Ignition Problems in Small Engines
In This Guide
When an engine cranks freely and has fuel but simply will not fire, small engine ignition problems are usually the reason, because without a spark at the right moment nothing burns. The good news is that the ignition system on a mower, a generator, a snow blower, or a pressure washer is the same simple setup, so once you learn to diagnose no-spark on one machine you can do it on any of them. Here is how the system works and how to find the fault.
How a Small Engine Ignition System Works
It is a beautifully simple system. As the engine turns, magnets in the spinning flywheel pass the ignition coil, inducing a high-voltage pulse. That pulse travels down the plug lead to the spark plug, which fires a spark across its gap at the right moment to ignite the fuel and air.
So a no-spark fault lives in one of a short list of places: the spark plug, the coil, the flywheel and its key, or the kill switch circuit that grounds the system to stop the engine. That short list is the whole diagnosis.
Start With the Spark Plug
The plug is the cheapest and most common cause, so always start here. Remove it and look.
| Plug looks | Meaning |
|---|---|
| Black and sooty | Running rich or oil fouled. Clean or replace. |
| Wet with fuel | Flooded, or not sparking. Dry and retest. |
| White or blistered | Running too hot. Check mixture and heat range. |
| Worn or damaged electrode | Replace it. Plugs are cheap. |
Test for Spark
The definitive test. Fit an inline spark tester between the plug lead and the plug, or hold the plug against bare metal on the engine, and crank it. You should see a bright blue spark.
- No spark: the fault is upstream, in the coil, flywheel key, or kill switch.
- Weak orange spark: a failing coil or a bad connection.
- Strong blue spark: the ignition is fine, so the problem is fuel or compression instead.
Check the Ignition Coil
The coil, also called the armature or magneto, is a frequent failure. Coils can fail with heat, so a machine that starts cold then cuts out once hot is a classic sign of a coil breaking down as it warms.
Two things to check: the air gap between the coil and the flywheel, which should be set to the manual's figure using a gapping gauge, and the coil's resistance with a multimeter. A coil out of spec, or one that fails only when hot, needs replacing.
The Sheared Flywheel Key
This one is easy to miss and very common after the blade or auger strikes something hard. The flywheel is located on the crankshaft by a soft metal key, and a sudden impact can shear that key, letting the flywheel slip out of position.
When the flywheel timing shifts, the spark no longer arrives at the right moment, so the engine will not start, kicks back, or runs terribly. Removing the flywheel to inspect and replace a sheared key restores correct timing.
Do Not Forget the Kill Switch
Before condemning expensive parts, check the simplest cause of no spark: the kill switch circuit. Every small engine has a wire that, when grounded, stops the engine by killing the spark. If that wire is chafed and shorting to the frame, or a safety switch has failed closed, the ignition is grounded out and you get no spark at all.
The Diagnosis Order
1. Inspect and gap the plug
The cheapest and most common cause.
2. Test for spark
Confirms whether the fault is ignition at all.
3. Disconnect the kill wire
Rules out a shorted kill circuit before blaming the coil.
4. Check the coil air gap and resistance
Including the hot-failure test.
5. Inspect the flywheel key
Especially after an impact.
Rather Have a Real Technician Handle It?
A Wildwood technician comes to your driveway, diagnoses the machine on site, and gets it running, usually in about an hour. More than 10,000 repairs completed since 2019, with flat rates quoted before we arrive so there are no surprises.