How to Check Lawn Mower Spark Plugs (Easy Guide)

Lawnmowers need spark plugs to run the engine, but they can rust, corrode, and fail. Without functioning spark plugs, your mower won’t get much done. You know it might be time to replace them when the engine misfires, consumes excess fuel, or sputters. But how do you inspect a spark plug?

To check lawn mower spark plugs, follow these steps:

1. Find the spark plug the engine
2. Pull the spark plug off of the lawn mower
3. Inspect the spark plug for rust, debris, and corrosion
4. Replace the spark plug if needed
5. Brush it with cleaning solution
6. Check and correct the gap
7. Use a spark plug tester
8. Put it back onto the engine

In this post, you’ll learn how you can check a lawn mower’s spark plugs, tips to know when it’s time to replace them, and how to locate them on your mower. You’ll also find out what tools and supplies you need to handle, clean, and replace the spark plugs.

Locate the Lawn Mower’s Spark Plugs

Start by finding where your lawnmower has its spark plugs. Most brands place the plugs at the front of the engine, which rests on top of the mower’s deck. The spark plugs usually have a bulky wire with a plastic or rubber housing over them. You’ll notice them by check for a cylinder with a wire coming off of it.

The spark plug’s wire protects the spark plug from water and debris, but it also provides a steady connection to the engine. Removing the spark plug wire will stop the electrical connection, but it’s important to turn off the ignition before trying to take anything off of the engine.

Once you locate the spark plug wire, it’s time to remove everything. Wear rubber protective gloves and proceed to the next step.

Remove It With a Spark Plug Wrench

Pulling the spark plug wire off of the spark plug is a delicate process. You don’t want to damage the spark plug, so it’s important to grab the wire underneath and put it straight out. Pulling the wire up or down while removing it can damage the spark plug.

Follow this method to remove the spark plug from your lawnmower:

1. Place a spark plug wrench over the spark plug. We suggest the Lexivon Swivel Magnetic Spark Plug Wrench since it hugs and holds the spark plug, preventing it from sliding around. This wrench has a 360-degree hinge and fits most lawnmower spark plugs.
2. Twist the spark plug wrench counter-clockwise until it budges, then slowly remove the plug when it’s loose. Spark plugs have deep threads that take a while to remove. Don’t pull it out until it’s fully unthreaded; Otherwise, you’ll risk stripping the spark plug.
3. Set the spark plug on a clean, soft cloth. Spark plugs are quite sensitive, so anything abrasive can scratch the threads and make it unusable. Many spark plugs can be cleaned and put back into the engine. Our goal is to help you bring your spark plugs back to good condition.

While it’s ideal to be able to reuse spark plugs, they don’t last as long as other lawn mower parts. Repair Clinic estimates most mower spark plugs last up to one year before it’s time to clean or replace them. Once the spark plugs are done, they can’t be refurbished.

Remember, damaged spark plugs often show similar symptoms to a worn lawnmower battery. Slow starts, failing to operate, or random rough ignition might not be your spark plugs. It’s best to test the battery with a multimeter if your spark plugs look and test fine.

Look for Signs of Wear and Tear on the Spark Plug

A brand-new lawnmower spark plug is typically white and silver or bronze. There are a few variations, but it definitely shouldn’t look dingy, debris-covered, or warped. If the gap from the tip to the corner on the piece of metal sticking off of the tip is too short, it might be time to either replace or trim it.

The good news is that your mower’s manufacturer guidelines should indicate the gap’s size, replacement model, and more information. It’ll also tell you what kind of spark plug wire you’ll need, so there’s no need to guess or pick parts that are too small or unfit for the mower.

Briggs & Stratton has a helpful guide that shows everything you need to know about inspecting, cleaning, and replacing spark plugs for their brand-made lawnmowers. While it’s not applicable to every lawnmower on the market, it’s always nice to have a visual representation of what they look like and where to find them.

Decide if It Needs to Be Replaced

Now that you’ve looked for signs of potential problems on the lawnmower’s spark plug, it’s time to find out if it needs to be replaced.

Keep these considerations in mind:

• If the spark plug’s threads or body are stripped, it needs to be replaced. These damages are irreplaceable, so you’ll have a problem if you try to start the mower after installing it under the spark plug wire. The loose connection will cause ignition problems.
• A loose, broken, or cracked O-ring needs a replacement. Not all spark plugs have O-rings, but many of them do to prevent water from entering the mechanism. A damaged O-ring is practically useless and stops a water-tight seal from forming.
• Check the spark plug’s electrode (the curved tip at the top of the piece) to see if it’s bent or broken. These problems are irreversible, which means you have to replace it with a new spark plug.

Many of the tiny components on a lawn mower’s spark plug are nearly as expensive as the whole unit. You might as well get a new one since they often don’t last longer than a year or so. If you can’t use the lawnmower choke, it could mean there’s a loose engine connection.

Clean the Mower’s Spark Plug

If you’ve determined that the spark plug is in usable condition, it’s a good idea to clean it. Most mower spark plugs can be cleaned with a wire brush, but it doesn’t hurt to downgrade to a stiff-bristle toothbrush to prevent abrasive scratches and corrosion.

Check your local automotive or hardware store for a spark plug cleaning solution. You can spray the spark plug and brush it until the rust and debris lift off of it, restoring it to a like-new condition. However, aggressive scrubbing can make it hard to thread the spark plug.

Always make sure the spark plug is entirely dry before putting it back into the wire and housing. Moisture can cause current problems, preventing the spark plug from establishing a reliable connection.

Check and Adjust the Spark Plug’s Gap

Champion Autoparts recommends keeping a spark plug’s gap between 0.028 to 0.060 inches. The gap refers to the space between the tip of the curved electrode to the right angle at the base of it. Check the gap with a Spark Plug Gap Tool. Slide the ring next to the electrode and turn it until it stops the ring.

If the spark plug’s gap needs to be lengthened, place the sharpened inner ring around the electrode. Turn it until the inner metal is shaved enough to fit within the aforementioned measurement’s range. If the electrode breaks off, the metal was too thin and the spark plug needs to be replaced.

After shaving the spark plug, you can either use a tester or simply place the spark plug back into its socket on the lawnmower.

Use a Spark Plug Tester to Inspect Its Condition

Spark plug testers offer a quick and easy way to know if your lawnmower’s spark plugs are in good condition. You can use them on any spark plug, all you have to do is attach the tester to the machine and you’re good to go.

Try the ENA Universal In-Line Spark Plug Tester. Plug it into a spark plug machine, then check if it’s illuminated. You can find these machines online, at an automobile store, or almost any big chain store you’ll come across. They’ll brighten up if the spark plug is working.

Put the Spark Plug Back into the Lawn Mower

Whether you’re using a brand-new spark plug or placing the old one back into the lawnmower, the steps are the same. All you’ll do is simply reverse the process you followed to remove the spark plug from your mower, ensuring the threads are aligned with the socket.

Here’s what you’ll need to do to replace the spark plug:

1. Thread the spark plug into the socket. Slowly rotate it with a spark plug wrench. Always rotate it gently to prevent the threads from stripping or breaking.
2. Attach the spark plug wire over the plug. Ensure it’s snug and doesn’t wiggle or fall off.
3. Start the lawnmower’s engine to check if the sparrk plug is working as it should. It shouldn’t sputter, skip, or anything else.

If you’re sure the spark plug is working but the mower still won’t work, inspect the fuel filter. Air or clogs in the filter can cause similar issues.