When you think about lawn mower maintenance, the first thing you probably think about is keeping the blades in good condition or lubricating the machine so that all the components run smoothly.
However, there is another maintenance task you need to know about if you want your lawn mower to work properly for as long as possible, and that’s gapping the spark plug.
The spark plug is a very important component of any lawn mower – it’s responsible for producing the initial spark in the mower’s combustion chamber during startup.
If any part of the plug isn’t adjusted correctly, you could find yourself with a malfunctioning mower on your hands.
While it’s easy to tell whether a lawn mower’s spark plug has been damaged by excess oil or soot, figuring out whether the gap needs adjusting is trickier.
In this guide, we’ll be outlining how you can tell whether your lawn mower’s spark plug needs gapping and how to do this quickly and efficiently, without damaging the plug.
Do Spark Plugs On Lawn Mowers Come Pre-Gapped?
If you’ve just bought a lawn mower, you may be wondering whether you need to gap the spark plug straight away.
After all, you might assume that lawn mowers come pre-gapped so that they work properly immediately when unboxed.
While it’s true that most manufacturers will claim that their machines come pre-gapped, you shouldn’t assume that this is true.
Often, even pre-gapped lawn mowers are not gapped accurately.
We’ve seen many lawn mowers come out of the box with a spark plug gap that is different from what the user manual says it should be.
Therefore, it’s best to measure the spark plug gap of any new lawn mower and check it against the specifications in the user manual. If the measurement is even slightly off, you should readjust it.
Signs You Should Gap Your Spark Plug
You probably won’t be able to tell whether the gap in your lawn mower’s spark plug is accurate just by looking at it.
However, there are other ways to determine whether you need to gap the spark plug yourself or not.
Often, people realise that there is a problem with their lawn mower’s spark plug when they run into performance issues.
It might be that the mower simply isn’t starting up, or the engine is misfiring.
Rough idling and running out of fuel sooner than it should are also signs that something is wrong with your mower’s spark plug.
If you suspect that your lawn mower’s spark plug gap isn’t right based on any of the problems above, there’s only one way to know for sure.
This process involves measuring the current gap of your spark plug, checking it against the measurement specified in the user manual, and adjusting the gap manually so that the measurements match up.
How To Gap Your Lawn Mower’s Spark Plug
As we just mentioned, the lawn mower spark plug gapping process sounds quite simple: all you need to do is measure the gap you have currently, see if it matches what the user manual recommends, and adjust it to the right size if not.
However, if you want to do this properly, you will need the right tools for the job, and you’ll need to know the most accurate ways of carrying out each step to ensure that the process goes smoothly.
You Will Need
When you decide to gap your lawn mower’s spark plug, you will need:
The User Manual
This is really important because this is what will tell you the correct gap measurement for your spark plug. Without the user manual, you won’t be able to tell whether the gap in the spark plug is currently correct or not.
If you have lost the physical copy of your user manual, you should be able to find a digital copy online via the manufacturer’s website.
If it’s an old model and the digital manual is no longer available online, you should contact the manufacturer directly, either by telephone or email.
A Spark Plug Gap Tool Or Feeler Gauge
This is what you will be using to measure and adjust the gap in your spark plug. It’s best to have purpose-built tools like these for the job so that you get an accurate measurement and adjustment in the end.
Step 1: Identify The Correct Gap
To find out how big the gap in your lawn mower’s spark plug should be, you should find the user manual that came with your lawn mower and look for the section about the spark plug.
You should find the gap measurement here.
Usually, the standard size for spark plug gaps ranges from 0.02 inches and 0.035 inches.
Most manuals require you to set the gap to 0.03 inches, but this can vary, and it’s very important to note that the measurement has to be extremely specific, otherwise your lawn mower won’t work properly.
Step 2: Measure Your Gap
If you’re using a feeler gauge (which is the gentlest method for the electrodes but also the most expensive tool), you’ll need to select the wire with the correct thickness based on the measurement you found in the user manual.
