If your brush cutter cranks over fine, has fuel in the tank, and simply refuses to fire, the ignition system is the first thing to isolate before you touch the carburettor. The good news is that the ignition circuit on a two-stroke brush cutter is short and simple - plug, cap, coil, kill switch, flywheel - so a methodical spark test will point you at the fault rather than leave you guessing.
Before you start: rule out the obvious
Five minutes here can save you an hour of unnecessary teardown.
- Check the on/off switch is actually in the “run” or “I” position, not “stop” or “0”.
- Look at the kill wire (the thin wire running from the switch to the coil) for chafing or a bare patch touching the shroud or fuel line - a shorted kill wire is one of the most common no-spark causes and costs nothing to fix.
- Confirm the spark plug boot (cap) is pushed fully onto the plug terminal - vibration works these loose over a season.
- Pull the plug and look at it: wet and black means it’s fouled with fuel/oil and won’t spark reliably even if the coil is healthy; bone dry after cranking points to a fuel problem, not ignition.
Safety note: Remove the plug lead or ground the plug before any other work on the cutting head, clutch, or blade. An engine that fires unexpectedly while your hand is near the blade is a genuine amputation risk.
How to do a proper spark test
- Remove the spark plug with a plug socket (usually 13 mm or 5/8 in on most 25-45 cc units).
- Reconnect the plug cap to the plug terminal.
- Hold the plug body firmly against bare, unpainted metal on the cylinder fins or the engine block - not against a plastic shroud. Use pliers or a plug tester with an insulated handle so your hand isn’t near the electrode.
- Keep the plug at least 15 cm from the spark plug hole and away from any spilled fuel - unburned vapour around an open cylinder can ignite.
- Pull the starter cord briskly 2-3 times with the switch in “run” and watch the gap between the electrodes.
- Judge what you see:
| What you observe | Meaning |
|---|---|
| Strong, sharp blue-white spark, snaps audibly | Ignition is fine - move to fuel/carburettor/compression checks |
| Weak, orange or yellow, or spark only sometimes | Failing coil, worn plug, or moisture/carbon in the cap |
| No spark at all | Work through cap, kill switch, air gap, coil, flywheel key in that order |
| Spark at the plug but engine still won’t fire in the cylinder | Plug is fouled or gapped wrong - clean or replace it and retest in place |
Safety note: Never do this test with the plug still threaded into the cylinder. You can’t see the gap clearly, and if there’s fuel vapour in the cylinder it’s a direct ignition source right next to your hand.
Working through the ignition chain
1. The spark plug itself
A fouled, oil-soaked, or badly gapped plug is the single most common cause of “no spark” and the cheapest to rule out.
- Standard gap for most brush cutter plugs is around 0.5 mm (0.020 in) - check the manual, as some small engines want 0.6-0.65 mm.
- If the electrode is black and wet, it’s fouled from a too-rich mix, choke left on, or worn rings; clean it with a wire brush and re-gap, but replace if the porcelain is cracked or the electrode is rounded off.
- Plugs are cheap (a few euros) - if in doubt, swap in a known-good one before condemning anything else.
2. The cap and kill (stop) switch
- Inspect the cap boot for cracks or green corrosion inside - this lets spark leak to earth instead of jumping the plug gap.
- Trace the kill wire from the switch to the coil. Disconnect it at the coil spade terminal and retest for spark: if spark suddenly appears, the switch or wire is shorted to ground (bare wire touching metal, or a failed switch stuck in the “off” position internally). This is a very common fault after a machine’s been dropped or the shroud has been off a few times.
- A shorted kill wire is usually a €5 fix - a new switch or a bit of heat-shrink and electrical tape over the bare spot.
3. Coil-to-flywheel air gap
The ignition coil sits just outside the flywheel and is triggered as the flywheel’s magnet sweeps past. If the gap is wrong, the coil either doesn’t get a strong enough field to fire, or the flywheel physically strikes the coil.
- Typical air gap is about 0.3 mm (0.012 in) - check your model’s spec, as it can range 0.25-0.4 mm.
- To set it: loosen the two coil mounting screws, slip a feeler gauge (or a piece of business-card stock cut to roughly the right thickness if you don’t have gauges) between the coil leg and the flywheel magnet, rotate the flywheel so the magnet sits against the coil, push the coil down onto the gauge, then tighten the screws while holding it in place.
- Rotate the flywheel a full turn afterwards to make sure nothing catches - a gap set too tight will have the flywheel clip the coil on every rotation, which is both a no-start and a coil-killer.
4. The ignition coil
- If gap is correct and spark is still absent or weak, test the coil with a multimeter. Note that many modern CDI-style coils on brush cutters are hard to test meaningfully with a basic meter (the primary winding’s low resistance and the electronic trigger circuit don’t give a simple pass/fail reading), so treat this as a rough guide, not gospel.
- Rough reference figures: primary winding resistance is typically 0.4-2 Ω; secondary winding (through the plug cap) is typically 4,000-8,000 Ω, depending on the model.
- A winding reading open circuit (infinite resistance) or dead-shorted (near 0 Ω on the secondary) confirms a failed coil.
- In practice, most home mechanics confirm a bad coil by elimination: plug good, cap good, kill switch isolated, air gap correct, still no spark - swap the coil. They’re a bolt-on part, usually 20 minutes including resetting the air gap.
5. Sheared flywheel key
If the engine used to run fine and suddenly lost spark after the blade hit something hard (a rock, a fence post, a buried root), suspect the flywheel key before the coil.
- The flywheel key is a small metal half-moon or rectangular key that keys the flywheel to the crankshaft taper, keeping ignition timing correct relative to piston position.
- A sudden blade strike can shock-stop the flywheel while the crank keeps turning under momentum, shearing the key. The flywheel then sits at the wrong angle relative to the crank - timing is off enough that the coil may fire when the magnet isn’t aligned to actually spark, or spark at completely the wrong point in the stroke.
- To check: remove the flywheel (requires the correct puller - do not lever it off with screwdrivers, you’ll damage the taper) and inspect the key for shearing or rounding.
- This is a straightforward fix if you have the tools (flywheel puller, correct socket for the flywheel nut, a new key costs under €5) but easy to get wrong - overtightening the flywheel nut or seating it off-taper causes bigger problems, so if you’re not confident with a puller, this is worth a dealer visit.
Quick diagnostic checklist
Work top to bottom - each step takes a couple of minutes and rules something out completely.
- Switch in “run” position - confirmed.
- Plug removed, inspected, gapped to ~0.5 mm or replaced.
- Spark test done outside the cylinder, grounded to bare metal, 15 cm clearance.
- Kill wire disconnected at coil and retested - rules in/out a shorted switch circuit.
- Cap inspected for cracks/corrosion.
- Air gap checked and set to ~0.3 mm if adjustable.
- Coil resistance checked with a multimeter if you have one and the model supports it.
- Flywheel key inspected if the fault followed a hard blade strike.
When to call a dealer
Take it in rather than persisting solo if: you don’t have a flywheel puller and suspect a sheared key, the crankshaft taper looks damaged once the flywheel is off, you’ve replaced the coil and set the gap correctly but still get no spark (points to a wiring or module fault deeper in the harness), or the machine is still under warranty - opening the flywheel housing yourself can void it. For everything else on this list - plug, cap, switch wiring, air gap, coil swap - it’s well within reach of a Sunday afternoon with basic hand tools.