By: Author John Cunningham. Published: 2023/07/05 at 1:11 pm
An engine needs compression, fuel, and spark to burst into life. In this post, we’ll learn all about the exciting life of a small engine ignition system and how it conjures up a spark. Ground yourself, then strap yourself in!
A small-engine ignition system generates a spark to ignite the air-fuel mixture in the combustion chamber. Components of an ignition system include a spark plug, ignition coil, and flywheel with an embedded north-south magnet.
In this article, you’ll learn about the components of an ignition system, what they do, what they look like, and where they live. We’ll also cover how the whole system works.
We’ll cover ignition fault symptoms, troubleshooting, and common repairs.
Index
- Small Engine Ignition Components
- How Does Ignition System Work?
- Troubleshooting Common Ignition Issues
- Ignition System Maintenance & Repairs
- Small Engine Ignition System FAQs
Small Engine Ignition System Components
Let’s look at all the critical ignition system components necessary to make a small engine do its thing; we’ll briefly describe their function and location before later describing how the whole system works.
We’ll cover the following:
- Ignition switch
- Spark plug
- Coil
- Flywheel with magnet
Really! Only four main components in an ignition system? Yep
Surprising. I know!
Ignition Switch
We could control an engine by controlling any of the three critical ingredients it needs to run – compression, fuel, or spark. But since the spark is the easiest to manipulate, it makes perfect sense to use voltage control of the spark plug to control the engine state.
In most small engines, this is done with a simple switch controlled by a bail lever (lawnmower), but for some machines, the ground path to the coil is controlled by a key and or a computer module.
Where’s the ignition switch located? I hate answers that start with “It depends.” But sometimes, nothing else fits. It depends on the machine to which the small engines are fitted. Riding mowers, Zero turns, large Snowblowers, and Generators will likely have a regular ignition switch with a key on a control panel or dashboard.
For walk-behind mowers, the ignition switch is a simple on-off mechanical switch located at the engine flywheel and connected by a braided cable to the handlebar-mounted dead man’s lever.
Spark Plug
We are all familiar with a spark plug; it’s where the spark is made and is critical for combustion chamber ignition.
The spark plug is low technology; that said, it works bloody hard, and if you’ve had a faulty plug, you’d know about it pretty quickly. A spark plug consists of a centre electrode, a ground electrode, and a ceramic insulator, all enclosed in a metal shell.
Where’s the spark plug located? Typically located at the front of the engine, in the centre of the cylinder head.
Ignition Coil:
The ignition coil is the source of the voltage needed to create a spark. The coil is a magical bit of kit. It takes a small, almost insignificant voltage and transforms it into thousands of volts, enough to make the spark plug produce a spark. We’ll get to how that happens a little later, and you’ll see what I mean.
Where’s the coil located? Coil is located under the blower housing and crucially adjacent to the flywheel.
Flywheel
I love machinery. It’s why I became a mechanic; I mean, take something as seemingly simple as a cast-iron flywheel and consider its role. In the ignition system, the flywheel plays a critical role in initiating and maintaining ignition timing. (more on this later)
And if that isn’t important enough, the flywheel has a few other tricks. When the flywheel rotates, it stores kinetic energy; its rotational momentum helps maintain engine speed during the resistance of the compression and exhaust strokes; this reduces fluctuations between strokes and makes for smoother operation.
The flywheel also cools the engine and auxiliary components; fan blades attached to the flywheel move the hot air away from the engine and introduce cool air.
Where’s the flywheel located? The flywheel is located under the blower housing, directly in line with the crankshaft.
How Small Engine Ignition System Works
Where there’s smoke, there’s fire. But how did it start? Well, that’s what we’re about to find out. Let’s follow the sequence of events that leads to sparking in a small-engine ignition system.
But first, let’s get up to speed on the coil itself.
A typical small-engine coil consists of windings, which are copper-insulated wires wound in a circular fashion. More windings (turns) produce more voltage.
