Stihl replacement chain.

[ericm979]

Going from 20:1 to 50:1 is less oil. (that's 5% oil vs 2%).

The old engines recommended high oil ratios because the oil that most people used back then was much poorer quality than what we have now. I have run vintage two stroke motorcycles on modern synthetics at modern ratios with no problem. It can require rejetting. Changing the oil ratio does not change the compression ratio. But it does change the air:fuel ratio. The carb meters X amount of premix per Y amount of air. It does not know how much of that premix is oil. But oil is not fuel. Less oil in your premix means more fuel, thus a richer air:fuel ratio.

If the carb is tuned for 20:1 it will be running rich on 50:1. That's ok for seeing if it will run, but it might not run well or rev out like it's supposed to until it's tuned.
Being old it probably should not be revved as high as modern saws because the bearings and rings are not as good as modern ones. The port timing may not let it rev.

This saw is old enough that its fuel system may not like modern gas with ethanol in it. Anything that's about 25-30 years old or newer will have fuel system materials that can handle ethanol but older than that it might not unless someone updated it.

 

[John_Mc]

Going from 20:1 to 50:1 is less oil. (that's 5% oil vs 2%).

The old engines recommended high oil ratios because the oil that most people used back then was much poorer quality than what we have now. I have run vintage two stroke motorcycles on modern synthetics at modern ratios with no problem. It can require rejetting. Changing the oil ratio does not change the compression ratio. But it does change the air:fuel ratio. The carb meters X amount of premix per Y amount of air. It does not know how much of that premix is oil. But oil is not fuel. Less oil in your premix means more fuel, thus a richer air:fuel ratio.

If the carb is tuned for 20:1 it will be running rich on 50:1. That's ok for seeing if it will run, but it might not run well or rev out like it's supposed to until it's tuned.
Being old it probably should not be revved as high as modern saws because the bearings and rings are not as good as modern ones. The port timing may not let it rev.

This saw is old enough that its fuel system may not like modern gas with ethanol in it. Anything that's about 25-30 years old or newer will have fuel system materials that can handle ethanol but older than that it might not unless someone updated it.

ericm979 hit the nail on the head with this post. The 20:1 requirement was what it took to get reliable lubrication with the oils from that era. I don't personally own and equipment that calls for 20:1, but have owned equipment that calls for 32:1. I run it at 50:1 on good quality modern oil with absolutely no problem. My former brother-in-law who runs an old saw which calls for 20:1. He runs 40:1 or 50:1 all the time in it with no problems. A friend who collects chainsaws runs everything he owns at 40:1. (Even he admit to running 50:1 without any problems, he just runs 40:1 because as he says "old habits die hard".

I would definitely be running non-ethanol gas in the older equipment, since the ethanol attacks older rubber compounds (modern saws have changed the formulation to something more ethanol tolerant).

The key in all cases is to have your saw tuned for the mixture you are running. Madsens has a great page on tuning saws by ear. It includes a sound file so you can hear what they are talking about when tuning. You'll need good speakers (better than what most laptops have) or a quality pair of headphones to hear what is going on: Madsen's Saw Carb Tuning. Note that they give instructions for using a tach as well, but it is not needed (and there are those who argue that properly setting by ear is a better way to go). IF you do check with a tach, note that the example RPMs shown are for more modern saws, and are likely quite a bit higher than what you should be aiming for with your older saw. In the absence of any RPM spec, the best thing to do with an alder saw is to set by ear.

One of the better YouTube videos I've found about tuning saws by ear: How to tune the carburetor on a chainsaw

 

[tude]

Thanks for information on tuning the saw and fuel mixture. I was able to get an electronic copy of the instruction and parts manual for the chainsaw by contacting STIHL USA headquarters. I ordered the chain last Friday and it arrived in the mail Monday. Very quick service.

 

[ericm979]

I thought about this some more. I know motorcycles better than saws so bear with me. On some older vintage two stroke bikes there were no seals on the crank bearings, and the bearings were plain bearings (bushings) not ball bearings like on newer engines. For those engines, the oil in the bushing also seals the crankcase. Saw engines are in some ways quite a ways behind bikes- for example the ports even in the larger pro Stihls are like 1960 or early '70s motorcycles, when they're not plain weird like the super long transfers in their strato saws. So its possible that while motorcycles were beyond the plain mains are the seals stage by the '60s, Stihl might not have been.

Anyhow, with these type of bearings I'd want to run a decent amount of oil, even with modern synthetic oils, to maintain the crank seal. If it was mine I'd look for crank seals to be sure.

If you do run a heavy oil mixture, you can use a synthetic that's low deposit and low ash, like Redline Allsport. For race engines I'd use something else like one of their race oils (or someone else's) but saws, even modern ones, and some motorcycles as well, do not stress their engines that much and benefit from the low deposits. Allsport also makes the exhaust smell good.

 

[tude]

The owners manual tells the operator to use a 25-1 mixture of SAE 30 oil and non high test gas after 50 hours of operation. It also tells you that 16-1 won't hurt the saw. View attachment S10 Diagram.pdf

 

[ericm979]

That diagram clearly shows real crank seals and ball bearing mains. So my concern about plain bearing mains with no seals is unwarranted. SAE30 four stroke oil from the 60s is a far cry from modern synthetic two stroke oils.

 

[Popgadget]

2 stroke oil ratio is really a function of the oil used, not the engine.
Keep in mind that there is no spec for two cycle oil other than the boating industry’s BIA certification. This really doesn’t apply to high performance air cooled engines that run at much higher crankcase temps than a water cooled outboard.
For this reason, stick with an oil formulated for use in a chainsaw like those sold by Stijl, Echo, etc. Obviously, they don’t manufacture the oil, but they do set the specifications to meet the specific needs of a chainsaw engine.