Get the ratio, the mixing order and the storage right, and a 2-stroke brush cutter will start on the second pull and run cleanly for years. Get any one of them wrong and you’ll be chasing hard starts, smoke, fouled plugs, or — in the worst case — a seized engine. None of this is complicated, but it has to be done the same way every time.
Why you can’t just use petrol
A 2-stroke engine has no oil reservoir, no oil pump, and no dipstick. The only lubrication the piston, cylinder wall, crankshaft bearings and connecting rod ever get comes from the oil suspended in the fuel as it passes through the crankcase on its way to combustion. Run the engine on neat petrol, even briefly, and there’s nothing between the piston and the cylinder wall — friction and heat build almost instantly, and a seizure can happen within a minute or two of running under load. This is not a “might shorten engine life” issue; it’s a “the engine may not run again” issue.
Safety note. Never siphon fuel by mouth, never mix or refuel near an open flame, a lit cigarette or a hot muffler, and always mix outdoors or in a well-ventilated space. Petrol vapour is heavier than air and can pool in enclosed spaces.
Step 1: Confirm your ratio
Check the engine cover, the owner’s manual, or the fuel cap of your machine — the ratio is usually printed somewhere on the unit. There are two ratios in common use:
| Ratio | Where it applies | Petrol : Oil (litres) |
|---|---|---|
| 50:1 | Most modern brush cutters (post-2000s, air-cooled, low-emission engines) | 5 L : 100 ml |
| 25:1 | Older engines, some cheaper imports, a few specific manufacturers | 5 L : 200 ml |
If you genuinely can’t find the ratio anywhere on the machine or in the paperwork, 50:1 with a good quality, ashless 2-stroke oil is the safer modern default — but it’s worth five minutes checking rather than guessing, because running a 25:1-rated engine on 50:1 mix will starve it of lubrication. This page is about the mixing process itself; if you want the full range of ratios and litre-by-litre quantities laid out, that’s covered in our separate ratio reference table.
Step 2: Pick the right container and oil
You need three things before you start:
- A dedicated mixing container — either a proper 2-stroke mixing bottle with graduated markings (cheap, widely available, and by far the easiest way to get this right), or a clean fuel can plus a separate measuring jug rated for small volumes (50-200 ml).
- Fresh petrol — regular unleaded is fine for most machines; check your manual if it specifies a minimum octane. Avoid E10-heavy fuel that’s been sitting in a can for months; ethanol-blended petrol degrades faster and absorbs moisture.
- 2-stroke oil rated for air-cooled engines — not the oil sold for outboard motors or other water-cooled 2-strokes, and never 4-stroke engine oil. Air-cooled 2-stroke oil is formulated to burn cleanly at the higher temperatures an air-cooled engine reaches; the wrong oil leaves gummy deposits on the piston crown and exhaust port.
Step 3: Measure and mix — the actual procedure
- Measure the oil first, into the mixing bottle, using the graduated scale or a proper measuring jug. Don’t eyeball it — even 10-15 ml out on a small batch shifts your ratio meaningfully.
- Add a small amount of petrol (roughly a quarter of the final volume) and fit the cap.
- Shake vigorously for 20-30 seconds. This is the step most people rush, and it matters: oil and petrol don’t blend on contact, they need agitation to emulsify. A half-hearted swirl leaves pockets of unmixed oil.
- Top up with the remaining petrol to your target volume, then shake again for another 15-20 seconds with the cap on.
- Label the container immediately — ratio, date mixed, and “2-stroke mix ONLY” in permanent marker. This single step prevents the single most common and most expensive fuel mistake: someone else (or you, in a hurry) grabbing the can and pouring it into a 4-stroke mower, or worse, pouring straight petrol into your brush cutter because the cans looked the same.
- Shake again before every refuel. Oil can settle slightly over days of storage even in a well-mixed batch — a quick 10-second shake before you pour ensures an even mix goes into the tank each time, not just the first pour.
A simple worked example for a 5-litre batch at 50:1: measure 100 ml of oil into the bottle, add about 1.25 L of petrol, shake hard, top up to 5 L, shake again, label it, done.
Safety note. Mix outdoors or somewhere with good airflow, keep the container away from any ignition source, and wipe up any spilled fuel immediately — a wet patch on a workbench or in a garage is a fire hazard until it’s fully evaporated.
Step 4: Storage and shelf life
Mixed fuel is not shelf-stable indefinitely. Two things degrade it:
- Petrol oxidation — even without ethanol, petrol starts to break down chemically once mixed and exposed to air/heat cycles. Oxidised fuel smells slightly varnish-like, runs rough, and can leave gum deposits in the carburettor.
- Oil separation — over time (especially in heat), the oil can begin to separate out of suspension, meaning the last fuel poured from an old can may be leaner or richer than what you originally mixed.
Rule of thumb: use mixed fuel within 30 days, and store it in a cool, dark place — not in direct sun, not in a hot shed roof space, not in the boot of a car in summer. If you only use your brush cutter occasionally, mix small batches (1-2 L) rather than a full jerry can, even if it means mixing more often. Fuel stabiliser additives can extend usable life somewhat, but they’re not a substitute for mixing fresh when you can.
| Storage condition | Realistic usable life |
|---|---|
| Cool, dark, sealed container | Up to 30 days |
| Warm shed / direct sun exposure | 1-2 weeks before quality drops |
| Left in the machine’s tank, unused | 2-4 weeks before hard starting/carb issues appear |
If you’re laying the machine up for winter or any period longer than a month, run the tank dry (or as close to empty as you safely can) and run the carburettor out of fuel by letting the engine stall on its own, rather than leaving mixed fuel sitting in the tank and fuel lines.
Quick checklist
- Confirmed the correct ratio (50:1 or 25:1) for this specific engine
- Used a graduated mixing bottle or accurate measuring jug — no guessing
- Oil measured first, partial petrol added, shaken hard, then topped up and shaken again
- Container labelled with ratio and mix date
- Stored cool and out of direct sun
- Batch small enough to use within 30 days
- Shaken again immediately before every refuel
If your brush cutter is running rough, smoking heavily, or hard to start and you’ve double-checked the mix is correct and fresh, the problem has likely moved on to the carburettor or spark plug rather than the fuel itself — that’s a job for our troubleshooting guides rather than a remix.