, a dirty carburetor, or a faulty ignition coil.
Engine Runs Rough or Misfires
This could indicate a loose spark plug wire connection, incorrect spark plug gap, or a plug that is the wrong heat range for your engine. Verify your plug model.
Spark Plug Gets Dirty Very Quickly
Rapid fouling points to other issues. Check and replace your air filter, ensure you’re using fresh fuel, and check the engine’s choke operation. An engine running too rich will foul plugs fast.
FAQs About Changing Lawn Mower Spark Plugs
How Often Should You Change a Lawn Mower Spark Plug?
For optimal performance, change your lawn mower spark plug once a year. This is a standard part of springtime maintenance before the mowing season begins. If you use your mower extensively, a mid-season check or replacement is a good idea.
What Happens If You Don’t Change a Spark Plug?
A worn spark plug causes weak or inconsistent sparks. This leads to hard starting, rough engine operation, reduced fuel efficiency, and a loss of power. In severe cases, it can prevent the engine from starting at all, leaving you with an uncut lawn.
Can You Clean and Reuse an Old Spark Plug?
While you can clean light deposits off a spark plug with a wire brush, it is not recommended for long-term reliability. The electrodes wear down over time, increasing the gap. A new plug is inexpensive and guarantees a strong spark for easy starting and smooth running.
What Size Socket Do I Need for a Lawn Mower Spark Plug?
Most common lawn mower spark plugs require either a 13/16-inch or a 5/8-inch deep-well socket. A dedicated spark plug socket has a rubber insert to protect the plug’s porcelain insulator. If you’re unsure, bring the old plug to the store to test fit the socket.
Is It Safe to Change a Spark Plug Myself?
Yes, changing a spark plug is one of the safest and easiest DIY lawn mower repairs. The key safety rule is to always disconnect the spark plug wire before you begin. This ensures the engine cannot start accidentally while you are working on it.
Replacing your lawn mower’s spark plug is a quick and satisfying task that has immediate benefits. With a new plug installed, your mower should start on the first or second pull and run with more consistent power. By incorporating this simple change into your annual maintenance routine, you extend the life of your mower and ensure it’s always ready for the job. Remember to always consult your specific mower’s manual for the most accurate part numbers and specifications.