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How To Tell If A Lawn Mower Spark Plug is Bad

Quickly determine whether your Spark Plug needs Replacing

When I was having problems with my gasoline powered lawn mower I realized I may have a bad spark plug.  I needed to know how to tell If a lawn mower spark plug is bad, and if it was bad, exactly what spark plug to use for lawnmowers.

I was experiencing all sorts of problems with starting my mower, and it didn’t perform at the level it once did. Like me, you may also be asking when will I know I have a Bad Lawn Mower Spark Plug or how often do I need to change my lawn mower spark plug.

After spending many hours researching articles, manuals, and watching videos, as well as a lot of trial and error I finally discovered what the problems with my lawn mower could be, and how common they actually were. Then I knew how to tell if I had a bad lawn mower spark plug.

The most common way to tell if your lawnmower spark plug is bad, is the engine being more difficult than normal to start.  Since the engine only starts due to two principles: Gas & Spark, and assuming you have filled the gas tank – its most likely the spark, or a lack thereof.

Your push mower’s engine may stutter and take several pulls of the cord to start. If you have a sit on tractor mower the engine may not start on the first turn of the key. When it’s running the engine may cut out, causing you to have to attempt to restart your lawn mower’s engine again.

You may also experience loss of power and slower acceleration. These may appear subtle at the start but over time they can become more and more apparent, and these signs definitely point to a bad lawn mower spark plug.

What does a bad spark plug look like?

To quickly determine the condition of Your Spark Plug: Check out This Photo Chart From Champion – which illustrates all Spark Plug Conditions; then compare the photos to your own Spark Plug.

What Does a Spark Plug in a Lawn Mower Do?

To know why a well-conditioned spark plug is so important to run a lawn mowers engine effectively it’s helpful to know the job of a spark plug. The spark plugs job is basically the same job as it would in any other small engine.

A spark plug is an electrical gadget which fits in the engines cylinder head, and its job is to spark ignition of the fuel to create combustion within your engine. If it’s damaged or in bad condition, you may experience all sorts of issues.

Where does a Spark Plug get Power From?

The spark plug is connected to high voltage generated by an ignition coil or magneto. As the electrons flow from the coil, a voltage develops between the center electrode and the side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. The ionized gas becomes a conductor and allows the electrons to flow across the gap. Spark plugs usually require a voltage in excess of 20,000 volts to ‘fire’ properly.

How to tell if a Spark Plug is Bad?

We touched on the spark plugs job above and it’s important to know this to help us understand why we may be experiencing some of the common symptoms of a bad spark plug. We may have thought our lawn mower was experiencing engine problems and it could cost you hundreds or require buying a new mower, but it could simply be down to a damaged or faulty spark plug.

As we mentioned once the lawnmower engine is running, it may cut out unexpectedly and for what appears to be no reason. Now we know the job of a spark plug it may be easier to understand what is happening. The reason for the engine cutting out is often due to the spark being created from the spark plug being too weak to ignite the fuel which mixes with air in the carburetor. The result can often lead to the engine become flooded as more fuel enters the cylinder, due to it not being burnt off effectively.

If the engine has been started for a while it becomes hotter, which can increase the gap between the spark plug center and the firing electrodes, further weakening the ignition spark. This can be why you may need to leave the lawn mower to cool down if you’ve been trying to start it for a while but it keeps cutting out.

Some other tell-tale signs that your lawn mower spark plug is bad are excessive fuel consumption. You may have noticed you need to fill the engine with gasoline more often than usual. This can often be caused by faulty spark plugs. If the gasoline isn’t being completely burnt off or inefficiently so your fuel consumption will increase and your fuel efficiency will plummet. As the gasoline isn’t being burned off as it should you may also notice the smell of fuel in the air – often it’ll smell like raw gasoline.

Is it your Spark Plug or Something Else? – Let’s check out”The Top 6 Reasons Your Lawn Mower Won’t Start”. 

 

Top Reasons Lawn Mower Not Starting — Lawn Mower Troubleshooting

How often do you change spark plugs in a Lawn Mower?

Most experts claim you should replace the lawnmower spark plug in a standard mower once per season, or after around 25 hours of use.

I believe that may be overly cautious and you may get a little more use out of it before you have to change the spark plug.  A good gauge would be how well your lawn mower is working and whether you’re having any issues.

In any case, I’d probably look to change your lawn mower spark plug a maximum of every two years just to be safe, even if it’s still performing well. They’re very cheap, and you can usually pick them up for only a few dollars.

How to clean a lawn mower spark plug

When you remove the spark plug from the lawn mower it’ll give you some clues as to whether it needs to be replaced or whether it can be cleaned up.

The center electrode should have a flat top. If you find it is rounded then you should replace it. You should also look for any cracks, chips or physical damage to the porcelain sheath on the plug, as well as it’s pitting on its firing electrode.

If there’s any damage at all or you’re unsure then it’s best to replace it, especially as they’re so inexpensive.

If the plug is in decent condition, but has some black discoloring due to carbon or is wet with gasoline then this can usually be cleaned up.

To clean the spark plug spray it with some brake cleaner and let it soak in for around 10 or so minutes. Then wipe it with a clean cloth or rag to make sure any excess residue is removed. You can repeat this step 2 or 3 times if needed. Once clean you should inspect the plug and make sure it’s clean and clear from the threads to the point of contact. If you notice at this stage there’s some previously unseen damage then it’s best to replace it.

