What 2-Stroke Mix Ratio Does a String Trimmer Use?

String trimmers are used in short bursts a few times a month, which means the same tank of premix often sits for six to eight weeks. That is well past the point where untreated two-stroke mix starts to go off, and it is why trimmers are the tool most often brought in with a gummed carburettor.

Mix small batches, use a stabiliser if the can will sit, and pour off anything older than about a month rather than running it. A trimmer carburettor is small enough that a little varnish blocks it completely.

Stihl, Husqvarna, Echo, Shindaiwa and Makita trimmers are 50:1 across their current ranges. Budget and older tools are where it varies: some Ryobi, Homelite, Troy-Bilt and Poulan models call for 40:1 or 32:1, and a few older units were sold with the maker's own oil at a specific rate.

Check the fuel cap decal first. If the tool has an attachment-capable powerhead shared across a brand's line, the ratio belongs to the powerhead, not the attachment, so the same mix runs the edger, pole saw and brush cutter heads.

Echo is the brand people ask about most for trimmers, and the answer is the same across the range: every Echo handheld two-stroke is a 50:1 machine, which is 2.56 oz of oil per US gallon. That covers the straight-shaft SRM series (the SRM-225 is the one in most sheds), the curved-shaft GT series, and the PAS attachment powerheads. Model-by-model detail, tank sizes and what to do with an old manual that says 32:1 are in the Echo weed eater gas oil ratio guide.

The PAS point is worth repeating, because it is where people get confused. On a PAS-225 or any other attachment-capable powerhead the ratio belongs to the engine, not to whatever is bolted on the end. The edger, pole saw, brush cutter and articulating hedge attachments all run the same 50:1 mix out of the same can.

Echo sells its own oil in 2.6 oz single-use bottles sized for exactly one gallon at 50:1, and ties an extended consumer warranty to using Echo-branded oil, so check the terms before you switch brands on a tool that is still in warranty. Any JASO FD air-cooled oil is technically a safe substitute: the Echo oil guide compares Power Blend against Red Armor and covers what else is acceptable.

One terminology note, since the queries run together: "weed eater", "weed wacker" and "string trimmer" all mean the same machine here. Weed Eater is also a real brand, now part of the Husqvarna group, and a genuine Weed Eater badged trimmer carries its own decal. Read the cap rather than the nickname.

Stihl is the other name on most weed eater searches, and its rule is even simpler than Echo's: every Stihl gasoline tool runs 50:1, the same 2.56 oz of oil per US gallon. That covers the homeowner two-strokes (FS 38, FS 40, FS 50, FS 56, FS 70) and the KM 56 KombiEngine. It also covers the FS 91, FS 111, FS 131 and the KM 91, KM 111 and KM 131, which use Stihl's 4-MIX engine. 4-MIX is a four-stroke with valves, but it has no oil sump: the premix is its only lubrication, so straight gas will ruin it.

Stihl's own guidance adds three things worth following: mid-grade gas of 89 octane or higher, oil into the can before the gas, and no more than a few days' worth mixed at once, with stored mix never kept longer than three months. The Stihl weed eater gas mix guide goes model by model through the FS and KM trimmers and the BG and BR blowers, with tank sizes and batch amounts.

• Hard starting after a few weeks idle, then running only with the choke half on, is almost always stale fuel rather than the mix ratio.
• Heavy smoke plus a wet plug right after mixing a fresh can means too much oil, usually a full 2.6 oz bottle dropped into less than a gallon of gas.
• Loss of power under load with a clean plug is often a clogged spark-arrestor screen, which a rich mix accelerates. It unscrews from the muffler and cleans with a wire brush.
• A trimmer that revs but will not hold high rpm can be a fuel line or filter perished by ethanol rather than anything to do with the oil.