To set the governor at its proper position, loosen the screw on the governor arm so the governor arm will turn on the governor shaft. Move the governor arm so that it stretches the spring as much as it can. Turn the slotted end of the governor shaft clock wise as far as it will go. While holding the governor arm and stretching the spring, tighten the screw that holds the governor arm to the governor shaft.
Be careful when you start the engine and do not let it over speed. If we turned the governor shaft the wrong direction the engine will run wide open when you start it. If this happens go back through the adjustment and turn the governor counter clock wise this time.
If you need more information send another question and include the model number of the engine or the mower. Also include any pertinent information concerning exactly what the engine is doing and when it started.
It is possible you have a broken governor gear inside the engine and you won't be able to adjust it.