For a given Cruise Setting: Manifold set / RPM set / Mixture set according to P.O.H. values: what happens to the Mixture if you were to reduce power or increase Power by varying throttle? Does a change in power setting always require a change in mixture to maintain desired stoechiometric ratio (e.g.: best power)? Why? I somehow know it is the case but cannot figure out why. Thanks
All things equal, the throttle setting has no meaningful impact on the stoichiometry of the fuel-air mixture, it just changes the volume of that mixture that's allowed to flow into the cylinders.
There are reasons to enrich the mixture beyond stoichiometric efficiency that have to do with assisting the engine with managing its temperature, but this isn't about power, it's about not breaking the engine - allowing uncombusted fuel to exit the engine, carrying heat with it (fuel has more density, and is therefore a useful thermal mass).
Under cruise conditions, however, if you increase throttle, you're going to gain altitude (unless you re-trim for a higher speed). The reverse if you cut throttle. Fuel-Air-Mixture is, as you observe, stoichiometric but higher altitude air is lower pressure and therefore lower density - requiring fewer molecules of fuel to achieve stoichiometric equilibrium.
So if you reduce power, you're going to need to enrich the mixture sooner or later, because you're now descending, not because you reduced power per se.
1$\begingroup$ So the bottom line is that “No” a mixture adjustment is not required when power is changed, it it is often desirable. $\endgroup$– JimApr 16, 2022 at 15:23