Are you wondering How to Read a Spark Plug? If so, you’ve come to the right place.
This article explains how to diagnose spark plug conditions and boost gasoline engine performance.
Referring to the spark plug photos below helps you identify the condition of any spark plug and whether it’s worth saving or should be replaced.
How to Read a Spark Plug
Step1: Learn The Parts of a Spark plug
Refer to our Spark Plug diagram below – Familiarize yourself with the name of each spark plug component so you can quickly inspect spark plugs for the condition:
Step 2: Inspect the Condition of Spark Plugs:
What do bad spark plugs look like? After you remove a spark plug and scrutinize it, you will likely see one of these 7 Spark Plug conditions:
- Carbon fouled
- Wet (gasoline)
- Burnt
- Oil deposits
- Worn out Electrode
- Broken Electrode
- Normal
Step 3: Look for Permanent Spark Plug Damage:
Once you remove and examine the spark plug, look for any signs of visible damage, such as a broken or worn electrode, terminal damage, or a crack in the spark plug insulator.
Step 4: Check Spark Plug “Gap”:
Use a Spark Plug Gauge like this one from Gapper to ensure the spark plug gap is consistent with your engine’s specifications.
Insert the electrode over the Gap tool and slide until snug; this will determine the spark plug gap you are holding.
Use the Gap Opener portion of the gap tool, and open or close the side electrode (prong) – to match the gap specifications for your engine.
If the Spark Plug Gap is incorrect, the spark delivered will not match the spark intended for cylinder combustions for your engine, resulting in poor performance.
Diagnosing Spark Plug Condition:
As you examine it, determine what the condition of your spark plug is by matching your spark plug to the photos below:
Carbon Fouled Spark Plug (image)
If you see black deposits on the electrode or insulator of your spark plug, you have a carbon build-up or carbon-fouled plug.
Carbon-fouled plugs indicate that your fuel-to-air carburetor settings are too rich in gas or that your air filter is dirty and needs to be cleaned or replaced.
You may also consider using a “hotter” or different recommended spark plug brand.
Wet Spark Plug (image)
When lawn equipment doesn’t start, and you’ve pulled the start rope four or five (or twenty) times, you may have flooded your engine and created a wet spark plug.
If that is the case, the wet spark plug can’t deliver a spark to your engine. Remove the spark plug, dry it off, wait a few hours, or try it another day after it dries out.
Burnt Spark Plug (image)
It is a sign of a burnt spark plug if you see melted electrodes, blisters on the insulator, or any white deposits. It simply runs way too hot for its application.
This can result from using the wrong spark plug, an incorrect gas/air mixture, or a loose spark plug in outdoor lawn equipment. Correct the problem, then replace the spark plug.
Oil Deposits Spark Plug (image)
Suppose you discover black oily deposits on the insulator thread or the electrode. In that case, it usually indicates oil leaks past the pistons and into the engine’s cylinder.
If you are mechanically inclined, you can try to pinpoint the problem or take it to a qualified small engine mechanic for diagnosis.
If it’s a significant repair, you should weigh the costs; it could be time for a new mower vs. fixing the old one.
Worn-out Electrode Spark Plug (image)
If you find a spark plug similar to this photo, you’re looking at a spark plug with a worn-out center electrode. Replace the plug, and you should be fine.
Broken Electrode Spark Plug (image)
If you or someone else installed the wrong-sized spark plug, which was too long for your engine, you may find a flattened or broken side electrode.
This type of spark plug can cause significant engine damage, and installing a spark plug that is too short will cause poor gas usage.
Normal Spark Plug (image)
Congratulations – If you see brownish or gray deposits on the electrode like in this photo, your electrode isn’t worn, the ground armature isn’t broken, and your spark plug is Normal. You can go ahead and check the spark plug gap and reinstall it.
Recommendation: If your spark plugs all check out Normal, check your gas, fuel, and air filters*. Please ensure they are clean and regular, and replace them as needed. These last two repair checks can significantly impact your engine performance.
What Your Spark Plugs Can Tell You:
Spark plugs are the all-important device that delivers the necessary Spark to your engine and, when combined with the proper gas-air mixture, provide the combustion needed to run your motor.
A spark plug’s condition can provide tell-tale signs about the condition of your engine.
