⚡How to Replace a Lawn Mower Spark Plug: A DIY Guide
A lawn mower is a simple machine: a powerful engine with a lethal blade that spins around, cutting grass as long as there's gas in the tank and a spark plug that's properly combusting the fuel.
But if your spark plug is overworked and corroded, like the one pictured below, it's probably not firing correctly, and it's definitely time for a replacement.
The Science of the Spark: Why a Healthy Plug Matters
A spark plug's job is to create a high-energy electrical spark to ignite the compressed fuel-air mixture inside your engine's cylinder. This controlled explosion is what drives the piston and powers the mower.
The "gap" between the plug's center and ground electrodes is critical. If the gap is too wide due to wear, the spark may be too weak to ignite the fuel consistently, leading to misfires.
If it's too narrow, the spark may not be large enough for efficient combustion. A healthy, correctly gapped plug ensures a strong, consistent spark for optimal power and fuel efficiency.
How to Tell if Your Spark Plug Needs Replacing
Your mower will give you some clear signs that its spark plug is failing:
- Unusual Noises: You might hear rattling, pinging, or other odd sounds that suggest your engine is "knocking" or misfiring.
- Increased Fuel Consumption: If you find you're using more fuel than normal, it's a key sign that your plug isn't burning the fuel efficiently.
- Difficulty Starting: The classic problem. If your engine is hard to start or won't start at all, a failed spark plug is a prime suspect.
- Poor Performance: If the engine feels rough or lacks power, it could be due to an inconsistent spark from an aging plug.
How to Replace the Spark Plug in Your Lawnmower
This is a simple job that can make a world of difference. Here’s how to do it:
- Safety First: Work on a cool engine and disconnect the spark plug wire.
- Clean the Area: Clean any debris from around the spark plug to prevent it from falling into the engine.
- Remove the Old Plug: Use a spark plug wrench to gently unscrew and remove the old plug. If it's stuck, a little WD-40 can help.
- Check the Gap: Use a spark plug gauge to ensure the gap on your new plug matches the manufacturer's recommendation. Our guide can show you how.
- Install the New Plug: Screw in the new, correctly sized spark plug by hand first to avoid cross-threading. Then, use your wrench to give it a final, firm turn. Do not overtighten.
- Reconnect and Test: Replace the spark plug wire, ensuring it's securely connected. Then, start the engine to test your work!
Here's a handy video tutorial on how to replace a spark plug:
❓Frequently Asked Questions
What is the correct spark plug for my lawnmower?
While several different spark plugs may screw into your mower, using the correct type is crucial for engine performance. Look for the number on the ceramic part of your old plug and match it.
When in doubt, check your owner's manual.
It's not imperative that you use your mower's specific brand; a generic substitute from a quality brand like Briggs and Stratton will work well in most engines.
How sharp should mower blades be?
Your mower blade should be sharpened to be like a good pair of scissors or a butter knife, not razor-sharp like a boning knife. An overly sharp, thin edge will dull much faster than a correctly sharpened blade.
Can I sharpen a lawnmower without removing the blade?
Yes, you can, but it's tricky. Disconnect the spark plug, tilt the mower on its side (with the air filter and carburetor facing up to avoid oil spillage), and use a file at a 45-degree angle.
This is a "quick and dirty" method, and you run the risk of sharpening the blade unevenly, which can cause it to become unbalanced. We always recommend taking the blade off for a proper job.