How to Tell When it Is Time to Change Bad Spark Plug in a Lawnmower

A lawnmowers spark plug plays a critical role in how the engine operates. If it fails to fire, or doesn’t work properly, your lawnmower will not work.

But how to tell if a lawn mower spark plug is bad?  

There are a few ways to diagnose a bad spark plug. The following article will explore Symptoms Of bad lawn mower spark plug, and When & How To change a spark plug in a lawn mower.

How To Tell When It Is Time To Change Bad Spark Plug In A Lawnmower

What Does A Sparkplug Do?

It’s essentially an electrical device that directly draws power from a special induction coil that is connected to the flywheel of the engine.

When it is operating properly it ignites the fuel and air mixture in the combustion chamber. Without it, there is no way to burn the fuel and the piston that drives the mower’s blades simply will not work.

While spark plugs are designed to be durable, and their construction is quite simple, they do have a finite lifespan.

As they start to degrade, or physically deform from excessive use, the spark plug tends to fire intermittently.

What Are Some Signs Of A Bad Spark plug?

As time goes on a failing spark plug could make it much harder to start. This could manifest as a frustrating number of pulls on the drawcord.

In the case of a riding lawnmower, it could take a lot longer for the starter motor to turn the engine over, which could also start to tax it or even affect the life of the battery.

What Are Some Signs Of A Bad Spark Plug

A badly degraded or deformed spark plug could cause the lawnmower’s engine to stall. This is especially likely to happen if it bogs down in tall grass.

A riding lawnmower with a hydrostatic transmission and a bad spark plug could potentially stall out completely when the engine is being taxed by doing something like cutting while going uphill.

how to tell if a lawn mower spark plug is bad?

If you are paying attention, you can usually notice if a lawnmower’s sparkplug is starting to go bad.

A pull mower might take an increasingly longer number of pulls, or a riding mower might take longer than usual to start when you turn the key.

How To Tell If A Lawn Mower Spark Plug Is Bad

A lawnmower with a bad sparkplug might also start to develop performance issues or consume more fuel than usual. This could even cause a lingering smell of raw gasoline while you are mowing.

A truly bad sparkplug might also show visible signs. Physically the center of the electrode should have a flat top. If you take the plug out and it looks rounded on top or there are some cracks, then it’s probably wise to replace it.

At the same time, some bad spark plugs will also look black from carbon or degraded excess gasoline.

In a pinch, you might be able to lightly clean the plug and gently adjust the gap between the electrodes.

While this might work once or twice, to get you through a mowing session, you should still consider replacing the plug.

how to change a spark plug in a lawn mower?

Changing a lawnmower’s sparkplug might seem a little intimidating the first couple of times you need to do it.

Once you get comfortable with it, you will find it’s actually pretty easy, and relatively straightforward.

How To Change A Spark Plug In A Lawn Mower

You will need to get a few tools first

  • A ratcheting socket driver
  • A spark plug socket (13/16 or ¾-inch)
  • A spark plug gauge

Just about any reputable hardware or automotive store can point you in the right direction.

Step 1: Disconnect the lawnmower’s sparkplug wire.

While doing so, take an extra minute or two to clean around the plug and wipe away any debris with a paper towel or shop rag.

Step 2: Use the spark plug socket to carefully remove the spark plug.

Sometimes the plug can be a little stubborn. If it doesn’t move with a reasonable amount of force, you should spray it down with a little WD40 or some other type of penetrating lubricant.

Give it ten minutes or so to soak into the tight spaces around the plug. Then try again to loosen it.

Step 3: Examine the sparkplug, paying close attention to the electrode.

If the electrode looks dry, or there is a strange powdery material on it, there might be a problem with the carburetor, which could be depriving the engine of the fuel it needs.

Step 4: Double-check the new sparkplug.

Most are “Pre-gapped” to match your specific engine. Still, it would be wise to double-check it with a spark plug gauge to make sure it matches the manufacturer's specifications.

Step 5: Install new spark plug

Insert the new spark plug into the clean hole and then finger-tighten. At that point, use the appropriate sparkplug socket and ratchet to tighten the plug down until it stops.

Then give it another quarter turn to make sure it is sealed tightly. Don’t over-tighten it beyond this point, or you will risk cracking the plug.

If you no longer have the lawnmower’s owner’s manual, you should write down the model number and put it somewhere safe. It’s not uncommon for the model numbers to wear off on a sparkplug over time.

How Can I Maintain My Lawnmower’s Spark Plug?

Spark plugs have a limited lifespan. So, it’s unrealistic to expect it to last forever.

However, making sure that you properly maintain the lawnmower’s engine and other components can reduce the conditions that can prematurely shorten the spark plug’s lifespan.

