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.

• 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.