If it is broken then you risk major engine damage by continuing to use the engine. That should bring up afew videos which will explain exactly how it works and how you adjust it.

SO the shaft moves which moves the lever with moves the wire rod which moves the butterfly. The governor arm is clamped onto this shaft. In order to check the oil level you need to remove the vent plug (28 in the attached diagram). Consequently they did not make the transaxle easy to do maintenance on either. There are very limited repair parts for it and Deere suggests that it be replaced when drive components fail. The shaft, called a governor shaft pokes out of the engine. The transaxle in your L120 was not meant to be serviceable. Inside the engine are some bob weights which rotate a shaft as centrifugal force causes them to be flung out. The governor and the throttle are connected together by a spring, called the governor spring and the strength of this spring determines how fast the engine spins. Our online catalog of John Deere D120 Lawn and Garden tractor parts allows you shop shop by. You can also enter your part number into our search bar. Or, choose from the many popular options below. Shop our illustrated catalog for D120 parts here. Working against it is the lever you move which I will call the throttle. Green Farm Parts has all available John Deere D120 Lawn and Garden tractor parts online. The faster the engine spins, the harder it closes down the butterfly, How much it closes the butterfly is determined by how fast the engine is spinning. The governor slows the engine down by closing the throttle ( which from here on in I will call the butterfly to avoid confusion ) The thing you are having trouble with is the governor.

John deere l120 parts diagram