Lawn Mower Troubleshooting: Dealing With a Bad Spark Plug


Jeffrey Tung on Feb 16, 2024

blue lawn mower

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Imagine this.

It's the weekend. You're ready to tend to your lawn, but your mower wouldn't start and the engine keeps spluttering. The culprit may be a spark plug that has gone bad

What is a lawn mower spark plug?

Spark plugs are tiny electrical devices that serves as ignition sources for gas-powered mowers. They create the spark needed to ignite the fuel-air mixture in the combustion chamber, which powers up the engine. Lawn mower spark plugs that go bad causes incomplete fuel combustion, preventing the engine from working. 

How do I know if I need to change my lawn mower's spark plugs?

To know if you have worn spark plugs that need replacing here are some bad spark plug symptoms you need to watch out for.

  1. The engine doesn't start . One of tell-tale signs of damaged spark plug is a mower that doesn't start or requires multiple attempts to start. You may need to change spark plugs or have your mower repaired. 
  2. Decreased engine performance. If you notice a reduction in power or performance when mowing, your spark plug may need to be replaced. Engine misfires during operation, stalling, or rough idling are also common signs your spark plug has gone bad.
  3. Excessive exhaust smoke. A foul smell or black emission coming from your lawn mower can mean an incomplete combustion in your machine's ignition system. Check your spark plugs to ensure engine's performance. 
  4. Increased fuel consumption. If you notice that you keep refiling the gas tank more frequently than normal, a faulty spark plug may be causing your lawn mower to use more fuel than necessary.
  5. Worn out and dirty spark plug. Physically check if there are visible signs of wear such as soot build-up around the spark plug or worn electrode. These can affect your plug's performance and in turn, affect how your lawn mower engine works.


What tools do I need to replace a bad spark plug?

Replacing a spark plug is fairly simple and requires minimal tools. You'll need a spark plug socket or a normal wrench, gap guage, anti-seize, and a new spark plug. You may also need a knife, spray-on plug cleaner, and wire brush to scrub and scrape deposits on the spark plug. 

How do I replace a lawnmower's spark plug?

  1. Before starting anything make sure the lawn mower's turned off and cooled down.
  2. Then, find where the spark plug is on your lawn mower. Usually, it's located near the engine's cylinder head.
  3. Remove the old spark plug using the socket wrench. Move counterclockwise to loosen (0:31). 
  4. Before installing your new plug, inspect if the gap follows manufacturer standards. Adjust as necessary using the gap guage.
  5. Place a small amount of anti-seize compound on the spark plug to make it easier to remove next time. 
  6. Install the new plug by carefully threading it into place by hand. Use a spark plug wrench to keep it secure (1:34). 
  7. Double-check if it's properly tightened before starting the lawn mower. If properly installed, the engine should run well and everything should be in working order.


Can old spark plugs be cleaned and reused?

Yes, spark plugs can be cleaned and reused. This is especially useful when you don't have a new one in hand and need to continue with your mowing. However, most of the time, it's better to replace your spark plug with a new one to ensure the best performance for your lawnmower. Old plugs have worn edges that make it more difficult to create electrical discharges. In the long run, it may lead to poor engine performance and more repairs. 

How do I choose the right spark plug replacement

Small engine sparkplugs aren't universal, so use the right plugs to keep your mower operating smoothly. When buying a replacement, check with your owner's manual for the type of plug your lawnmower uses. Spark plugs from reputable manufacturers also have code numbers on the side of the ceramic insulator, indicating which model you need to buy. If you've lost your manual or if the number isn't readable anymore, you can always bring your old plug to the shop and they'll help you find the right model.

What causes spark plugs to wear out?

Spark plugs go bad for several reasons. Prolonged use, high engine temperature, fuel quality, and oil leaks all contribute to  spark plug wear.  A regular maintenance schedule can help with these issues and keep your spark plug and lawn mower in top condition for a long time.

To end

Don't let faulty spark plug slow down your mowing. It may be small, but its impact on performance is significant. Checking the signs of bad spark plugs is one of the most important maintenance for mowers. With the right knowledge and tools, you can easily keep your mower running and your lawn pristine. Clean your spark plugs to avoid poor performance and spark plug fails.