to keep the mixture stoichiometric i.e. in the correct ratio, as the partial
pressure of oxygen falls with increasing altitude.
First of all, the stoichiometric value cannot be used at high power because the burn is too hot. Neither is it anything to do with oxygen partial pressure.
The power output of a piston engine depends upon the
weight of charge in the cylinders. This in turn depends upon
air density.
As altitude is gained the air density (mass per unit volume) reduces. If the mixture were not leaned then the amount of fuel relative to the amount of air would increase.
Thus the mixture has to be progressively leaned to maintain the required
air/fuel ratio.
When the density of the air decreases, the suction acting on the fuel will
decrease
The depression in the carburettor ("suction" does not exist) venturi depends upon the
velocity of the air flow through it (Bernoulli).
