The maximum power occurs under conditions when the largest amount of fuel can be most rapidly consumed at the proper crank angle. Generally, this occurs nearer stoichiometric conditions and at higher temperatures and often rpms than most engines are able to sustain for long. Therefore, the functional maximum occurs at high fuel flow rates (throttle nearly wide open) and at high rpms, at least up to the point that the governor or other rpm manager limits the conditions to avoid engine damage. 

Do not confuse maximum power with maximum efficiency. The latter occurs at low fuel flows and relatively low rpms.