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Is Your Lawn Mower Spark Plug Bad? Find Out Now! (2023)

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how to know if lawn mower spark plug is badAre you having trouble getting your lawn mower to start? Have you noticed a decrease in power or an increase in fuel consumption? You may be dealing with a bad spark plug.

Being able to diagnose the issue before buying new parts will save time, money, and effort when trying to get your garden equipment back up and running.

Key Takeaways

  • Difficulty starting, difficult rewinding, loss of power, and increased fuel consumption are common signs of a bad lawn mower spark plug.
  • To diagnose spark plug issues, use an ignition tester by disconnecting the spark plug lead, attaching the tester, starting the engine, and interpreting the results.
  • Cleaning and reinstalling a spark plug involves disconnecting the lead, cleaning it with a soft brush, checking the gap, applying anti-seize compound, and reinstalling it carefully.
  • The lifespan of a lawn mower spark plug is typically around 25 hours of use, and common causes of spark plug failure include carbon buildup, oil fouling, electrode wear, heat, and vibration.

Signs of a Bad Lawn Mower Spark Plug

Signs of a Bad Lawn Mower Spark Plug
If your lawnmower is giving you trouble, it’s essential to consider the spark plug as a potential culprit. A bad lawnmower spark plug can manifest in various ways, from causing the engine to refuse to start, to making the rewind process a laborious task, sapping power during operation, and even leading to increased fuel consumption.

Failure to Start

If your mower’s engine just won’t kick into gear, you might find yourself stuck in frustration, pulling that cord again and again. One of the common culprits for this headache is a bad spark plug. Spark plug problems can manifest as a stubbornly silent engine.

To diagnose, use an ignition tester. If there’s no spark, it’s likely that your spark plug needs replacing. Remember, regular spark plug maintenance and knowing the signs of a bad spark plug can keep your lawn mower running smoothly.

Difficult Rewinding

When your mower’s cord becomes stubborn, making you struggle to rewind it, it’s like tugging at a tangled fishing line – a frustrating knot in your outdoor chore. Difficult rewinding can often signal spark plug issues in your lawnmower. Spark plug troubleshooting is crucial here.

A worn or fouled spark plug can lead to incomplete combustion, causing starting problems. To resolve this, consider spark plug maintenance or replacement for smoother starts and a more hassle-free mowing experience.

Loss of Power

Experiencing a noticeable decrease in performance? When your lawn mower starts losing power, it’s a clear indication that your spark plug might be the culprit. Spark plug performance is crucial for efficient combustion in your engine. If you’ve ruled out other issues like fuel or air filters, it’s time to focus on spark plug maintenance.

Check for spark plug damage and carbon buildup. A simple spark plug test and potential replacement can restore your mower’s power and efficiency, ensuring you maintain mastery over your lawn.

Increased Fuel Consumption

When your mower seems to be thirstier than a marathon runner on a scorching day, it’s like your wallet doing the heavy lifting, as increased fuel consumption could be a silent cry for spark plug attention.

A bad spark plug can lead to inefficient combustion, causing your mower to guzzle more fuel than necessary. To improve fuel efficiency and save money, consider regular spark plug care and replacement as part of your maintenance routine.

It’s a simple troubleshooting guide for performance improvement in your lawnmower.

Common Symptoms of a Bad Spark Plug in Garden Equipment & Lawn Mowers

Common Symptoms of a Bad Spark Plug in Garden Equipment & Lawn Mowers
If your garden equipment or mower is giving you trouble, you might notice a loss of power during operation, which can be frustrating when you’re trying to keep your outdoor space looking its best.

  • Loss of Power: Your equipment may struggle to maintain its usual performance, making it challenging to mow your lawn effectively.
  • Increased Fuel Consumption: A faulty spark plug can lead to inefficient combustion, causing your mower to guzzle more fuel than usual.
  • Stalling During Operation: If your equipment frequently stalls while in use, it’s a clear indication of potential spark plug problems.
  • Difficulty Starting: An engine that cranks but doesn’t start is often due to a bad spark plug.

While addressing spark plug issues can improve your equipment’s performance, it’s essential to remember that other factors, such as proper maintenance and cleaning, also play a crucial role in extending the spark plug lifespan.

So, keep an eye out for these common symptoms and ensure your spark plug is in good condition for a well-maintained lawn.

How to Use an Ignition Tester to Test a Bad Lawn Mower Spark Plug

How to Use an Ignition Tester to Test a Bad Lawn Mower Spark Plug
To check for a problematic spark plug, grab an ignition tester, a tool designed to detect issues with the electrical component that initiates combustion in your equipment.