Can the wrong spark plug damage your engine?

The short answer is yes! It’s important you research which spark plug is needed for your mower as different models require different plugs.

A common sign the wrong spark plugs have been installed is if the electrodes are flattened or broken off. A spark plug which is too long can cause extensive damage to your lawnmower’s engine, whereas a plug which is too short can cause poor performance and fuel efficiency.

A spark plug left is left in the engine for an extended period of time can cause problems with the piston rings.

What causes spark plugs to fail?

There are various reasons for spark plugs failing but a common one is from carbon buildup. This can be caused by a cracked distributor case, clogged air filter, a dirty fuel injector or improper fuel to air mixture. Another reason they fail is due to oil contamination. If oil seeps onto the spark plug, it will damage the tip over time. If the seals begin to fail then oil can leak into the combustion chamber.

Once You’ve determined the Spark Plug needs Replacing – Refer to the Instructions below:  How To Change a Lawn Mower Spark Plug.

How to change a lawn mower spark plug

Replacing your lawn mowers spark plug is pretty straightforward and one of the easier lawn mower maintenance tasks. The only tools you’re likely to need are a ratcheting socket driver, a spark plug socket and gauge. Make sure the engine is cool before you attempt to replace the spark plug.  For the novice who may be asking where is the spark plug on a lawn mower, the spark plug can be found beneath the black spark plug wire. Simply, unsnap (disconnect) the spark plug wire and you can now remove and inspect the lawnmower spark plug.

  • The first thing you should do is check your replacement plug is the same model and shape/size as the one that’s been replaced. You can usually match them using the reference numbers on the white insulator.
  • Disconnect the spark plug lead and clean around it
  • Remove the old spark plug using a socket driver or wrench. If you’re having trouble removing the old spark plug you can try adding some lubricant and leave it to soak for 10 minutes.
  • Once the old spark plug is removed inspect the electrode to see how the engine is running. If it’s too wet there may be a problem with the engines choke system. If it’s powdery and seems too dry there could be a problem with the carburetor mounting gasket or it could be starved of oil.
  • The final step is to insert the new spark plug into the hole of the engine. Remember to always turn the spark plug by hand initially, and until it won’t turn anymore to make sure you don’t cross thread it. Then use the spark plug wrench, a deep socket wrench or socket driver to tighten the plug. Don’t overtighten, just make it snug, then you just need to connect the spark plug wire back securely on the top of the plug.

Spark Plug Installation video: “How to Check, Clean, And Replace a Lawn Mower Spark Plug”

How To Check, Clean & Replace Lawn Mower Spark Plug

What size wrench do you need for a spark plug?

You’ll need a socket wrench to remove a spark plug. They have rubber inside to protect the spark plugs ceramic exterior. They can come in sizes from 13/16 inches to ¾ inches.

Check the size of your spark plug and match the socket to that. They’re readily available online or in hardware stores.

What does a white spark plug indicate?

If your spark plug is white in color or looks to be blistered it means the plug is becoming too hot. This could be down to the engines cooling system or incorrect ignition timing.

What does gap a spark plug mean?

In order for the spark plug to fire effectively, there must be an exact distance between the center and side electrodes of the spark plug.

When you adjust the distance between the two electrodes it is called gapping your spark plugs. You’ll need a feeler gauge to gap your spark plug properly.

The gap must be a specific width if the gap is too small the spark doesn’t have enough space to generate the power needed to ignite the fuel.

What is my lawn mower spark plug gap?

You’ll need to go to the manufacturer’s website or read the manual to see the exact gap of your lawn mowers spark plug. However, the gap usually ranges between .020” and .030”.

Where can I buy replacement lawn mower spark plugs?

Once you’ve got the model number from your spark plug or have referred to your user manual you can go about searching online for a replacement.

Amazon usually stocks pretty much all shapes and sizes so that could be your first port of call.

Need a recommendation on a Fantastic Cordless Electric Mower – Without a Spark Plug? Check out our favorite one here – the Greenworks 21″ 40V Brushless Cordless Mower

There’s also the option of going to your local hardware store, where you should be able to find a suitable replacement, assuming the stock a wide range of engine parts.

Can a faulty spark plug cause an engine to catch fire?

In theory yes but it’s very unlikely in the case of a lawnmower. There’s less than a 1% chance of this ever happening, even if it is leaking fuel. The engine is much more likely to fail before it catches on fire. If there are any warning signs though or you’re worried it’s best to consult an engine specialist for your peace of mind.

Further Reading

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Kevin

As a Homeowner, Builder, and Property Developer/Manager for 30+ years, I’ve truly worn a lot of different “hats”, and In that capacity, owned or used about every tool or piece of home equipment I can think of. Managing properties in Michigan, Arizona, and Florida, has also allowed us to test many of the products we review firsthand, in vastly different climates and conditions. Our goal at BestHomeGear.Com is to share that first-hand experience by providing exceptional Home & Garden techniques, Product Reviews, and useful DIY Guides for our faithful readers - Homeowners like you. We hope you find our Site extremely helpful and informative!

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2 Comments

    1. Hi and thank you for posting a comment – I hope the article helps you quickly determine how to remove, inspect and change spark plugs in the future.

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