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What does a spark plug do?
A spark plug provides a source of ignition for small engines, like the engine that powers your lawn mower.
It consists of a threaded shank to install it into an engine block, a center electrode which is encased in a porcelain sheath, and a firing electrode, which is a metal tab that makes an arch above a center electrode. 
How can you tell when it’s time to change a lawnmower’s spark plug?
- Your mower is harder to start than usual – It might take a few more pulls on a mower’s starter rope, or your lawn tractor won’t start when you turn the key. The spark generated by the spark plug might be too weak for igniting the air and fuel mixture that the mower’s carburetor has delivered to its cylinder. That will cause the engine to flood when more mixture enters the cylinder
- If the engine starts and then quickly dies out immediately, that’s a good sign that your spark plug is shot. Bad spark plugs can cause misfires, causing your engine to pop, miss, or sputter
- If your lawnmower is guzzling more gas than usual, it could be because of the spark plug. Gasoline doesn’t burn entirely or efficiently if it had a weak spark, causing your mower’s fuel consumption to increase and its efficiency to nose-dive
- You’ll need to remove the spark plug to examine it. The center electrode on the plug should sport a flat top. A rounded top means the spark plug should be replaced.
Here are a few tips that will show you how you can check for a damaged spark plug:
- Disconnect your spark plug lead – Clean all around the plug so no debris will fall into the combustion chamber when you’re ready to remove the plug
- Take out the spark plug with a spark plug socket
- Clean off any light deposits you can see on the plug with spray-on plug cleaner and a wire brush – Make sure you use a special spray-on plug cleaner and NOT an abrasive or a shot blaster. The plug might still be good if its shape is OK, but it’s wet with gas or black with carbon. Brush it good with a wire brush, adjust the electrodes gap, and try to start the motor again. No start? Then it’s time to go on to the next step
- Carefully inspect the plug for electrodes that have burnt away, cracked or chipped porcelain, any pitting on the firing electrode, or any extremely stubborn deposits
- Examine the spark plug gap and then adjust as necessary – The gap in between the side and center electrodes needs to be an exact distance apart, or the plugs won’t fire efficiently
- If all looks well, you can replace the spark plug, making sure you don’t over-tighten it (a maximum of 15 ft. lbs.) by re-attaching your spark plug lead
- Now it’s time to start up your engine
If your lawn mower spark plug didn’t pass the test, it’s time to replace it.
I recommend using a small-engine tune up kit that contains spark plugs, air filter, oil filter, and fluid that’s engine specific (fuel stabilizer, lawnmower oil, etc.).
Here are the next steps:
- Find the correct spark plug and then adjust the gap settings
- Disconnect the spark plug lead and use your spark plug socket to remove it
- Replace the old plug with a new plug, making sure you do not overtighten it and then simply reattach your spark plug lead
Test your mower and you should be good to go!
If the engine is still having issues, you may want to take it to a dealer or local small engine repair shop for further diagnosis.