The engine fires on the second pull every morning, runs all day, but the moment you stop for five minutes and try to restart it, nothing. This pattern — easy cold start, hard hot start — points almost entirely at fuel vapour and starting technique, not a worn-out engine.
Why a hot engine is harder to start than a cold one
It sounds backwards. Cold engines need choke, prime and patience; you’d expect a hot one, already warmed through, to fire on the first pull. In practice the opposite often happens, for three reasons that usually combine:
- Fuel vaporises in the lines. Petrol boils at a much lower temperature than most people assume, and a fuel line or carburettor sitting against a hot cylinder and muffler can easily reach 60–80°C. At that temperature, fuel in the line and in the carburettor turns to vapour instead of staying liquid. The primer bulb and fuel pump are built to move liquid; they push vapour around uselessly, and no usable fuel reaches the cylinder. This is vapour lock, and it’s the single biggest cause of hot no-starts on air-cooled 2-strokes.
- Fuel percolation in the carburettor. Even without full vapour lock, heat soak makes fuel already in the carburettor’s metering chamber “boil” gently and bubble out through the jets, over-filling the throat with raw fuel — effectively flooding the engine from the inside, without you touching the choke at all.
- Habit. Most people start a brush cutter with choke and prime by reflex, every time, hot or cold. On a hot engine that reflex floods it immediately, because a warm cylinder already holds enough fuel vapour to fire without any choke at all.
Safety note. Never attempt any of the checks below with the engine running or the cutting attachment fitted and spinning. Stop switch off, let the blade or line come to a complete stop, and keep hands clear of the muffler — it’s easily hot enough to burn skin on contact.
The correct hot-start technique
This is the fix for the vast majority of hot no-starts, and it costs nothing.
- Leave the choke fully open. Do not touch it. A hot engine doesn’t need enrichment — it already has plenty of fuel vapour in the cylinder.
- Do not use the primer bulb. Priming pushes more fuel into a system that’s already fuel-rich from heat soak.
- Set the throttle to its starting position — the half-throttle lock on most machines — while pulling the cord. The extra air through the carburettor throat helps clear a rich mixture. Don’t hold the trigger wide open: the head spins up to full speed the instant it fires.
- Pull the cord firmly, 3–4 times. Most hot restarts fire within this many pulls once choke and prime are left alone.
- If it doesn’t fire, stop and check for flooding (see below) rather than repeating the same pulls another ten times.
Once it catches, ease off the throttle to let it settle to idle before engaging the cutting head.
If it still won’t fire: check for flooding
(The full clearing method is in how to unflood a brush cutter.)
If pulls 4–8 produce nothing, the engine is likely flooded from vapour condensing back to liquid fuel, or from an earlier attempt with the choke closed.
- Move the stop switch off.
- Remove the spark plug — expect it to be wet or smell strongly of fuel.
- With the plug out and choke open, pull the cord 5–6 times to clear excess fuel and vapour from the cylinder.
- Wipe the plug dry (or fit a spare if you carry one) and refit it.
- Restart with choke open, throttle held partway, no priming.
Let the heat and vapour dissipate
Sometimes the fastest fix is simply patience. If repeated hot-start attempts aren’t working:
- Loosen the fuel cap a quarter-turn for 30 seconds to release built-up vapour pressure in the tank, then retighten it.
- Stand the machine in shade, off the hot ground and away from the muffler’s direct heat if possible.
- Wait 5–10 minutes. This is usually enough for the fuel line and carburettor to drop below the temperature where vapour lock forms.
- Try again with the choke-open technique above.
Standing in direct sun on tarmac makes this worse — the machine has nowhere to shed heat and the fuel line runs even hotter than the engine itself.
When it’s the ignition coil, not fuel
A small number of hot no-starts are electrical rather than fuel-related (a coil that has stopped sparking leaves the plug wet, because fuel keeps arriving with nothing to light it): the ignition coil generates a spark when cold but the coil’s insulation breaks down as it heats up, weakening or killing the spark entirely. This is a genuine, if less common, cause and it’s worth ruling in or out if the fuel-based fixes above make no difference over several outings.
How to tell the difference:
| Symptom | Points to |
|---|---|
| Won’t fire hot, plug is soaked with fuel | Flooding — or fuel arriving with no spark, so test the spark hot |
| Won’t fire hot, plug is dry, no fuel smell | Fuel not arriving — vapour lock or a blocked vent |
| Fires hot after 5–10 min rest, every time | Vapour lock — fuel issue |
| Fires hot occasionally, no clear pattern | Worth a spark test hot vs cold |
| Runs fine, then cuts out mid-cut once hot | Coil breakdown more likely than fuel |
Testing for a heat-sensitive coil: with the plug removed, refitted into its cap, and the metal body earthed against the cylinder fins (insulated pliers, never bare hands), pull the cord and check for a crisp blue spark — first cold, then again once the engine has run and is hot. A strong spark cold that visibly weakens or disappears once hot confirms the coil. A failing ignition coil needs replacing, and setting its air gap correctly afterwards is why many owners leave it to a dealer or small-engine workshop (see brush cutter no spark).
Other things that make hot starting worse
- Old or wrong-ratio fuel. Stale or badly mixed fuel makes every start harder, hot or cold. Fresh fuel mixed at the correct ratio (commonly 50:1, check your manual and our ratio chart) helps here as much as anywhere else.
- A clogged fuel filter or pinched line, restricting flow so the pump has to work harder — worth checking if hot starts are consistently bad even after resting the machine (see brush cutter won’t start for the full fuel-system checklist).
- Running the engine too lean at high speed (what the H screw does), which raises cylinder and muffler temperature and makes heat soak worse between cuts — a job for correct carburettor adjustment, not something to chase by trial and error on the H screw.
Quick checklist
- Hot restart: choke open, no prime, throttle held partway, pull 3–4 times.
- If nothing after that: check for a flooded plug, clear the cylinder, retry.
- Still nothing: loosen the fuel cap briefly, rest the machine 5–10 minutes in shade, retry.
- Persistent hot no-start with a fuel-wet plug even after clearing it: test spark hot vs cold — a coil that weakens with heat needs replacing. A dry plug instead points to fuel not arriving (vapour lock or a blocked vent).
- Keep fuel fresh and correctly mixed — stale fuel makes every one of these problems worse.