Learn to Fix your Lawn Mower: Fix Majority of No-Start Lawn Mower Issues Pertaining to Spark, Yourself!

Understanding the Relationship Between Fuel and Spark

Many lawn mower owners unknowingly face a no-spark issue that can develop into a fuel delivery problem. This often happens when the unit is left idle with gas in both the tank and carburetor. The gas can become stale and create blockages within the carburetor. Therefore, it’s essential not to overlook the importance of spark in the combustion equation. Ignoring this can compound the problem and make matters worse!

In case you didn’t know, a lawn mower engine needs three ingredients to run: air, fuel, and spark. Simply put, air is mixed with fuel, which is provided to the engine by the carburetor. This mixture is sent into the cylinder, where it is ignited by the spark plug. When all these ingredients are provided to the engine at the right time and amount, it will run successfully.

This is a spark plug removed from a lawn mower.

The threaded portion of the spark plug goes into the hole in the cylinder head, while the other end, visible from the outside, connects to the spark plug wire.

Spark plug wire shown here.

Assess Your Lawn Mower's Ignition System, and Fix Your Lawn Mower Yourself!

If you have read and followed the instructions in one of our previous posts found here: Learn to Fix your Lawn Mower: Fix Majority of No-Start Lawn Mower Issues Pertaining to Fuel, Yourself! and still can’t get your lawn mower running again, it’s wise to investigate your ignition system next. A simple and free method to verify if you have spark is to remove your spark plug from the cylinder head of your lawn mower using an appropriate spark plug socket and ratchet.

Safety First! Always allow the engine to cool before removing the spark plug. Be careful not to damage the delicate aluminum threads in the cylinder head. Once the spark plug is removed, reattach it to the spark plug wire. To quickly assess whether you have spark, place the electrode end of the spark plug against the metal cylinder head of the engine. This grounds the ignition system, which will make more sense later.

If you’re alone when performing this test, place a phone or digital camera near the plug, where you can clearly record the spark plug's electrode end resting against the engine's cylinder head. Next, pull the starter rope as if trying to start the engine. Follow all the steps you normally would to start your lawn mower, including holding down the blade brake bar.

This is where you want the spark plug to be situated before you test for spark. The engine cover has already been removed to allow a picture to be taken.

If Spark Is Discovered

If you see a spark jump across the electrode of the spark plug while it is touching the cylinder head, you can move on to troubleshooting either fuel or air (compression) as possible culprits for your no-start condition. Please note that the condition of the spark may change after the lawn mower has started to run and is under compression (when screwed into the cylinder head inside the combustion chamber). This test checks whether your ignition system can produce a spark.

Video Sample: Here’s what you should see if you do have spark:

A healthy spark jumping across the spark plug

If No Spark is Discovered

If you pull the starter rope and do not see a spark, the next step is to examine the spark plug wire. Clean the wire if it is dirty, and look for potential tears or cracks. If no obvious damage is visible, proceed by removing your lawn mower's engine cover.

Follow These Steps to Recover Spark

Remove the engine cover to reveal the flywheel underneath.

This is an ignition coil removed from an engine. Note the two oval-shaped holes allowing for adjustments to be made before securing it down; this adjustment allows the coil to be moved into or away from the flywheel.

This is the bottom side of the ignition coil; note the label 'cylinder side,' meaning it must face down when reinstalling.

With the flywheel exposed, you have a clear view of the ignition system of a lawn mower. To put it simply, as you pull the starter rope, the recoil assembly (normally built into the engine cover) spins the flywheel by engaging the flywheel cup (when the engine cover is attached). The spinning flywheel has magnets built into its sides that pass by the laminations or arms of the ignition coil.

Pro Tip: Many people think that if their pull rope becomes stuck, their engine is seized. Usually, this condition can either be a mechanical issue with the recoil assembly or a lack of necessary gap between the ignition coil and the flywheel. If the flywheel scrapes across the arms of the coil, it will result in a no-spark condition.

To remedy this situation, consult the service manual of your lawn mower's engine manufacturer to reference the necessary air gap that must be set between the ignition coil and the flywheel. If the service manual is not readily available, a business card will work as its thickness is generally required for a push mower.

Begin by removing the spark plug from the cylinder head. Ensure you place a clean cloth inside the spark plug hole to prevent anything from getting inside the combustion chamber. This will make spinning the flywheel easier. To set the ignition coil gap, first loosen the two bolts holding down the ignition coil.

Next, move the coil away from the flywheel. Spin the flywheel by hand until the magnets engage the ignition coil's armatures, pulling it into contact with the flywheel. Visually note where the magnets are on the flywheel. Remove the ignition coil from the flywheel. When free, place a business card between the ignition coil arms and the flywheel magnets. Allow the ignition coil to be pulled into the flywheel with the business card wedged between the coil's arm and the flywheel.

Next, tighten the bolts holding the ignition coil down and remove the business card. There should now be a small air gap about the thickness of a business card between the flywheel and the ignition coil's laminations. The flywheel should be able to spin freely past the ignition coil without coming into contact with it.

Business card is placed between the ignition coil laminations and the flywheel magnets.

With the ignition coil laminations being pulled into the flywheel magnets, tighten down the coil's fasteners. Ensure both laminations are being magnetically pulled into the coil to ensure proper air gap is set.

Examine The Kill Switch Wire

If you have set the gap as explained above and examined the spark plug itself for carbon buildup at the electrode gap (refer to this post for more details: Your Spark Plug is Telling You Something...and it Never Lies: Read Your Spark Plugs Like A Pro), and its wire for any obvious tears or cracks, another potential culprit for a no-spark condition is the kill switch wire. The kill switch wire connects to the ignition coil.

Examine the wire that is attached to the coil for breaks or tears.

You can repeat the above steps for testing for spark after simply removing the kill switch wire attached to the ignition coil. If, after you remove this wire, you suddenly see a spark jump across the spark plug gap, you can isolate your no-spark diagnosis to an issue with the kill switch wire and ground.

When you pull the blade brake bail to start your lawn mower, it moves the kill switch wire away from a ground, allowing spark to be produced. When you release the blade brake bail to shut off the engine, the wire comes in contact with a ground, preventing a spark and shutting off the engine. So, when you depress the blade brake bail to start the lawn mower, the ignition coil kill wire is removed from ground; when you release it, the wire is grounded.

The above procedures do not replace a proper diagnosis with a spark tester. However, they provide a free and easy way to initially test for spark in your lawn mower without using specialty tools!

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Never the Norm!