When the engine is not running, what is the only force on the throttle shaft?

When the engine is not running, the only force acting on the throttle shaft is the governor spring. This component is designed to control engine speed and, in the absence of engine operation, it is the primary force that maintains the throttle's position. The governor spring ensures that the throttle returns to a default position when not influenced by other forces, such as those created by engine vacuum or operator input.

The throttle cable, which connects the throttle to the operator's controls, generally would not exert force unless the operator is actively engaging it. Fuel pressure is generated by the fuel system when the engine is running, allowing fuel to be delivered to the combustion chamber; it is not a contributing force when the engine is off. Lastly, while the operator's hand can influence the throttle position, it only acts as a force when the operator is physically interacting with the throttle lever. Consequently, out of all these options, the governor spring remains the sole force acting upon the throttle shaft when the engine is not in operation.