Hard starting or sluggish running can have several causes, including a worn or fouled spark plug. These symptoms alone do not prove that the plug needs replacement.
But before changing the spark plugs, you want to be sure that they’re the actual guilty party.
This guide teaches you how to test a spark plug on a lawnmower when you suspect that you have bad spark plugs.
Before we go further, perhaps it’s necessary I tell you about what these little accessories do to ensure we are on the same page.
What does a spark plug do?
Spark plugs are crucial for your lawn mower’s starting.
They make powerful electrical sparks – they have electrodes- that help the mower ignite and the engine may fail to start if the spark plugs are in a bad state.
And if you’re lucky to crank it to life, the gas won’t burn efficiently leading to an extremely sluggish performance.
You’ll be facing loads of other issues if your spark plugs are damaged including increased fuel consumption, nasty engine misfires, oil dilution, and more.
For this reason, you should learn how to test spark plugs and read the signs that hint that it’s time to clean the part (sometimes it’s just dirt making your life hard) or replace it.
How to test a spark plug on a lawnmower
The following steps will teach you how to test a spark plug on a lawnmower:
Tools you need:
- Spark plug removal tool such as a spark plug socket.
- A compatible inline ignition tester, such as the Lisle 20610 below.
- A wire brush (for cleaning)
Preparatory measures:
- Park on level ground, stop the engine and let it cool before handling the ignition lead or plug.
- Follow the mower and engine manuals. Work outdoors, away from spilled fuel or fuel vapors.
- Keep people clear of the mower and moving parts. An inline test can allow the engine to start; do not bypass safety controls.
Stage 1- Remove the spark plug for inspection
Remove the plug if you need to inspect or replace it. The inline test described below uses an installed plug, so reinstall it to the engine manufacturer’s gap and tightening specifications before that test.
Tip: These steps are general to most mowers and you may want to check what your owner’s manual says even as you follow them.
Step 1: Find the spark plug.
On most lawnmower engines, the plug is located at either the front or on the side of the mower.
Again refer to your operator’s manual if you can’t figure out the correct location.
Step 2: Remove the connected spark plug wire.
With the engine stopped and cool, disconnect the plug lead by holding the insulated boot rather than pulling on the cable. Follow your model’s instructions for access.
Of course, there’s no way you’re going to get the spark plug out with the wire still attached.
Tip: Sometimes the wire could be stuck and you might have to move up it and down (or from one side to the other) a bit to make it come off.
If the boot will not release, use the removal method specified for your engine rather than pulling hard on the cable or damaging the insulation.
Step 3: Clear the spark plug surroundings
Clean loose dirt from around the plug before removal, using a suitable brush or the method specified in the engine manual.
You don’t want this to fall into the combustion chamber because it can mess the engine.
Step 4: Pull the spark plug out
Next, unscrew the spark plug from the mower engine.
A spark plug wrench should work fine- fit it over the plug then turn counterclockwise until you get the plug out.
Tip: If the plug is seized, stop rather than forcing it. Follow the engine manufacturer’s service procedure or ask a service technician to avoid damaged threads.
Stage 2- Check ignition activity with an inline tester
The Lisle 20610 below provides a visible indication of ignition activity. It is not a bench tester for a removed spark plug.
Its flashing indicator helps with diagnosis, but it does not measure ignition voltage or prove that the spark plug fires correctly under compression.
Next we look at how to use the tool:
How to test a spark plug on a lawnmower with a spark plug tester
- With the engine stopped and cool, make sure the correct spark plug is installed and tightened as specified in the engine manual.
- Disconnect the plug lead and connect the Lisle 20610 in line between that lead and the installed spark plug. Use the straight or angled boot as appropriate; both connections must fit securely.
- Position the tester where the indicator can be observed without touching the ignition system or approaching moving or hot parts. Do not leave cables where they can be caught.
- Follow the mower’s normal starting procedure using its original recoil starter or electric starter. Leave guards and safety controls in place; the engine may start.
- Observe the indicator from a safe position. Flashing indicates ignition activity in the circuit; it does not by itself prove that the plug is good.
- Stop the engine before disconnecting the tester or adjusting a connection. If no flash is visible, check the setup and follow the engine’s ignition-diagnostic procedure; wiring, a stop switch or an ignition coil fault may also be involved.
This is the connection method for the Lisle inline tester. A different grounded-gap tester used for coil diagnosis may require a different procedure. Do not combine those instructions or improvise a rope around an exposed flywheel.
Lisle 20610 Inline Spark Tester
- Visual indicator for ignition activity.
- Connects between the installed spark plug and its lead.
- Double-ended design with straight and 90-degree boot options.
- Diagnostic aid; flashing alone does not establish spark-plug condition.
Do a Visual check
The simplest test is actually visual.
Take a good look at the plug for signs of erosion, corrosion, being burned, and cracking (in the porcelain sheath).
Damage such as a cracked insulator or badly eroded electrodes is a reason to replace the plug with the correct type for the engine.
Deposits can require cleaning or replacement, depending on their condition and the engine manufacturer’s instructions. Appearance alone does not confirm the whole ignition system is working.
Tip: If the plug is serviceable, reinstall it with the specified gap and tightening torque. Avoid overtightening, which can damage the plug or cylinder-head threads.
Test with a working spark plug
Another diagnostic step is fitting a known-good plug of the exact specified type and gap, then following the mower’s normal starting procedure.
Be sure to measure the gap accurately when mounting it as it is unlikely to spark if not set correctly (confirm the recommended gap measurement for your model from the owner’s manual).
If the mower starts without problems with the correct known-good plug, that suggests the old plug may have contributed to the fault. It is not a substitute for checking other causes if the symptom returns.
If the engine still fails to start, the plug may not be the only problem. Distinguish an engine that cranks but does not start from one that does not turn over, and follow the relevant troubleshooting procedure.
Like other small engines, a lawn mower’s engine is quite simplified and you should have minimal problems testing and replace the spark plug.
What is important is to recognize the signs and take action before the worn out spark plugs causes more issues.