The chainsaw I bought a few months back was the first 2-stroke engine I’ve ever personally owned. 4-stroke engines, like the one in your car, or, probably, your lawnmower, have a separate oil reservoir and inject fuel and oil into the engine separately, as needed. 2-stroke engines don’t have a separate oil reservoir, but of course, they still need lubrication. What you do is mix the oil into the fuel at a specified ratio and the pour that into the gas tank. The oil lubricates the engine as the fuel/oil mixture burns during normal combustion.

The ratio of fuel to oil is important. Too much oil and the soot from the burning oil will foul the spark plug and the engine will lose power and stop running. This is an easy fix: just take out the spark plug and clean it with a stiff brush and an appropriate cleaner, or even gasoline if you don’t have anything better on hand. Too little oil, and the engine will, in the worst case, overheat and warp the piston, basically permanently destroying it. If you’ve got a chainsaw that’s expensive enough to make replacing the piston worth the price, then you probably don’t need to read anymore!

The chainsaw (or weed-whacker, or whatever) probably comes with a manual that specifies a certain oil-to-fuel ratio, such as 30:1, 40:1, or 50:1. These ratios are commonly expressed as the number of ounces of oil that have to be added to one gallon of gas to get the desired ratio: 4.3 oz, 3.2 oz, and 2.6 oz respectively. When you buy oil, the oil typically comes either in single-dose containers, or it comes in a big container with a measuring spout that is calibrated to the single-dose amount. The idea here is that you measure out a single dose and mix it with a gallon of gas and then away you go!

Herein lies the first confusing thing to people (like me) who are new to 2-stroke engines. If my owner’s manual says I need a 40:1 mixture, but the oil that I bought is pre-measured to give out 2.6 oz shots (50:1 mixture), what do I do? The answer to this one is actually pretty simple: the oil’s number takes precedence. The proper mixture for oil depends entirely on the lubricating properties of the oil itself. So a modern, synthetic oil, can provide adequate lubrication at a 50:1 ratio, whereas if you go back far enough, oils had as low as a 16:1 ratio! Forget what your owner’s manual says and just put the amount of oil in the gas that the oil container says.

Fuel goes “stale” after a couple of months. What, exactly, happens to it is explained different ways by different people, and I have never been curious enough to really search out good answers. Some people say it takes on moisture. Other people say it “gums up”. You can buy fuel stabilizer to mix in with the gas and extend the life of the fuel to maybe six months or more, but I have, twice now, had a chainsaw absolutely die on me when I was 30 minutes’ drive from home, cutting up a tree for firewood, and let me tell you, that’s a waste of time and effort that I don’t need. What was the problem? The fuel was less then a month old, so it should have been fine. I even shook it up really well, to mix the oil back in (it settles out if it sits for a while). That fuel ought to have been fresh, but the minute I mixed up a new batch, the saw came right back to life, so you’ll excuse me if I’m only interested in burning freshly-mixed fuel in my 2-stroke engines.

And that brings us to issue number 2: I don’t burn through a lot of 2-stroke fuel. Each time I take out the saw, I might burn through a tank or three and then put it away for another month or so, especially during the summer, when gardening is more of a priority and cutting firewood less. Getting rid of excess 2-stroke mixture is easy: just pour it into your car’s gas tank. The amount of oil in the mixture is minute, but if you’re really concerned, take the 2-stroke mix to the gas station with you and pour it into your (empty) tank just before filling up. This will ensure the smallest possible ratio of oil to gas, and thoroughly mix the oil into the fuel.

Even though this prevents the fuel from going to waste, the oil still goes to waste, and that’s not ideal. So today, I figured out how to mix up just one tank’s worth of 2-stroke mix at a time. This means less wasted oil and less 2-stroke mix being burned in my car’s tank. Here’s how to do it:

1. Determine the capacity of your tool’s tank. I did this by filling the chainsaw’s tank with straight gasoline and then emptying the gas tank into a large metal bowl (large to catch the spillage). I set the bowl on a scale and determined that my chainsaw’s tank holds about 9.25 oz of fuel.
2. Determine the appropriate ratio for your oil. This is easy to do because the oil is marked with the right amount to use for 1 gallon of fuel. 3.2 oz per gallon is 40:1. 2.6 oz per gallon is 50:1, and these are the most common ratios seen today. If you don’t know the correct ratio, a gallon is 128 oz. Divide the “one-shot” amount for your oil into 128 and then take 1/ the result. Example: 2.6 / 128 = 0.2. 1/0.2 = 50. 2.6 oz per gallon is a 50:1 ratio.
3. Divide the capacity of your tool’s tank by the desired ratio. Example: 9.25 oz capacity with a desired ratio of 50:1. 9.25 / 50 = 0.185. At a 50:1 ratio, a full tank of my chainsaw’s gas would require 0.18 oz of oil.

Having calculated the single-tank amount of oil, the only remaining task is to devise a method for measuring out that much oil. My preferred method is a turkey-baster or other such object. a large syringe, such as is used in food prep might be good too, and more precise, but I happen to own some extra turkey-basters that I used to use to fill kerosene lanterns, so there you go. My turkey baster is pre-marked at 1/4 oz, 1/2 oz, and 3/4 oz, so I was tempted to just eyeball the 0.18 oz of oil, but there I was with a scale just sitting there, so I weighed it out. Scales are your friends, boys and girls, in all sorts of tasks, from the kitchen to the garage. In the future, I will probably mark on the turkey-baster where the 0.18 oz point is to make future measurement easy. With this technique, it’s not much harder to fill the tank, but I never have more than a tank’s worth of extra fuel.

One final note: I’d suggest squirting the oil into your empty gas tank first, and then adding fuel to fill. This will ensure that the oil is mixed thoroughly with the fuel, which might not happen if you put the fuel in first and then the oil.