How do I mix two-stroke fuel correctly?

A two-stroke engine has no sump. All of its lubrication comes from oil suspended in the fuel, which is why the ratio is not a suggestion. Too little oil seizes the engine, usually permanently. Too much fouls the plug, coats the exhaust port with carbon and makes it run badly. Both are avoidable with a measuring jug.

Safety

What you'll need

• Two-stroke oil suitable for the machine
• A fuel container marked for petrol
• A measuring jug or a graduated mixing bottle
• Fresh petrol

Step by step

1. Find the ratio the machine requires.In the handbook, and often on a sticker on the machine or the fuel cap. Most modern strimmers, hedge trimmers, chainsaws and blowers are 50:1. Older machines are frequently 25:1 or 40:1. Do not assume — the difference matters.
2. Do the arithmetic once and write it down.At 50:1, 20ml of oil per litre of petrol. At 40:1, 25ml per litre. At 25:1, 40ml per litre. Write it on a label on the fuel can so you never have to work it out again.
3. Use oil intended for air-cooled two-stroke engines.It is formulated to burn cleanly at the temperatures these engines reach. Outboard two-stroke oil and car engine oil are not the same thing and should not be substituted.
4. Measure the oil properly.A graduated mixing bottle or a measuring jug, not a splash from the bottle. Two-stroke oil is often dyed so you can see it is present; the colour tells you nothing about the proportion.
5. Put the oil in the empty container first.Then add roughly half the petrol and shake hard. Oil poured onto a full container of petrol sits on top and mixes poorly.
6. Add the rest of the petrol and shake again.Thoroughly. Unmixed fuel means the first tankful is nearly pure petrol and the last is mostly oil, which is the worst of both problems.
7. Label the container clearly with the ratio and the date.Mixed fuel and plain petrol look identical. A can of 50:1 poured into a four-stroke mower will make it smoke heavily; plain petrol poured into a two-stroke will seize it. Label both cans.
8. Mix only what you will use in a month.Two-stroke mix degrades faster than plain petrol because the oil separates as well as the fuel going stale. Mixing a litre for an afternoon's work is better practice than keeping five litres.
9. Shake the can before every refill.Even correctly mixed fuel separates a little on standing. Ten seconds of shaking before each fill costs nothing.
10. Consider pre-mixed alkylate fuel for occasional machines.It comes ready mixed at the correct ratio, keeps for years, does not gum carburettors, and produces far less unpleasant exhaust. It costs several times as much, which makes sense for a chainsaw used twice a year and not for daily use.
11. Dispose of leftovers properly.Mixed fuel that is over a few months old should go to a household waste site. Do not tip it on a bonfire, into a drain or onto the ground.

Tips

• Buy a graduated mixing bottle. They cost very little and remove the arithmetic entirely.
• Keep a different coloured or clearly marked can for two-stroke mix so it cannot be confused with mower petrol.
• If you cannot find the ratio and the machine is modern, 50:1 is the common standard — but confirm before running it.

Common mistakes

• Guessing the oil quantity — A splash is not a measurement, and a two-stroke engine run lean on oil seizes, usually within an hour of running and usually terminally. The cost of a measuring bottle is a fraction of the cost of the engine.
• Adding more oil than specified to be safe — Excess oil does not lubricate better. It fouls the plug, builds carbon in the exhaust port and the muffler, and makes the machine run badly and smoke. Two-stroke ratios are specified for a reason in both directions.

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