Briggs & Stratton is a small engine manufacturer whose engines are commonly found in lawn and garden equipment, including lawnmowers, garden tractors, Rototillers and more. These small engines are protected by a governor that is connected to the throttle. When the engine is under a considerable workload, the governor opens the throttle to compensate. Conversely, if the engine's workload lightens, the governor closes the throttle slightly. If the engine is running roughly, you may want to adjust the governor on a Briggs & Stratton 12 HP engine.
Pull on the spark plug wire to disconnect it from the spark plug.
Inspect the governor linkage and spring between the governor arm and the throttle on the carburettor. If the linkage is broken or the spring is overly stretched, replace them with Briggs & Stratton parts.
Rotate the governor arm back and forth and to the left and the right. The arm should move easily, with the linkage opening and closing the throttle as it moves.
Loosen the nut at the base of the governor arm one full turn with an open-end wrench.
Open the throttle linkage so that the throttle is in the fully open position. Notice which direction the governor arm moves as you open the throttle. Push the governor arm by hand as far as it will go in this direction, and tighten the nut at the base of the governor arm with a wrench.
Release the throttle control, and recheck the movement of the arm, as you did in step 3.
Reconnect the spark plug wire, start the engine and let the engine warm up for a few minutes. Put the engine under a relatively heavy workload, and then lighten the workload. The engine should adjust and run smoothly under either circumstance.