A brush cutter needs three things to run: fuel, spark and air. When it won’t start, one of those three is missing. Work through the checks below in order — they’re arranged from most to least likely, so you’ll usually find the fault in the first few minutes.
Safety first. Before any check, move the stop switch to off, let a hot engine cool, and never spin the cutting head by hand with the plug connected.
1. Stale or wrong fuel (the number one cause)
Petrol goes off. After about 30 days the light fractions evaporate and, in a 2-stroke mix, the oil begins to separate. After a winter in the shed it leaves a sticky varnish in the carburettor jets.
The fix: drain the tank completely, then refill with a fresh mix at the ratio in your manual (usually 50:1 for modern 2-strokes). If it starts and runs roughly, run it for a few minutes — fresh fuel often clears light deposits on its own.
Also check you mixed it at all. Running a 2-stroke on straight petrol will seize the engine. If you suspect that happened and it ran even briefly, stop and have the cylinder inspected.
2. Flooded engine
Too much choke, or too many pulls with the choke closed, fills the cylinder with fuel and drowns the spark.
The fix:
- Move the choke to the open (run) position.
- Remove the spark plug — it will be wet.
- Pull the starter cord 5–6 times to blow the excess fuel out of the cylinder.
- Dry the plug with a cloth (or fit a new one), refit it.
- Start with the choke open and no priming.
3. Fouled or dead spark plug
A plug that’s black and sooty, oily, or has a damaged electrode won’t spark reliably.
The fix: remove the plug and inspect it. A light tan colour is healthy. Black and dry means it’s running rich; black and oily means too much oil in the mix. Clean it with a wire brush, or simply fit a new one — they’re cheap and it’s the single most cost-effective part to replace.
Test for spark: refit the plug into its cap, hold the metal body against the cylinder fins (use insulated pliers), and pull the cord. You should see a crisp blue spark. No spark points to the plug, the plug cap, or the ignition coil.
Also check the electrode gap — typically 0.5 mm on most brush cutters. Set it with a feeler gauge.
4. Blocked air filter
A filter clogged with dust and grass chokes the engine of air, making it run overly rich and refuse to start.
The fix: open the filter cover and inspect. A foam filter can be washed in warm soapy water, squeezed dry (never wrung) and refitted once completely dry. A paper or felt filter should simply be replaced — don’t try to wash it.
5. Stop switch left off
Obvious, but it catches everyone. The stop/ignition switch must be in the on / I position. On some machines the throttle trigger must also be held for the engine to fire.
6. Blocked fuel filter or fuel line
The small felt filter on the end of the pickup hose inside the tank clogs over time, and fuel lines go hard and crack.
The fix: hook the filter out of the tank with a piece of bent wire and inspect it. If it’s discoloured or stiff, replace it — it’s an inexpensive part. Check the fuel line for cracks and hardening while you’re there, and squeeze the primer bulb: if it never fills with fuel, you have a blockage or an air leak on the suction side.
7. Carburettor out of adjustment or gummed up
If you have spark and fresh fuel but it still won’t run, the carburettor is the next suspect. Most brush cutter carbs have L (low speed), H (high speed) and T (idle) screws.
The fix: start with the factory settings in your manual — commonly about 1½ turns out from lightly seated on both L and H. If the machine has sat with old fuel, the jets are likely varnished and need a proper clean with carburettor cleaner, or a carb rebuild kit.
Don’t run an engine with the H screw set too lean — it causes overheating and can seize the piston.
8. Primer bulb not filling
If the bulb stays soft or won’t fill, air is leaking in, or the return line is blocked. A cracked bulb is a common and cheap failure.
The fix: replace the primer bulb and check both small hoses that connect to it.
9. Loss of compression
If the starter cord feels unusually light and easy to pull with the plug fitted, the engine may have lost compression — a worn piston ring, a scored bore, or a seized engine from running unmixed fuel.
The fix: this one isn’t a driveway repair. A compression test (a healthy small 2-stroke reads roughly 6–8 bar / 90–120 psi) will confirm it, and the repair usually means a new piston and rings — at which point, on an older budget machine, replacement is often more sensible than repair.
Quick diagnostic table
| Symptom | Most likely cause | First thing to try |
|---|---|---|
| Turns over, never fires, smells of petrol | Flooded | Clear the cylinder (section 2) |
| Turns over, no smell of fuel | No fuel delivery | Fuel filter / primer bulb |
| Fires once then dies | Stale fuel or blocked jets | Fresh fuel, then carb clean |
| No spark at the plug | Plug, cap or coil | New spark plug |
| Cord pulls very easily | Low compression | Compression test |
| Started fine last season, dead now | Stale fuel | Drain and refill |
Cordless brush cutter that won’t start
Battery machines fail differently. Check, in order:
- Battery charge and seating — push it in until it clicks; a partly seated battery won’t power the tool.
- Safety lock-off — most cordless models need the lock-off button pressed before the trigger will work.
- Thermal cut-out — if you’ve been cutting heavy growth, the tool or battery may have shut down to cool. Wait 15 minutes.
- Battery terminals and contacts — clean, dry and free of grass debris.
- Battery health — if it charges but dies within seconds, the cells are worn out and the pack needs replacing.
When to give up and call a dealer
Take it in if you have no spark after fitting a new plug (likely an ignition coil), if the cord pulls with almost no resistance (compression loss), or if the engine ran on unmixed petrol. These need parts and tools that rarely justify a DIY attempt on a domestic machine.
If the repair quote approaches half the price of a new machine, it’s usually worth putting the money towards a replacement instead.