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

Before you toss your lawn mower to the curb make sure you try this free and simple test to check for the health of your lawn mower's ignition system, all without the use of specialty tools! As you may already know from our previous posts, the majority of lawn mower no-start issues are related to fuel-delivery problems, which can usually be pinpointed to a dirty carburetor. However, no matter how well your fuel system is functioning it will fail to start without a spark, to ignite the air-fuel mixture in the combustion chamber. That is why it is imperative that you verify your lawn mower has spark, before you misdiagnose it! The following post will show you how to determine whether your lawn mower has spark, and if not, how to potentially get it back and get your lawn mower running again!

The Relationship of Fuel and Spark

Many lawn mower owners unknowingly have a no spark issue to start, that further develops into a fuel delivery issue, after the unit is left to sit idle, with gas left in both the tank and carburetor. This gas that is left in the lawn mower, soon becomes stale and creates blockages within the carburetor. That is why it is imperative that you do not overlook the importance of spark in the combustion equation as doing so 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. To put it simply, air is mixed with fuel and is provided to the engine by the carburetor and sent into the cylinder where it is ignited by the spark plug. When all of these ingredients are provided to the engine at the proper time, and amount, it will successfully run.

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, and the other end that is visible from the outside, is connected to the spark plug wire.

Spark plug wire shown here.

Assess Your Lawn Mower's Ignition System, 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! on how to solve fuel-related issues, and still can't get your lawn mower running again, it is wise to next investigate your ignition system. 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. Use caution when removing; never remove a spark plug from a hot engine, allow it to sufficiently cool first. Be careful not to damage the delicate aluminum threads in the cylinder head. Once the spark plug is removed, reattach the spark plug to the spark plug wire. In order 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 are 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, that is 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 lawnmower. This includes holding down the blade brake bar to start it.

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, the condition of the spark may be impacted after the lawn mower has started to run and is under "compression," (as in when screwed into the cylinder head inside the combustion chamber.) This test is testing whether your ignition system is capable of producing a spark.

Video sample of 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 you do not see a spark, next examine the spark plug wire. Clean the wire it if it is dirty, and look for potential tears or cracking of the wire. If no obvious damage is visible proceed by removing your lawn mower's engine cover.

Procedure 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 (that is normally built into the engine cover itself,) spins the flywheel by engaging the flywheel cup (when engine cover is attached.) The spinning flywheel has magnets built into its sides that pass by the laminations or arms of the ignition coil. Since their is a small gap between the ignition coil and the flywheel magnets, a spark is created each time the magnets pass the coil. Pro Tip: Many people think because their pull rope becomes stuck that their engine is seized. Usually such a condition can either be a mechanical issue with the recoil assembly, or there is no longer a necessary gap between the ignition coil and the flywheel, so the flywheel essentially scrapes across the arms of the coil. Such a condition will result in a no-spark condition.

To remedy this situation it is best to 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 also work as its thickness is the general thickness/distance 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. In order to set the ignition coil gap, first loosen the two bolts holding down the ignition coil.

Next move the coil away from the flywheel. Then spin the flywheel by hand until the magnets engage the ignition coil's armatures, and pull it into contact with the flywheel. Now visually note where the magnets are on the flywheel. Remove the ignition coil from the flywheel and 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 actually 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 lamintations 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 cracking, another potential culprit for a no spark condition is the kill switch wire. The kill switch wire is connected to the ignition coil. Its connection to the ignition coil is shown below.

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 that is attached to the ignition coil. If after you remove this wire and 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. In order to understand this, please know that when you pull the blade brake bail to start your lawn mower, it essentially moves the kill switch wire away from a ground allowing spark to be produced. When you release the blade brake bail in order to shut off the engine, you allow the wire to come in contact with a ground which prevents a spark, thus shutting off the engine. So in essence when you depress the blade brake bail to start the lawn mower=ignition coil kill wire is removed from ground, when you release the blade brake bail to stop the engine=ignition coil kill wire moved into ground (grounded.)

The above procedures in no ways replace a proper diagnosis with a spark tester, however it is a free and easy way to initially test for spark in your lawn mower without the use of specialty tools!

If you found this post helpful consider enrolling in our free Online Program designed to transform novice lawn mower operators into knowledgeable and independent troubleshooters, and maintainers of their own equipment! Avoid the expense and hassle of untimely breakdowns! All that is required to enroll is an email, and it is completely free of charge! Click here to be directed to the enrollment page: Online Program

Never the Norm!