What 2-Stroke Mix Ratio Does a Chainsaw Use?

This is the mistake that ends the most saws. A chainsaw has two separate reservoirs: the fuel tank, which takes the gas and 2-stroke oil premix, and a bar-and-chain oil tank, which takes a thick tacky oil that lubricates the chain as it runs over the bar. They are usually side by side under similar-looking caps.

Bar oil must never go in the fuel, and 2-stroke premix oil must never go in the bar oiler. Bar oil is not formulated to burn and will foul the engine quickly; premix oil is far too thin to stay on a moving chain. If the caps are unmarked, the fuel cap is the one closest to the starter and carburettor side.

Modern saws are almost universally 50:1. The exceptions worth knowing are older saws, which ran 40:1 or 32:1, and a small number of professional saws sold in markets where the manufacturer specified its own oil at a different rate. The decal on the fuel cap or tank is the authority; the manual is second best.

Where a saw has no decal and no paperwork, mixing 40:1 is the safer guess than 50:1. The extra oil costs you some smoke and a dirtier spark-arrestor screen, while guessing lean costs you a piston.

Brand-specific searches for chainsaw mix mostly land on the same answer. Current Stihl saws from the MS 170 up to the professional MS 500i, Husqvarna saws from the 120 Mark II to the 572 XP, and Echo saws from the CS-310 to the CS-590 Timber Wolf all specify 50:1, which is 2.56 oz of oil per US gallon or 20 ml per litre.

Where older saws differ, it is usually the oil that changed rather than the engine. Echo printed 32:1 in its manuals until the late 1990s and now says its older air-cooled engines can run 50:1 on a modern oil. Older Stihl and Husqvarna manuals often gave a richer ratio for other brands of oil than for their own bottle, which is why 40:1 and 33:1 figures still turn up in forum answers about the same saw. With a current JASO FD or ISO-L-EGD oil, 50:1 is what all three makers ask for.

The brand oils are compared one by one in the Stihl, Husqvarna and Echo guides, and the same can of 50:1 runs the trimmer and leaf blower from those brands too.

• Bogging or four-stroking at full throttle, with heavy smoke, usually means too much oil or stale fuel rather than a carburettor fault.
• Gradual power loss over a tank or two, then a saw that stops and will not restart, is the classic lean-on-oil seizure. Pull the muffler and look at the exhaust side of the piston: vertical scoring is the confirmation.
• A saw that runs but will not hold idle after sitting all winter is usually stale premix, not the ratio. Two-stroke mix begins to degrade within about 30 days unless it is stabilised.
• A spark-arrestor screen that clogs every few hours points at a mix that is too rich or an oil that is not a proper low-ash 2-stroke oil.

Use fresh 89 octane or higher, and ethanol-free fuel if you can get it. Ethanol attracts water and separates in storage, which is the leading cause of small-engine carburettor trouble. Mix only as much as you will burn in a month, label the can with the ratio and the date, and store it out of sunlight.

Before a saw goes into storage for the season, either run the tank dry or fill it with stabilised mix. Leaving a half-empty tank of untreated premix over the winter is what turns a working saw into a carburettor rebuild.