A brush cutter that starts on the second pull in August and fights you for twenty pulls in January isn’t broken — it’s telling you something is marginal, and cold is exposing it. Below about 10°C, several things stack up against the engine at once: fuel vaporises less readily, the engine turns over more slowly on the cord, and the battery-free ignition coil generates slightly less spark energy at that lower cranking speed. Get the starting sequence right first (the full routine is in how to start a brush cutter), then check the parts that cold weather is quietly punishing. If the plug comes out wet, see brush cutter flooded.
Safety first. Cold or warm, keep the cutting attachment clear of the ground and anyone nearby before pulling the cord — a machine that suddenly fires can spin the head.
1. The cold-start sequence, done properly
Most cold no-starts are simply the wrong sequence, not a fault at all. This is the order that works on the vast majority of 2-stroke brush cutters:
- Fuel tap/valve (if fitted) to on.
- Choke to full/closed.
- Prime — press the primer bulb until it is visibly full of fuel (Stihl’s manuals say at least five presses); more presses after that only add to the risk of flooding.
- Throttle at idle/start position (many models lock the throttle open slightly for starting — check your manual).
- Pull the cord firmly 3-5 times on full choke.
- As soon as it fires, sputters, or even just “catches” briefly, move the choke to half immediately.
- Pull again until it runs, then ease the choke to open/run over 10-20 seconds as the engine warms.
The step people get wrong most often is step 6 — leaving full choke on after the first pop, or jumping straight from full choke to no choke. Both flood a cold engine fast, because cold cylinder walls don’t help fuel evaporate the way a warm engine does.
How long to warm up before cutting: run it at idle for 30-60 seconds in cold weather before opening the throttle. Revving a cold 2-stroke hard before the oil has circulated accelerates wear on the piston and rings.
2. Weak or intermittent spark in the cold
Ignition coils generate spark from the magnetism of the flywheel spinning past them, and a cold, stiff engine turns over slightly slower on the pull cord than a warm one. If the coil, plug, or plug cap is already marginal, that small drop in cranking speed can be the difference between a spark that jumps the gap and one that doesn’t.
Check:
- Spark plug gap — should be close to 0.5 mm. A gap that’s crept wide from wear needs more voltage to jump, and that’s exactly what a cold start doesn’t have to spare.
- Plug condition — a plug that’s fouled, oily, or has a worn electrode may just about manage a spark when warm and cranking fast, but fail a slower cold crank.
- Plug cap and lead — push the cap firmly onto the plug; a loose or corroded connection shows up first in cold, damp conditions.
The fix: if the plug is more than a season old, or you can’t remember when it was last changed, fit a new one — it’s the cheapest and most reliable fix for cold-start spark problems. Gap it to spec with a feeler gauge before fitting.
3. Old fuel in the tank
At 50:1 the oil is only 2% of the mix and is dissolved in the petrol, so its own thickness makes little difference to a cold start. What matters far more is the petrol: fresh fuel contains light, volatile fractions that evaporate readily on a cold cylinder, and those are the first thing old fuel loses. A mix that started a warm engine happily in September can be too flat to light a cold one in January.
What helps:
- Store your fuel mix somewhere above freezing if possible, not in an unheated shed or the boot of a car overnight in winter.
- Ready-mixed alkylate fuel keeps far longer than pump petrol mix and is a common choice for machines used occasionally in winter (see our 2-stroke oil and fuel guide).
- Mix fresh — fuel more than 4-6 weeks old starts to separate and lose the light fractions that help cold starting, regardless of oil type.
- Never top up an old, partly separated mix with fresh fuel; drain and remix instead.
4. Carburettor L screw set too lean for cold
The L (low speed) screw controls the fuel mixture at idle and low throttle — exactly the range the engine runs in during a cold start. If the L screw has been set (or has drifted) slightly lean, the engine may run acceptably once warm and turning at full RPM, but struggle to catch on a cold, slow crank because there simply isn’t enough fuel in the mix at low speed.
Signs the L screw is too lean:
- Cold starts take many more pulls than warm restarts on the same day.
- The engine catches briefly on full choke but dies the moment you move to half choke.
- Idle is unstable or the engine stalls when you release the throttle, even once warm.
The fix: with the engine warm and idling, turn the L screw 1/8 turn counter-clockwise (richer) at a time, waiting a few seconds between adjustments, until idle smooths out and throttle response is crisp. Don’t go past roughly 1½ turns out from lightly seated without a tachometer to check idle RPM (typically 2,800-3,000 rpm) — full carburettor adjustment procedure is outside the scope of this article; see how to adjust a brush cutter carburettor if the engine also runs poorly once warm.
Only adjust the L screw a small amount at a time. Running too rich wastes fuel and fouls the plug; running too lean risks overheating and seizure.
5. Decompression valve or compression release
Some larger brush cutters and clearing saws have a decompression valve — a small button on the cylinder head that vents pressure to make the cord easier to pull, especially useful in cold weather when oil is thicker and cranking resistance is higher.
Check:
- If your model has one, press it before every pull until the engine starts. Forgetting it makes cold starts noticeably harder to crank.
- If pressing it makes no audible hiss of released pressure, the valve itself may be stuck or worn and not doing its job — this is worth having checked, as it isn’t a simple home fix on most models.
Machines without a decompression valve rely purely on cranking speed and technique, which is why steps 1-4 above matter more on those models.
Quick diagnostic table
| Cold-start symptom | Likely cause | Where to look |
|---|---|---|
| Fires once, dies when choke opens | Choke sequence too fast | Section 1 — hold half choke longer |
| No pop at all, no smell of fuel | Weak spark or dead plug | Section 2 — plug gap and condition |
| Fires eventually but takes 15+ pulls every cold morning | Old fuel or lean L screw | Sections 3-4 |
| Runs rough cold, fine once warm, unstable idle | L screw too lean | Section 4 |
| Cord feels unusually hard to pull cold | Missing decompression step (if fitted) | Section 5 |
| Starts fine warm, same-day cold restart also hard | Not genuinely warmed through, or choke released too early | Section 1 |
When to call a dealer
If a fresh plug and correct priming still won’t produce a spark, suspect the ignition coil — this is a straight swap for a dealer but not something to chase with home testing gear. Likewise, if the L screw needs more than about 1½ turns out from seated to run acceptably, or idle won’t stabilise at all, the carburettor likely needs a proper clean or rebuild rather than screw adjustment alone.
Quick checklist for next cold start
- Full choke, prime until the bulb is full of fuel, throttle at start position.
- Pull 3-5 times, drop to half choke the instant it catches.
- Let it idle 30-60 seconds before opening the throttle fully.
- Fresh mix, or alkylate fuel if the machine sits between uses.
- New plug if you can’t remember the last one.
- Press the decompression valve first, if your model has one.