To simplify matters, the governor and the governor spring are two opposing forces that want to reach equilibrium.
Let's say the engine is idling and you set the throttle at about halfway between idle and full speed. When you move the throttle lever, it pulls on the governor spring which is attached to the governor arm. The governor arm then moves toward the spring because you have upset the balance by pulling on the spring. Also attached to the governor arm is a rod that connects to the throttle plate on the carburetor. When the governor arm moves toward the spring, it also opens the throttle which should increase the engine speed. As the engine increases in speed, the force of the governor pulling against the spring increases until they are once again in balance.
Adding load to the engine causes the engine speed to want to decrease. A slight reduction in speed causes the governor arm to move toward the spring again and in turn open the throttle more until the load decreases and engine speed comes back up to the point of equilibrium with the governor spring. The opposite is also true - if load is decreased, engine speed tends to increase and the governor arm will move to close the throttle plate on the carburetor until the forces between the governor arm and spring are in balance once again.
It gets a little more complicated when you consider the geometries involved and the relationship between deviation from target rpm and proportional throttle response. A lawn tractor governor is set up to operate over a wide range of engine speeds while maintaining reasonable control over engine speed. In contrast, a governor on a generator only needs to work within a very small range of engine speeds but must maintain very tight control over that speed so as to maintain proper AC frequency.
The geometry of the generator governor force curve, spring rate, and all its associated linkages and ratios are set up to react with an aggressive amount of throttle response to only minor changes in engine rpm. The tractor governor is designed to have a much more linear response to changing speeds and over a wider range of throttle settings.
Mostly what has already been explained, but maybe in a different way.
Edited by Oo-v-oO, March 01, 2015 - 10:43 AM.