Choose the right thickness from the different wires and place it into the gap in your spark plug. If they are the same width, your spark plug’s gap is the right size.
However, if there is a difference between the thickness of the wire and the size of the gap, you’ll need to adjust it.
Alternatively, you could use a wire gap tool to take the measurement and work out the difference between the ideal gap and the current gap.
This tool is less expensive than a feeler gauge, but it does roughly the same thing except that it’s a circular shape and is surrounded by rectangular wires as opposed to the long wires on a feeler gauge.
The rectangular wires are different widths, and you use these like the feeler gauge wires by putting them into the gap to check for size differences.
The third and final method of checking the difference between the gap size specified in the user manual and the size of the current spark plug gap is a coin gap tool.
This should probably be used as a last resort because coin gap tools can damage the electrode in the centre of the spark plug. However, if you are going to use a coin gap tool, it’s easy.
Just put the tapered edge into the gap in the spark plug where it’s the thinnest and turn it until it’s wedged in between both electrodes. This is where the potential for damage comes in, so be careful.
Step 3: Adjust The Electrode
Once you know what the difference between the ideal spark plug gap measurement for your lawn mower and the current gap is, you can begin the process of adjusting the gap to make the measurements the same.
You can use any of the tools listed in the section above to adjust the electrodes as well as measure the gap, so you don’t need to get any additional tools.
To adjust the gap, the grounded electrode is the one you will need to bend using the hole or slot in your chosen tool.
Make sure not to put pressure on the central electrode underneath the grounded electrode since this could damage your spark plug.
Keep bending the grounded electrode to make the gap as narrow or wide as you need it to be, but do this slowly and gradually.
Move the electrode a little at a time and be as gentle as possible to avoid accidentally breaking the electrode.
When you think you have bent the grounded electrode to the right position, use your tool to measure the gap again.
It might take a few tries to get it exactly right, so be prepared to adjust and measure a couple more times.
After several tries, you should end up with a gap that exactly matches the measurement listed in the user manual.
Frequently Asked Questions
What Happens If A Spark Plug Gap Is Too Wide?
If the gap in your lawn mower’s spark plug is too wide, the spark produced in the combustion chamber will be weak.
This could mean that your lawn mower doesn’t start up properly, so if you’re noticing that ignition isn’t going as smoothly as normal when using your lawn mower, it’s possible that an overly wide spark plug gap is to blame.
Is A Smaller Spark Plug Gap Better?
A smaller gap in your lawn mower’s spark plug may make the engine more responsive so that it starts up more easily.
However, you shouldn’t make the gap smaller than the user manual recommends since this can also cause the mower to malfunction.
An overly-responsive ignition system can be dangerous, so stick to the measurement listed in the user manual at all times.
How Often Should You Change the Spark Plug In A Lawn Mower?
The spark plug in your lawn mower is something you should replace on a regular basis to keep your mower working correctly, even if you keep the gap at the right size.
For every 25 hours that you use your lawn mower, you should replace your spark plug, or at the very least inspect it for damage.
Final Thoughts
The gap in your lawn mower’s spark plug plays an integral role in the functioning of your mower overall, so it’s important to ensure that the gap is accurate to what the user manual recommends.
You can use a feeler gauge, a wire gap tool, or a coin gap tool to measure the gap and adjust the grounded electrode in the spark plug.
You should adjust the electrode very slightly and measure between adjustments until the measurements match up exactly.
Be careful when using a coin gap tool since this is the most likely to damage your spark plug.
Check your mower’s spark plug every 25 hours of use to make sure it’s still in good condition and that the gap is still accurate.
Hi, my name is Brian Freis and I have been a local gardener in my hometown for almost 20 years now. There’s nothing I’d rather do with my time than tend to a much-needed garden, whether that’s shaping hedges, planting flowers, weeding, mowing lawns, or more.
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