Two important things to note about windings;
- A magnet with north-south poles induces an electrical current as it passes copper windings.
2. Windings that carry current and are wound around an iron core become magnetic.
Most coils consist of three internal windings:
1 Trigger – A small winding known as a trigger or transistor is responsible for starting the sequence within the coil that ends with a spark.
2 Primary winding – consists of hundreds of turns capable of producing hundreds of volts and is wound around an iron core.
3 Secondary windings – consist of thousands of turns capable of producing thousands of volts and is independent of the primary windings.
Flywheel Rotation
The story of a spark begins with the cranking of an engine; for most small engines, that’s the operator yanking on the starter cord, and for others, it’s when the electric starter engages.
Either way, what’s important is that the flywheel rotates sufficiently fast enough for the flywheel magnet to excite the coil – anything over 400 revolutions per minute (RPM) works.
The flywheel with an embedded permanent magnet is coupled with the crankshaft, and as the operator cranks over the engine, both the electrical and mechanical engine starting sequence begin simultaneously.
On the mechanical side, the engine passes through the stages, or strokes, of the engine cycle, and just as it completes the second stroke, “Compression stroke,” and begins the third stroke, “Power stroke,” the flywheel magnet passes the coils’ trigger.
(Note, unlike car ignition systems, most small engine ignition systems make a spark with every revolution of the flywheel, even though a spark is only required once in every two flywheel revolutions.)
And as the magnet passes the trigger, it induces an electrical current in the trigger’s coil, which feeds to the primary coil windings; and because the primary windings are wrapped around an iron core, they become magnetic. A magnetic field strong enough to induce an electrical current in the secondary windings of the coil.
But as the rotating flywheel magnet moves beyond the coil, the trigger senses current loss and triggers the primary windings – the sudden collapse of its magnetic field induces a voltage spike in the secondary winding.
(Some coil triggers can adjust (advance & retard) ignition timing to allow for RPM)
And since the plug wire is connected directly to the secondary windings, this high voltage is transferred down the plug wire to the spark plug in search of ground.
As the voltage travels through the spark plug’s centre electrode, it meets resistance in the form of the plug gap.
The ground is just the other side of the gap, and the push of the voltage is so great (thousands of volts) that the current jumps the gap to the ground side of the spark plug and, in doing so, creates a beautiful, bright blue spark.
The plug is positioned inside the combustion chamber where the air-fuel mix is waiting, the spark ignites the mixture, and the engine bursts into life, pushing the piston downward under power.
Great, but how do we shut her down? Since the ignition coil voltage is constantly seeking the shortest path to ground, we can divert the voltage flow by offering it a better ground source, thereby starving the plug of voltage.
When the ignition switch is in the run position (Open), the spark plug offers the shortest, near-perfect path; that’s what causes it to spark. And when we offer ignition system voltage an alternative shorter perfect ground (Kill), the voltage is diverted away from the spark plug and to our perfect ground. As a result, the engine stalls.
That’s it; you are all up to speed on how a simple ignition system works.
If you want to understand how a small engine carburetor works, you can check that out right here.
Troubleshooting Common Ignition System Symptoms
If you own a small engine at some point, you will need to check the ignition system for a good spark. That’s what this section is all about. I’ve been a mechanic for over twenty years, and here I’ll share my top tips and links for checking ignition system components.
Faulty Spark Plug
Spark plugs are basic technology, but they still cause trouble. Hey, I don’t blame them. It’s a tough job, and most of the time, they do it faultlessly.
A spark plug is easy to access and test, so it is the go-to component we test when diagnosing a no-start or intermittent starting issue.
Typically, I advise small engine owners to have a new spare spark plug on hand. Swapping out the plug is a fast way to troubleshoot, especially if you are unfamiliar with what a good spark plug should look like.
I also advise self-sufficient small engine owners to invest in an inline ignition system tester.
