Most people can stick with the manufacturer’s recommendation for chainsaw fuel and be absolutely fine. There’s a reason why the manufacturer recommends a certain fuel be used, and usually, that comes down to safety (liability for the company) and performance (who wants a bad product).
Modern gasoline chainsaws run at 50:1 mixture, 50 parts gasoline to 1 part oil. Older chainsaws can require 16:1 mixtures or 32:1 (most common). Two-stroke engines are much more efficient now, as well as having cleaner combustion.
At most automotive stores, pre-mix fuel can be bought in 50:1 and 40:1 mixtures. Interestingly, Husqvarna recommends 33:1 fuel mix for 75cc and up engines in “heavy-use” situations.
Chainsaw things you may need:
Outboard motors for boats generally use a fuel mix of 100:1. This is for environmental reasons (since the exhaust goes towards the water) and the use of the motor. Outboards are also used at a steady speed and thus need less lubrication since there’s less wear on bearings for example. For a dirtbike or chainsaw, the throttle is on and off frequently and we need that lubrication for things to run smoothly (literally).
As such, oil has changed as well. We have gradually shifted from conventional oil to synthetic.
Synthetic oil also has fewer impurities than conventional meaning possibly lower emissions. It is also more resistant to extreme heat and cold weather conditions, as conventional oil does not flow as freely under cold weather conditions.
In 2020, I picked up a Remington Mighty Mite for $40. a while back and it had “16:1” on the gas cap. A tiny little saw, it always started on the first pull. I purchased it from the original owner, who bought the saw in the late ’70s. Unfortunately, the cord catch broke on the flywheel and sourcing a part would have been too difficult.
What About Using 40:1 Instead?
40:1 mix has slightly more oil, so you will have to adjust your carburetor to compensate. This would mean making it run richer. But why 40:1 over 50:1, wouldn’t the change be negligible?
Pretty much, yeah.
For chainsaws, we want the highest RPM at WOT. We want to cut as fast as we can. The more we cut, the hotter our saw gets. A 40:1 ratio has more oil to lubricate the piston and rings. The lubrication helps the ring seal better, creating more compression and power.
But oil only partially burns in the engine and can leave residue behind (fouling). More oil means more residue. Some people swear by Sea Foam to clean their engine. I don’t know how it works, but it somehow reduces the residue build-up as well.
One thing I always recommend is to make sure your spark plugs are gapped correctly. You can check by using a spark plug gap tool.
You can also help reduce fouling by proper carb tuning of your chainsaw. Fuel mix can matter, only if you have tuned your carb accordingly.
Check your spark plugs, like any sane person.
On the far left, you can see how “burnt” the spark plug looks by running to lean (too much air). In the middle, you can see the carbon deposits by running too rich (too much fuel). What we’re after is a “coffee” brown colour of the spark plug ground electrode.
What happens if you use 50:1 fuel instead of 40:1?
Since the fuel mixture has changed, you must adjust your carb. The fuel is now richer (less oil) and can potentially run hotter (again, less oil).
