If your lawn mower is hard to start, running rough, or lacking power, learning how to change a spark plug in lawn mower is an essential skill. Replacing a faulty spark plug is a quick and inexpensive way to restore your lawn mower’s engine performance.
This simple maintenance task can make a huge difference. You don’t need to be a mechanic to do it successfully.
With a few basic tools and about 15 minutes, you can have your mower running smoothly again. This guide will walk you through every step, from gathering supplies to the final test start.
How To Change A Spark Plug In Lawn Mower
This section provides the complete, step-by-step procedure. Before you begin, always ensure the mower is on a flat, stable surface and the engine is completely cool. Safety is the first priority.
Tools And Materials You Will Need
Gathering the right items before you start makes the job faster and easier. You likely have most of these in your garage already.
- A new spark plug (correct type for your mower)
- A spark plug socket wrench (usually 13/16″ or 5/8″) with a ratchet and extension
- A gap gauge tool (feeler gauge)
- A small amount of rubbing alcohol and a clean rag
- A pair of needle-nose pliers (optional, for stubborn wires)
- A torque wrench is recommended but not essential for most homeowners
Step 1: Locate And Disconnect The Spark Plug Wire
The spark plug is typically found on the side of the engine cylinder. It has a thick, rubber-insulated wire attached to its top.
Firmly grasp the rubber boot (not the wire itself) and pull it straight off the plug. A slight twisting motion can help if it’s stuck. Be gentle to avoid damaging the wire.
Step 2: Clean The Area Around The Spark Plug
Use your rag and a bit of rubbing alcohol to wipe away any grass clippings, dirt, or debris from around the base of the spark plug. This prevents anything from falling into the engine cylinder when you remove the old plug, which could cause serious damage.
Why This Step Is Critical
Debris in the combustion chamber can score the cylinder walls or get stuck on the piston. Taking a minute to clean can save you from a very costly engine repair down the line.
Step 3: Remove The Old Spark Plug
Fit your spark plug socket over the plug. Attach the ratchet and extension if needed. 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 straight out of its hole.
Step 4: Check And Gap The New Spark Plug
Even new plugs need to be checked. The “gap” is the distance between the center and side electrodes. Your mower’s manual specifies the correct gap, usually between .028″ and .035″.
Insert your gap gauge between the electrodes. If the gap is too small, gently pry the side electrode open with the gauge’s tool. If it’s too large, carefully tap the electrode on a hard surface to close it slightly. Re-check until the gauge slides through with a slight drag.
Step 5: Install The New Spark Plug
Begin threading the new spark plug into the hole by hand. This is crucial to avoid cross-threading, which strips the aluminum threads in the engine head.
If it doesn’t turn in smoothly, back it out and try again. Once it’s hand-tight, use your socket wrench to tighten it further.
Proper Tightening Technique
If you don’t have a torque wrench, a good rule is to tighten the plug until it seats, then give it an additional 1/8 to 1/4 turn. Do not over-tighten. Over-tightening can crack the plug’s ceramic insulator or damage the engine threads.
Step 6: Reconnect 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’s seated completely to prevent arcing, which can cause poor performance.
Step 7: Test Your Work
With the wire securely connected, you can now test start your mower. It should start more easily and run more smoothly. If it doesn’t, double-check that the wire is fully seated and that you used the correct spark plug type.
Choosing The Correct Replacement Spark Plug
Using the wrong spark plug can lead to poor performance or even engine damage. There are several reliable ways to find the right one.
Consult Your Owner’s Manual
The owner’s manual is the best source. It will list the exact spark plug model number recommended by the manufacturer, such as a Champion RJ19LM or NGK BPR5ES.
Check The Old Spark Plug
The model number is usually stamped on the ceramic insulator or the metal hex body. Take the old plug to the store to find an exact match.
Use An Online Lookup Tool
Most major spark plug brands and lawn mower parts websites have lookup tools. You enter your mower’s make, model, and engine number, and it gives you the correct plug options.
Common Spark Plug Types For Lawn Mowers
- Resistor Plugs: Most common. They suppress radio frequency interference.
- Copper Core Plugs: Standard, affordable, and reliable, but may have shorter service life.
- Iridium or Platinum Plugs: Longer-lasting and often provide more consistent performance, but are more expensive.