  1. Disconnect the Spark Plug Lead: Ensure your mower is off, then disconnect the spark plug lead from the plug itself.
  2. Attach the Ignition Tester: Connect the ignition tester to the spark plug. It should fit snugly over the plug’s terminal.
  3. Attempt to Start the Engine: Now, try starting the mower while observing the ignition tester. If the spark plug is in good condition, you’ll see a visible spark at the tester’s electrode.
  4. Interpreting the Results: If the ignition tester shows no spark, it’s a strong indicator of a bad spark plug. In this case, you’ll need to clean or replace the plug to restore proper ignition.

Using an ignition tester provides valuable insights into your lawn mower’s ignition system and helps diagnose spark plug issues accurately.

Tips for Cleaning and Reinstalling a Spark Plug

Tips for Cleaning and Reinstalling a Spark Plug
For effective maintenance, make sure your cleaning area is free of debris. Use a spark plug socket to safely remove the plug. Clean it with a soft-wire brush before carefully reinstalling it without overtightening.

When cleaning and reinstalling a spark plug:

  • Disconnect the plug lead and remove it using the proper spark plug socket. Be careful to avoid cross-threading.
  • Use a soft-bristled brush or commercial cleaner to gently remove deposits from the electrodes and porcelain.
  • Check the gap using a feeler gauge and adjust it if necessary according to the specifications.
  • If you’re replacing a worn plug, apply anti-seize compound to the threads.
  • Carefully reinstall the plug and tighten it just until the washer contacts the cylinder head.
  • Reconnect the lead, being cautious to avoid sparks near fuel vapors.
  • Consider replacing the spark plug in the spring and fall as part of preventive maintenance.

Regular and careful spark plug service maximizes performance and extends the lifespan of your equipment.

Lifespan of a Lawn Mower Spark Plug

Lifespan of a Lawn Mower Spark Plug
Keep up with your lawn mower’s hours of use, as spark plugs normally last around 25 before needin’ a replacement. To maximize lifespan, perform seasonal maintenance like checking gaps, cleaning fouled plugs, and replacing worn ones.

But even with care, factors like oil deposits, combustion heat, and electrode wear limit how long a spark plug lasts. If you notice issues startin’ up, stalling, or rapid fuel consumption, it likely means the plug needs replacin’.

Track mower hours and replace plugs about every 25. With basic maintenance like gap checks, cleaning, and replacement, spark plugs can reliably ignite the fuel mix to keep your mower runnin’ strong.

If troubleshootin’ doesn’t fix performance issues, new plugs are an easy, inexpensive DIY repair. Stay ahead of problems by knowin’ the signs of wear and replacin’ plugs around the 25 hour mark.

Proper maintenance keeps your mower’s ignition system hummin’.

Causes of Spark Plug Failure in Lawn Mowers

Causes of Spark Plug Failure in Lawn Mowers
You’ll shudder when you see the damage carbon buildup does to those delicate electrodes. Over time, the repeated combustion in a mower engine leaves carbon deposits that hinder the spark. Oil fouling from excessive crankcase oil also coats the plugs, disrupting ignition.

Electrode wear and gap widening are also common with extended use. The porcelain insulator around the central electrode can develop cracks or chips that allow voltage to leak. Excess heat and vibration can take their toll too. Even slight damage to the electrodes or insulator makes it tough for the plug to fire properly.

That’s why it’s so important to periodically check, clean, or replace your mower’s spark plugs. With some basic maintenance, you can catch issues before they leave you pushing a silent mower around the yard.

How to Determine if Your Lawn Mower Spark Plug is Bad

How to Determine if Your Lawn Mower Spark Plug is Bad
If your lawn mower fails to start, struggles when starting, performs poorly, or uses more gas than usual, the spark plug may be to blame. Examine the plug for signs of electrode wear, widened gap, or fouling to determine if replacement is needed.

The Lawn Mower Fails to Start

If your mower engine cranks over but won’t start up, it’s likely the spark plug. A weak or fouled plug prevents proper ignition. Disconnect the lead and remove it for inspection. Look for excessive electrode erosion, carbon buildup, cracks, or oil fouling.

A spark plug that fails to ignite the compressed fuel-air mixture will cause startup failure. Consider replacement after verifying the issue. Quality Champion or NGK plugs properly gapped will restore startup reliability.

Difficulty in Starting the Lawn Mower

Grinding away repeatedly before the mower roars to life taxes your patience. Frequent pull cord straining signifies potential spark plug issues. Hard starting indicates ignition problems – the spark isn’t firing properly to ignite fuel and start the engine.

Examine the spark plug for fouling or damage. Consider replacing it or testing the ignition coil if troubles persist.

