Governor - Most steam tractors used a fly ball governor to protect against overspeeding the engine, although a few freighting engines relied on the mass of the entire tractor to control speed, like a railroad locomotive. The governor consisted of a number of weighted balls on spring loaded levers that were belted to the crankshaft, so that they would spin at a speed proportional to the engine. Centrifugal force would cause them to move away from the center, and the levers they were mounted on would push down on a valve, closing off the flow of steam to the engine. As soon as the engine slowed down, the springs would move the balls back in towards center, releasing the valve. Adjusting spring tension and the valve stem length allow the operator to set the maximum speed with some degree of precision.
    The action of a governor is always a bit behind the engine, since it has to sense an increase or decrease in speed before it can act. In practice, an engine running at full governor speed has enough momentum to maintain that speed under all but the most variable loads (such as a sawmill operator who likes to ram the log into the blade), so the lag generally isn't much of a problem.
It is generally recommended to use the governor to control the engine, not the throttle. The reasoning behind this seems to be that the governor is fall more effective at maintaining a constant speed, thus saving wear and tear on the engine from sudden shocks.
    Difficulties with the governor generally can be traced back to improper oiling, belt slippage, or packing nut tension. A governor that is not properly oiled will wear or will be slow in acting. Belt slippage can cause the engine to race, and a valve stem packing nut that is too tight can cause either slow action or prevent release.