It’s also worth bearing in mind that seasonal maintenance is just as important as periodic in-season maintenance.

Beware Dust And Debris

Dusty, dry conditions can potentially affect how a lawnmower performs. Dust and even excessive airborne pollen can start to clog the air filter.

When this happens the fuel-air ratio in the engine’s combustion chamber can lead to premature carbon buildup in the carburetor or around the spark plug.

Can Dust And Debris In The Fuel Tank Cause A Problem?

At the same time dust that gets into the fuel tank through the breather on the gas cap or when you open the cap to refill the tank can become a problem.

As it settles lower into the tank these tiny particles can start to collect and congeal. If you run the tank low or completely dry during a long mowing session, this debris can be sucked into the fuel line.

Not only can this impede the passage of fuel into the system, but it can also start to clog the lawnmower’s fuel filter.

Cleaning Or Replacing the Air Filter 

Air filters naturally clog over time. In a certain light, it’s their job. When it is functioning properly, the air filter is tasked with catching any dust and airborne debris before it makes it into the carburetor.

As debris starts to buildup, it can rob the lawnmower of the air it needs to efficiently ignite the fuel-air mixture in the engine’s combustion chamber.

Excess fuel and other debris can then damage or corrode the spark plug.

Remove Air Filter Then Clean It

With some older, gas-powered push mowers, the air filter is little more than a piece of soft flexible foam.

To clean it you simply loosen the long, threaded bolt that holds the lid in place. You can then wipe off any noticeable debris with a clean piece of paper towel or a shop rag.

If you have a can of air or a shop air compressor, you might be able to blow off any stuck-on debris.

Some newer gas-powered push mowers and riding lawnmowers have more sophisticated air filters and air cleaner systems.

In a case like this, you should check the part number in the owner’s manual. If you don’t have it, the air filter itself might have something stamped or printed on it.

When in doubt, you should be able to easily take it out and bring it with you to the auto parts store.

There are even some discount retail chains that have an automotive department stocked with common lawn mower air filters.

Winter Lawnmower Maintenance Helps Preserve The Engine And Spark Plug

If you live in a part of the country where it gets cold in the winter, or the grass merely goes dormant, then you will need to take some measures to ensure the lawnmower is stored away properly.

Stabilize the Fuel

Fuel stabilizer is very important when storing a lawnmower for the winter. Not only does it help prevent the octane of the gasoline from degrading, but it also reduces the chances of water settling out of the fuel.

Personally, I try to run the fuel tank very low on the last mowing session of the fall. Then I take a small siphon and suck out the last quarter of a tank of gasoline.

How Do I Drain And Change The Lawnmower Oil_

This helps suck out some of the dust and other debris that might have found its way into the tank during the summer.

Then I mix fuel stabilizer with fresh premium gasoline in a separate gas can. At that point, I fill the tank until it’s roughly half full.

This allows me to add fresh gasoline with the first session next spring. It’s not something critical, but it helps hedge my bets!

Clean the Deck

I also make it a point to clean away any lingering grass clippings and other organic matter from the underside of the mower deck.

Inspect The Blade

Mice and other small rodents will sometimes be attracted to stuck on grass clippings during a particularly cold and harsh winter.

When they do, they have a knack for chewing on things that they shouldn’t. This includes spark plug wires!

In the case of a riding lawn mower or lawn tractor, I will pull the battery out and bring it into the house.

A battery that is exposed to freezing conditions without proper charging can slowly degrade.

Spring Lawnmower Preparation Is Essential

Spring weather can be fickle. When the trees don’t have any leaves, a rainy day that gives way to a few days of sun can bring your grass out of dormancy with a fervor.

One day you just look outside only to find that things have gotten out of control.

Change The Oil

In a moment like this, it can be tempting to run out to the garage, dump the old gasoline for the snowblower into the lawnmower and just pull the cord.

While you might get away with it once or twice, it’s likely not very good for the long-term lifespan of your lawn mower spark plug or your carburetor.

What Kind Of Oil Should I Use In My Lawn Mower_

After a long winter sitting in the garage, your lawn mower’s engine likely needs an oil change, the air filter needs to be cleaned or replaced, and it could do with a good dose of fresh fuel.

If you didn’t add fuel stabilizer to the tank when you put it away, you should add some Iso Heat or a similar fuel conditioner to address any potential water separation in the tank.

Lubricate the Moving Parts

If you have a lawn tractor or riding lawn mower, you should also grease the pulleys and bearings. A simple grease gun and two healthy pumps on every zerk fitting will improve operating efficiency.

While this won’t have an appreciable impact on the spark plug, it’s still a wise move for maintaining your investment.

If your riding lawnmower has a battery, you need to make sure it is properly charged before you install it, and try to start the mower for the first time.

Last Updated on by Lindsey Hyland

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