I have been told, and my manual says the same thing, that the knob under the seat is for locking the the 3PH in position. That is it's only function. There is a lever near it that is for controlling the lowering speed, but it doesn't have anything to do with raising the 3PH.. Moving the lever that controls the 3PH slower is not the answer. No matter how slowly the lever is moved the 3PH raises very fast. So even tiny increments of lever movement results in very fast rising of the 3PH. Even if it is a very small amount. This is why it seems to be jerking. I barely move the lever and the 3PH rises quite fast. I cannot raise the 3PH smoothly. I guess I will try closing the valve with the knob so I know it is bottomed out and then open it a little and then operate the 3PH to see what happens. I have been warned not to do this because it might dead head the hydraulic pump. Which may then break stuff. But I gotta try something. I am having a hard time believing that Yanmar did such a good job designing and building the rest of my tractor but neglected the 3PH.
Eric