Why do you tune a saw to “just at 4 stroke”?

[TnShooter]

Hey there back again with another question.
First I will start by saying this is NOT a “How to tune” question.

My question is
why is it that you want to tune a saw to “just starting to 4 stroke“. ( the sound )
With most other 2 stroke equipment like blowers and trimmers, I was taught you should find the medium between Lean and Rich. (Going by sound)
Set it in the middle, and then turn a it slightly more towards Rich. But NOT to the point of 4 stroke. ( I always tend to go rich, just to be safe)
Why is it that you tune a saw to Rich or “just as it starts to 4 stroke”?
Yes, I do this. But have never understood the actual reason for it?

Thanks for try to help me understand this.

 

[RED-85-Z51]

A blower, trimmer, hedger...has a constant load, the resistance it sees will generally be consistent for the majority of its life...and these units are geared less towards all out power...vs as much power as they need to make.

A saw...can see a variety of loads minute to minute, from a 4" limb to 25" buried. A saw is built more for power, its more aggressive because it needs to be. Much ths same way that the v8 in a Tundra isnt designed to operate the same as the v8 in a corvette.

4 stroking out of the cut, no load...is safe, it should just clean up when a load is applied, if it cleans up out of the cut its lean.

If you tune a blower, you make it rich...and lean it out until it peaks, then a bit rich, you are "cleaning it up" because its under load already. Same with a trimmer, feed the line...rev it wide open, richen it up then lean it in til it peaks...then back off a hair rich.



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[TnShooter]

I do understand the the differences.
My thinking is ( and likely wrong) is that the RPM would DROP in the cut.
And make make the saw run even Richer. Obviously it doesn’t as it does indeed clean up in the cut.
I never get it exactly right the first time, and usually have it set just a bit rich. As it burbles a bit in the cut.
Again, this is on a fresh tune. Starting from scratch.

If I’m understanding you, the saw needs the extra fuel in the cut to make “power”. (Under load)
Where as as outside the cut, when it doesn’t need to make power, it doesn’t need the extra fuel (not under load). Correct?

Thanks for the help.

I am the curious type.

 

[drf255]

Great question.

It comes down to physics.

When you are piss revving a saw, the 4 stroke sound just gives you some type of safety zone to tune a saw unloaded so that you are close to an appropriate loaded good stoichiometric ratio. It’s a good starting point. For peak power in the cut, many tune in the wood, much the way one needs to do with saws with limited coils.

When you rev a saw unloaded, you are not transferring much energy from the fuel to anything, only some heat and thermal energy-the cylinder isn’t that hot. Once you put the chain into wood, you are turning potential energy into kinetic energy and thermal energy is also produced. Cylinder temps go up almost instantly and change the needed Stoichiometric ratio to something richer than at no-load. This is why a saw will stop 4 stroking out of a long cut at piss rev, then 4 stroke again once it cools down. It’s all physics, and it needs to be taken into account when increasing saw performance.