How to test the spark plug? Go ahead and remove the spark plug, note the condition, and categorise and diagnose it using the image and chart.
| Plug Color | Mechanics Description |
| Tan | Running sweet |
| Black | Running rich |
| White | Running lean |
| Oily | Mechanical issue |
| Wet | Running rich |
Now we are ready to test for spark using both the DIY MacGyver style and the preferred inline tester.
Faulty coil
A coil is easy to test; they generally work without issue until one day they just stop. That said, occasionally I’ll test a coil that works without issue when the engine is cold but fails when it warms up.
How do we test the coil? Begin by running an inline spark test, or use the MacGyver-type test we covered above to check the spark plug.
Finding no spark usually means you found your problem, a failed coil.
But it is possible that both the coil cap and the ignition system are faulty, or that an ignition system issue is the root cause.
I have also found faulty safety sensors/switches on equipment like riding mowers that cause the coil to ground. On more sophisticated kit like that, I’d disconnect the coil control wire and test again, just to be sure it is the coil that has failed and not a sensor giving you a false reading.
I’ve covered removing the coil control wire right here – Riding mower won’t start
Ignition switch fault
Generally, ignition switches are robust; of course, they do wear out, but a spark plug or coil is more likely to cause trouble in an ignition system. Checking an ignition system on a small engine is simple, but it can get a little more complex when it’s fitted to a riding mower, for example.
A riding mower employs safety sensors on the blade controls, seats, transmission clutch pedal, and grass box, and some also have them on the hood (i.e., hood open = no start). A low-oil-level safety switch is common on small horizontal engines used in generators and power washers.
Sensors may be hard-wired inline, meaning they supply ground to the coil (preventing a spark) until they are activated, at which point the ground feed opens, and the coil sends voltage to the spark plug.
Computer-controlled ignition switch
However, some machines may wire the sensors to a control module instead, and the control module decides when it should remove the ground from the coil and allow the engine to start.
So some small engine ignition switches may be module (computer) controlled. I’ve covered testing that type set up here – Riding mower won’t start no click
Ignition System Maintenance & Repairs
There aren’t a ton of ignition system maintenance or repair chores, but below, you’ll find the most common ones.
Small Engine FAQs
Here are the most common questions I’m asked about ignition systems.
Can I use any spark plug in my small engine?
You can not use any spark plug in a small engine. Different engines require specific spark plugs designed for their ignition system. All spark plugs have a code printed on the ceramic insulator. Best to consult your engine manual or check your engine manufacturer’s recommendations online.
When troubleshooting small engine issues, never assume the plug removed from the engine is the correct type; best to verify.
A spark plug must have the correct heat range, electrode gap, and thread dimensions. Using the wrong spark plug can cause a ton of issues, from no-start to plug fouling, overheating, poor performance, intermittent misfires, poor fuel consumption, and the risk of engine damage.
What causes an intermittent ignition system spark?
A small engine intermittent ignition system spark is typically caused by the following:
- Bad spark plug – dirty or poorly gapped spark plug
- Loose spark plug cap – loose plug cap will offer a spark intermittently
- Coil wire chafing – a coil wire that grounds on the engine or body will divert the voltage needed to fire the plug
- Faulty coil – a failing coil often falters as the engine warms up
- Faulty ignition switch – loose wiring can cause grounding of the coil
- Faulty safety sensors – worn out or loose sensors can offer a coil ground when not wanted
- Faulty computer module – wet or failing computer module can offer an unwanted ground to the coil
Can a bad ignition system issue cause a lack of power?
A bad ignition coil will cause a lack of power in a small engine. So too can a range of other issues; some of the more common issues include a bad spark plug or wrong spark plug type, fouled spark plug, badly gapped spark plug, wet spark plug, bad gas, dirty carburetor, engine wear, overfull oil level, low oil level.
You may find the following links helpful:
- About the Author
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John Cunningham is a Red Seal Certified Tech with 25+ years of wrenching.
Bringing hands-on expertise to LawnmowerFixed.com, John writes practical repair and lawn care guides with a passion for keeping outdoor equipment running smoothly.