By simply reading spark plugs for wear and deposits, you can quickly discover whether your spark plugs are either worn out, getting too much gas (rich), too little gas (lean), running too hot (burnt), wet, cracked, and whether it’s time to change them.
A bad or fouled spark plug can also cause problems in your Car, Truck, Lawn Mower, Chainsaw, or any other gas-powered engine. Symptoms include the inability to start your motor, engine misfires, poor gas mileage, and lousy performance.
If you notice any of these performance issues with your vehicle or Lawn equipment, your spark plug is the best place to start.
Why Check your Spark Plug first?
1) A bad spark plug is often the leading cause of poor engine performance.
2) Spark plugs are very inexpensive to replace. It’s the cheapest place to start.
3) If you haven’t changed your spark plugs in 2 years or more, your problem will likely be a bad spark plug.
Routine Engine Maintenance:
Every vehicle owner, landscape professional, and homeowner must regularly check the performance of their lawn equipment. Examining spark plugs allows you to “Instantly Read” the condition of this vital piece of your engine’s operation.
Video from Motorweek: “How to Read Spark Plugs”
Frequently Asked Questions (FAQ)
Q: How Do I Know If a Spark Plug Is Bad?
A: The most straightforward way to determine if your lawnmower spark plug is wrong is to remove and inspect its condition. For instance, if the spark plug has a worn or broken electrode (prong), you must replace it.
Additionally, if the spark plug has oil deposits or is burnt, it should be replaced.
Sometimes, the spark plug needs to have the spark plug “Gap” set correctly.
When in doubt, we recommend replacing a spark plug if you suspect it is causing the engine to fail, as spark plugs are inexpensive.
Q: Should You Replace a Wet Spark Plug on A Mower?
A: A wet spark plug on a lawnmower typically indicates it has been flooded with too much gasoline. A wet spark plug will happen when you pull the pull cord many times without starting the lawnmower.
We recommend removing and inspecting the spark plug for damage or, if wet, drying it off and reinstalling it.
Note: Be sure you have “Fresh Gas” in your mower before starting it again, as this is a leading cause of power failure.
Q: How Often Should You Change a Lawnmower Spark Plug?
A: To keep a Lawnmower engine running smoothly, the spark plug should be changed at least every two years.
Q: How Do You Set the Spark Plug Gap on the Mower?
A: Knowing how to Use a Spark Plug Gauge is vital to ensuring the spark plug gap is consistent with the engine specifications for your lawnmower engine.
Step 1: Insert the electrode over the Gap tool and slide until snug; this will determine the spark plug gap you hold.
Step 2: Use the Gap Opener on the gap tool to open or close the side electrode (prong) and match the gap specs for your small engine.
If the Spark Plug Gap is incorrect, the spark delivered will not match the spark intended for cylinder combustions for your engine, resulting in poor performance.
Conclusion
Determining the condition of spark plugs is simple once you know how to read one. Now that you know what to look for, you must familiarize yourself with the simple steps to change a spark plug.
If you’re changing or inspecting multiple spark plugs on a vehicle, lawnmower, or multi-cylinder engine, we highly recommend checking and replacing one spark plug at a time. This will help you avoid re-connecting the wrong spark plug wire.
Check online or with your local hardware or auto parts store. Once you’ve located and replaced the correct model spark plug(s), gas, and air filters, your engine should perform as designed.
Note: If you’re inspecting Spark plugs on Outdoor lawn equipment and aren’t very familiar with minor engine maintenance – check out how to tell if a lawn mower spark plug is bad.
Notes: The contents of this article are designed for informational purposes and to help you quickly navigate the various conditions. Under no circumstances should you avoid the advice of your local mechanic or small engine repair professional.
Always seek the advice of a certified technician if you have any questions or concerns related to the content of this article.
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Further Reading
- Check out this handy guide from Briggs & Stratton to find the correct spark plug gap requirement for mowers.
- Infographic How To Inspect Spark Plugs
- Is Your Lawn Mower Surging – Besthomegear.com
- How To Tell If a Lawn Mower Spark Plug is Bad – Besthomegear.com
- Check out the Anatomy of a Spark Plug. Check out this article from Wikipedia
- How do you Clean and Change your spark plug? See the best way to do it here.
- Need a New leaf blower? Top 5 Best Leaf Blowers – Cordless and Electric
- How To Read a Spark Plug – Krockfd1, CC BY-SA 4.0, via Wikimedia Commons
