Why do you have to lean?
For a little open discussion... why do you have to lean the mixture when you increase in altitude?
Most people know the basic operation of a intake system where the air being sucked in by the pistons will create a partial vacuum and suck in a certain amount of fuel. When the density of the air decreases, the suction acting on the fuel will decrease and less fuel will be mixed with the air. But does the decrease in density/pressure cause the fuel coming in to decrease so that the fuel/air ratio remains constant? Is it because we're creating less power now that we don't need the extra fuel for detonation protection?