The big problem here is that it appeared that he tested these mixture changes all in one day.... the density-altitude (DA) likely didn't change too much. Nor was he using a large-displacement engine.... likely around only 30cc. The bigger the displacement and power-output, the harder it is to cool the engine properly. A proper F/A mixture helps cool the piston and helps to keep the piston from expanding too fast. If now the F/A mixture is lean because of a decrease in F/O mixture ratio, the additional heat generated due to a leaner condition is going to be harder to cope with by the piston and cylinder. In minor leaning, you'll see piston discoloration occuring from burnt oil sitting on hot piston surfaces -- top, sides, and bottom, and of course rings. With far-too-lean mixtures, you're looking at damaged pistons and cylinders.
If we're talking about small, 30+-cc engines, the piston deck's surface area is small compared to the piston-skirt/ring to cylinder surface area ratio. Remember elementary Geometry? Area of a circle = (Pi)r^2; Circumference of a circle = 2r(Pi), where r is the bore radius. Of course the skirt and rings have area, so let's say the side area of cylinder-wall contact is h*2r(Pi), where h would be the height of skirt and ring area that would be associated with heat transfer to the cylinder walls -- mostly surrounding the ring areas and not much below. So, the piston-deck area to heat-transfer area ratio would be r^2 / h*2r = r/2h. You can see that as the displacement (bore) increases, so does the ratio increase, reducing the amount of heat transferred to the cylinder via the rings and a small part of the piston sides. Obviously h increases as well with displacement, but engineers try to keep reciprocating masses to the absolute minimum, especially ring mass (preventing ring-flutter) so h likely does not increase linearly along with an increase of r.
As the displacement of the engine increases, the piston-deck to piston/cylinder contact area ratio also increases, thus the transfer of heat from the piston deck to the cylinder is retarded compared to the smaller displacement engines, and the likelihood of minor piston seizure increases with a non-optimum carb tune. Larger engines demand a more accurate F/A ratio to help keep internal heat under control. And that applies for when an operator changes the Fuel/oil mixture ratios, but even more so when the DA changes.
In my area in the foothills of Appalachia (NC), my general altitude is 1100' msl. However, the DA can be anywhere from (-)100' msl in the winter, through nearly 6000' msl on hot, humid summer days. That's if I am at my shop. If I go down to the ravine, does 150' elevation change mean anything? Not too much. But If I go to the coast, then yes, gotta re-tune. If I go up to Saueratown Mountain, then yes, gotta re-tune. If the last time (say yesterday) I re-tuned the saw the DA was at 1500'msl and today it's at 3500' msl, and I'm operating at the same physical altitude, then yes, I gotta re-tune. And yes, the DA can change that much within 24 hours or less. In fact, it can change in a matter of a couple hours if a large cold- or large warm-front blows through the area.
My point - re-tune your carbs when *anything* changes! It's not rocket science. It's damn easy and quick once you've done it a couple times.
PS. Ever run out of fuel with a small-displacement 2-stroke engine (30+_cc)? What happens? The rpm's don't really do anything but slow down to a stop. Anyone operating a 'big-displacement' (90+cc) 2-stroke knows that when the fuel tank runs dry, the engine starts to scream uncontrollably due to a very lean F/A condition. At least mine do. All the more reason that his testing on a small-displacement engine doesn't really prove a point, except perhaps in a small, careless way for small-displacement engines.
That’s a sensible approach! I generally tweak anything I use each time I take it out.
