How does a magneto work on a lawn mower

If the spark plug isn’t firing, check the flywheel system first. That compact coil-and-rotor setup near the crankshaft is usually where the issue hides. When the flywheel spins, it passes by a small armature. That movement generates electrical current through magnetic induction–no battery required.
The setup is surprisingly simple. A rotating metal disc with embedded magnets moves past copper wire coils. That sudden magnetic change produces high-voltage pulses, timed precisely to send current down the wire to the spark plug. If the coil is worn, corroded, or misaligned, you’re likely getting weak or no spark at all.
One mistake people make is replacing the plug before testing the coil gap. It should be less than a millimetre–close enough to generate voltage, but not so close it scrapes the flywheel. A simple feeler gauge helps here. Also, don’t forget to inspect the kill switch wire. If it’s shorting, nothing else matters–it cuts off spark entirely.
On older machines, wear on the contact points or frayed wiring near the ignition module might be the cause. But on most newer models, the sealed ignition block tends to either work or fail completely–there’s rarely an in-between.
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Generating Spark Timing Through Rotational Magnetism
Align the rotating flywheel so that the embedded magnets pass close to the ignition coil’s armature at the precise moment the piston approaches top dead centre. This alignment determines when the current peaks and collapses–timing the electrical discharge that jumps the plug gap. Don’t guess; check the keyway on the crankshaft. If that’s sheared or misaligned, your timing will be off no matter how new the parts are.
Magnetic Field Collapse Triggers Spark
As the magnetic poles sweep past the coil, they induce current in the primary winding. The flywheel’s velocity directly impacts the intensity. Faster spin, stronger induction. But the actual spark isn’t produced during the peak–it happens milliseconds later, as the magnetic field collapses. That’s when the rapid drop in current jolts the secondary winding into producing a high-voltage pulse.
If the coil gap is too wide, the induced current weakens. Too narrow, and you risk physical contact or overheating. Keep it around 0.010″ to 0.014″–a business card works in a pinch, though not perfect. I’ve done that myself once when I couldn’t find the feeler gauge. It ran, not great, but enough to finish the backyard. Probably not ideal for the long haul though.
Proper Gap, Proper Burn
Spark must ignite the air-fuel mixture at the right moment. Too soon and the combustion fights the piston; too late and power drops. With rotational magnetism, you’re working with mechanical certainty–no sensors or computers. Just steel, copper, and movement. That simplicity makes it reliable, but also less forgiving if something’s just slightly out of line.
If this feels like too much trial and error, you’re not alone. But once it’s dialled in, it’s solid. Or skip the guesswork entirely and get professionals like PROPERTY WERKS in Calgary to keep your machine–and your yard–running sharp.
Triggering the Ignition Coil to Fire the Spark Plug
To make the coil discharge high voltage to the plug, timing between the rotating flywheel and the coil’s magnetic field must be exact. As the flywheel spins, a small nub on its rim–called the trigger magnet–passes by a sensor embedded near the coil. That moment is what matters.
The sensor, usually a Hall effect switch or a similar magnetic pickup, notices this change in the field. It sends a low-voltage signal through the primary coil winding. This briefly energizes the coil before it’s cut off almost immediately. The sudden stop in current flow is what triggers the collapse of the magnetic field inside the coil, forcing it to jump across to the secondary winding.
Now, this is where the real punch happens. That voltage spike–easily upwards of 15,000 volts–travels down a short wire to the spark plug tip. And it needs to fire at exactly the right time: just before the piston reaches the top of its compression stroke. A few degrees too early or too late, and the whole cycle runs rough. Or doesn’t run at all.
If you’re dealing with weak spark or none at all, check that the flywheel key isn’t sheared. It might look intact but be twisted just enough to throw off timing. This happens more often than you’d think, especially after a hard stop or a blade hitting something solid.
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Diagnosing Common Magneto Malfunctions in Small Engines
Check spark output first. Remove the plug, keep the wire attached, and ground the threaded end to the metal housing. Pull the recoil cord and watch the tip–no spark usually means ignition failure. Use a gap tester if available, not your fingers.
Signs of Failure
- No-start condition: If the starter spins freely but the motor won’t catch, suspect a dead coil or broken flywheel key.
- Intermittent spark: Unreliable ignition points to weak insulation, overheating, or failing internal windings.
- Backfiring: Often traced to timing issues–usually a sheared flywheel key from blade impact.
What to Inspect

- Flywheel Key: Pull the flywheel and check the key. If it’s even slightly offset or cracked, replace it. Don’t file it down or guess at alignment.
- Air Gap: Measure the space between the coil and flywheel magnet. A typical spec is around 0.010″ to 0.014″. Too tight or too wide, and spark strength drops.
- Kill Switch Circuit: Disconnect the kill wire from the ignition coil. Still no spark? Problem’s inside. Spark returns? Then the switch or wiring is shorting to ground.
Don’t just swap parts. A failed component leaves traces–burn marks, brittle insulation, discoloured epoxy. Take time to check before replacing. And if you’re stuck or just want it done right the first time, PROPERTY WERKS handles more than mowing–they’ve got the tools and experience to sort out stubborn ignition trouble too.
Q&A:
How does a magneto generate the spark in a lawn mower engine?
The magneto works by converting mechanical motion into electrical energy. As the flywheel spins, a magnet passes by a coil of wire, inducing a high-voltage pulse. This pulse travels to the spark plug, causing it to ignite the air-fuel mixture in the engine cylinder. The timing of this process is precisely synchronized with the engine’s cycle to ensure proper combustion.
What components of a magneto are most likely to wear out over time?
Over time, parts such as the breaker points and condenser can degrade. Breaker points may become pitted or misaligned, which can interrupt the electrical flow. The condenser, responsible for storing and releasing energy, may lose its ability to function correctly. In modern systems, solid-state modules are used instead, but they too can eventually fail due to heat or moisture exposure.
Can a lawn mower run without a magneto?
No, it cannot. The magneto is the primary source of ignition in most small gasoline engines. Without it, there’s no way to produce the high voltage needed to create a spark at the plug, and without that spark, combustion can’t happen.
Why is flywheel speed so important to magneto function?
The magneto relies on the motion of the flywheel to generate electricity. The faster the flywheel spins, the more rapidly the magnetic field changes, which increases the voltage produced in the ignition coil. If the engine turns too slowly, the spark may be too weak to ignite the fuel properly, making it harder to start the engine.
Is it possible to test a magneto at home without special tools?
Yes, a simple spark test can be done. Disconnect the spark plug wire, insert a spare spark plug into the boot, and ground the plug against the engine block. Then pull the starter cord and look for a visible spark at the plug gap. If there’s no spark, the magneto or another part of the ignition system may need inspection. While this doesn’t give a full diagnosis, it can help confirm whether the magneto is producing voltage at all.
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