Poor Performance of the Lawn Mower

Fading power demonstrates the spark plug’s declining function. As oil fouling accumulates from overuse, the spark weakens, reducing combustion efficiency. This strains the small engine’s electrical system. When mowing performance lags, consult troubleshooting tips for maintenance.

Assess ignition problems. Replace a worn spark plug matched to your model for enhanced performance.

Increased Spark Plug Gap

You’re sensing arcing as the widened gap hinders conduction.

Over time and use, the gap between electrodes can widen, leading to:

  • Inconsistent sparking
  • Misfiring
  • Loss of power

This lack of continuity across the plug’s gap disrupts the ignition process. Reviewing spark plug maintenance best practices, like gap adjustment, helps optimize performance. An ignition tester can also isolate electrical issues needing repair. Replacing worn plugs provides a simple fix, improving ignition and restoring peak functionality.

Proper Removal and Replacement of a Bad Spark Plug

Proper Removal and Replacement of a Bad Spark Plug
Before removing the spark plug, make sure to disconnect the lead to avoid accidental electrocution. Then, utilize a spark plug socket to safely unscrew the plug for inspection and cleaning or replacement if necessary, being careful to reconnect properly.

Disconnecting the Spark Plug Lead

Disconnect the plug lead carefully before removing the old spark plug. When servicing your lawn mower’s ignition system, safely disconnect the spark plug wire by grasping and pulling the boot – not the wire itself.

Always reference your mower’s manual for the proper plug removal procedure. Once detached, you can thoroughly inspect, test, and replace the spark plug if needed. Proper lead disconnection allows safe access to diagnose potential ignition issues.

With the wire safely removed, fouled plugs can be cleaned or faulty plugs can be swapped out for new ones, restoring performance.

Using a Spark Plug Socket

Grasping the spark plug firmly, unscrew it with the socket to avoid cracking the porcelain.

  1. Select a socket specifically designed for spark plugs to prevent damage to the plug.
  2. Use a rubber insert inside the socket to securely grip the spark plug.
  3. Ensure you’re using the correct socket size for your plug – typically 5/8 or 13/16 inch.
  4. Avoid using pliers or vice grips, which can round off the spark plug hex.
  5. Gently turn the socket counterclockwise to remove the old plug.

When changing your spark plug, using the proper specialized socket is crucial to avoid cracking or stripping the plug. Investing in a designated spark plug socket set will make the process easier and prevent frustrating mishaps.

With the right tools and care, spark plug replacement can be a quick DIY maintenance task.

Inspecting and Cleaning or Replacing the Spark Plug

Take a close look at the spark plug to check for signs of wear or damage before deciding whether to clean or replace it. Examine the electrodes for corrosion, deposits, cracks, or abnormal gap widening.

Worn electrodes, oil fouling, or heavy carbon buildup signal the need for a new spark plug. Minor issues, such as some carbon fouling, may allow for cleaning with a wire brush. Severe damage or heavy wear requires full replacement. Spark plug inspection guides proper maintenance action.

Reconnecting the Spark Plug

You mustn’t overtighten that spark plug when reconnecting it, lest you damage the threads and cause even bigger problems later.

  • Gently thread the spark plug by hand until you feel resistance to avoid cross-threading.
  • Use a torque wrench or tighten 1/8 to 1/4 turn after hand tightening.

Reconnect the spark plug wire, taking care not to damage the boot.

After reconnecting, test the engine to confirm normal operation.

Proper maintenance and replacement of spark plugs help engines start smoothly and prevent premature wear. Perform routine inspections and replace worn spark plugs to keep outdoor power equipment running efficiently.

Troubleshooting tips can help diagnose issues and restore peak performance. With some basic tools and preventive care, you can handle spark plug replacement and enjoy reliable operation.


Like a well-oiled machine, a working spark plug is essential for a lawn mower to run smoothly. But, with wear and tear, this small part can eventually fail. To keep your lawn mower running in peak condition, it’s important to know how to tell if your spark plug is bad.

Signs of a bad spark plug include engine failure to start, difficult rewinding, loss of power, and increased fuel consumption. If you experience any of these symptoms in your garden equipment or lawn mower, it’s time to inspect and clean or replace your spark plug.

With proper removal and replacement, you can keep your lawn mower in great shape and maintain its performance. So, don’t wait until it’s too late – take the initiative and make sure your lawn mower spark plug is in optimal condition.

Avatar for Mutasim Sweileh

Mutasim Sweileh

Mutasim is a published author and software engineer and agriculture expert from the US. To date, he has helped thousands of people make their yards lush and thick.