When To Change Your Lawn Mower Spark Plug
Don’t wait for your mower to fail. Proactive replacement is part of good maintenance. Here are the key signs and schedules to follow.
Visual Inspection Signs
Remove and inspect your plug periodically. Look for:
- Heavy Carbon Deposits: Black, sooty coating indicates a rich fuel mixture or weak ignition.
- Oil Fouling: Wet, black deposits mean oil is entering the combustion chamber.
- Burnt or Eroded Electrodes: The electrode may be rounded or melted, a sign of overheating.
- Cracked Ceramic Insulator: Any cracks mean the plug must be replaced immediately.
Performance Symptoms
Your mower’s behavior tells you a lot. Watch for these issues:
- Difficulty starting, especially on the first pull
- Rough idling or the engine surging up and down
- Loss of power when cutting grass, particularly thick or wet grass
- Increased fuel consumption compared to normal
- Misfiring, which sounds like sputtering or popping from the exhaust
Recommended Replacement Schedule
As a general rule, replace your lawn mower spark plug at least once per mowing season, ideally in the spring during your annual tune-up. If you use your mower heavily (over 50 hours per season), consider changing it mid-season as well. Always check your manual for the manufacturer’s specific interval.
Safety Precautions You Must Follow
Working on small engines requires attention to safety. These simple rules protect you and your equipment.
Work On A Cold Engine
Always let the engine cool completely before attempting to remove the spark plug. A hot plug can cause severe burns, and heat can make the metal threads more prone to damage.
Disconnect The Spark Plug Wire
This is the most important safety step. With the wire disconnected, the engine cannot accidentally start while you are working on it. It’s a good habit to disconnect it whenever you perform any maintenance, like cleaning the deck or sharpening the blade.
Avoid Fuel Fumes
Work in a well-ventilated area, away from open flames or sparks. Gasoline vapors are highly flammable. Don’t smoke while working on your mower.
Troubleshooting Common Spark Plug Problems
Sometimes, a simple plug change doesn’t fix the issue. Here’s what to check next if problems persist.
Mower Still Won’t Start
If you’ve installed a new, properly gapped plug and the mower won’t start, the problem may lie elsewhere.
- Check for fresh fuel in the tank.
- Ensure the fuel valve (if equipped) is in the “On” position.
- Verify that the spark plug wire is firmly connected.
- The ignition coil or flywheel key could be faulty.
Engine Runs Rough After Replacement
A rough runing engine after a plug change suggests other issues.
- Double-check the spark plug gap. An incorrect gap is a common mistake.
- There could be old, degraded fuel in the carburetor.
- The air filter may be clogged, affecting the air-fuel mixture.
New Spark Plug Gets Dirty Quickly
If a new plug fouls with carbon or oil within a few uses, it indicates a deeper engine issue.
- Oil fouling suggests worn piston rings or valve guides.
- Carbon fouling often points to a dirty air filter or a carburetor that is set too rich.
- Continual problems warrant inspection by a small engine repair professional.
Frequently Asked Questions
How Often Should I Change My Lawn Mower Spark Plug?
For most residential mowers, once per season is sufficient. If you notice performance issues, check and replace it as needed. Heavy users should consider a mid-season change.
What Happens If You Don’t Change A Spark Plug In A Lawn Mower?
A worn spark plug causes incomplete combustion. This leads to hard starting, poor performance, wasted fuel, and increased exhaust emissions. Over time, it can also cause carbon buildup in the engine.
Can I Clean An Old Spark Plug Instead Of Replacing It?
You can clean a lightly fouled plug with a wire brush and solvent, but it’s often not worth the effort. A new plug is inexpensive and guarantees reliable performance. Severely worn or damaged plugs should always be replaced.
What Is The Correct Spark Plug Gap For A Lawn Mower?
The gap varies by engine model. Common gaps range from 0.028 inches to 0.035 inches. Always refer to your owner’s manual or the specification chart on your mower’s engine shroud for the precise setting.
Why Does My Lawn Mower Spark Plug Keep Getting Wet?
A wet spark plug is usually fouled by fuel (flooding) or oil. Fuel fouling happens from over-choking or a carburetor issue. Oil fouling indicates internal engine wear, allowing oil to seep into the combustion chamber.