How to Check, Clean, and Change Lawn Mower Spark Plugs

Manufacturers have been doing their best to give gas-powered lawnmowers a run for their money with increasingly powerful and long-lasting cordless models. If you’re in the market for a new mower, cordless offers many advantages, but according to Bryan Clayton, CEO of online lawn care service Greenpal: “Gas mowers offer more power and runtime, making them suitable for large lawns or properties with tough, dense grass.”

Comparisons aside, cordless mowers are still relatively new on the market, so if you already own a lawn mower, there’s a good chance it’s a gas one. Years of lawn mowing experience have taught me that a gas mower performs poorly without regular maintenance. It will sputter when you try to start it, stall out while it’s running or fill the air with smoke. Unless you’ve over-wintered your mower with a full tank of gas (which is a mistake that can cost you an afternoon of carburetor cleaning work), the maintenance it needs is really simple:

• Change the air filter
• Check the spark plugs
• Change the oil

It’s also important to sharpen the blade at the beginning and in the middle of the season — more often if you have a large lawn or cut the grass more frequently than once a week. However, the following instructions are focused on tuning up the engine itself.

Tools Required

• Screwdriver
• Stiff paintbrush
• Socket wrench
• 13/16″ spark plug socket
• Spark plug gapping tool
• Oil pan

Materials Required

• 120-grit sandpaper
• SAE 30 motor oil

Get Familiar with Your Machine

Lawn mower designs differ from manufacturer to manufacturer, and even some models from the same manufacturer have different designs. It should be obvious where the air filter and spark plug are, but some housing designs hide them really well. Consult the manual if you aren’t sure where to find them, and while you’re at it, note the type of filter and spark plug you need. Hardware stores stock most types, but in rare cases, you may have to order what you need online.

• Pro tip: Don’t worry if you don’t have the manual. When I lose mine (which is almost always), I just search for it on the manufacturer’s website.

Step 1: Check the Air Filter

Even if your mower has a brand-new spark plug, it won’t run smoothly if air can’t get into the carburetor and combustion chamber. Remove the air filter cover (you may need a screwdriver), pull out the filter and check its condition. If it’s just dusty, you can usually brush off the dust with a stiff paintbrush. But if it’s caked with dirt and old leaves, it’s better to spend a few bucks and replace it— especially at the beginning of the season.

Some machines may have a foam filter between the casing and the air filter that catches the larger debris. In those cases remove the foam filter, wash it in the sink and let it dry out.

• Pro tip: Air filters come in all sizes and shapes. Be sure to choose one that’s identical to the one you have.

Step 2: Check the Spark Plug

Pull off the spark plug wire and remove the plug using a socket wrench and a 13/16″ spark plug socket. Check the color of the electrode and the condition of the porcelain insulator. The top electrode should be gray or light brown, and the insulator around the bottom electrode should be white.

• Carbon deposits that turn the electrode black are fairly common. You can clean these off with 120-grit sandpaper. However, if the black deposits are wet, you’ve got an oil leak. Beyond needing a new plug, your mower needs to be serviced or replaced.
• A glazed, brown or broken insulator is a sign of a bad plug. Replace it.

It’s also important to check the gap, which is the distance between the upper and lower electrodes. For most mowers, it should be 0.030±.002 inches, but some manufacturers have different specifications, so check your manual.

Measure the gap using a gapping tool, which is a metal disk with a graduated taper around its circumference (available for a few bucks at any hardware store). If the gap is too wide, you can close it by setting the gapping tool flat against the top electrode and pressing down with your thumb. If it’s too narrow, the gapping tool has a hole you can use to pry the electrode up.

• Pro tip: When the engine runs hot, the upper electrode tends to deteriorate over time, and it gets smaller. Rather than trying to re-gap a plug with a worn-out electrode, I usually just buy a new spark plug.

Step 3: Change the Oil

Most gas mowers these days have four-cycle engines that circulate oil for lubrication. Like automobile engine oil, it wears thin and needs to be changed periodically. To get the old oil out, remove the dipstick, turn the lawnmower on its side and drain the oil into a pan. When that’s done, set the mower upright and add enough fresh SAE 30 oil to bring the level up to the top line on the dipstick.

Finish off by screwing the spark plug into the engine housing and using your wrench to tighten it. Replace the wire and push it down until it snaps onto the plug.

FAQ

Can using the wrong type of spark plug damage my lawn mower engine?

Although more than one type of spark plug may screw into your mower, they don’t all generate the same type of spark. The wrong one can cause performance problems and — yes — it can damage the engine. Stick with the plug recommended by the manufacturer.

How can I troubleshoot starting issues with my lawn mower?

Beyond the basic tune-up procedures described here, a few other issues may be preventing your lawn mower from starting:

• Matted, dried grass on the underside of the mowing deck could prevent the blade from turning. Clean off the grass with a putty knife.
• The fuel filter may be clogged. It’s either inside the fuel tank or connected to the fuel line near the tank. It’s an inexpensive part that’s easy to replace.
• The carburetor may be clogged, and the most common reason is for this is stale gas. Before adjusting and cleaning the carburetor, I usually drain the fuel tank, add fresh fuel, remove the air filter, spray starting fluid directly into the carburetor, and pull the cord. That often forces the engine to turn over, and once it does, it burns off what’s left of the old fuel, creating a cloud of smoke in the process.

About the Expert

• Bryan Clayton started mowing lawns in his neighborhood in Nashville, Tennessee at the age of 13 and has been in the lawn care industry for his entire working life. He is the CEO of Greenpal, an Uber-like app that connects homeowners to lawn care professionals in their area.