The linkage rod which crosses over the valve cover is supported by a stand on the injection pump side of the engine. The stand is held in place with two "self centering" nuts. These are also known as ball end nuts #127 990 01 51. The curved "ball end" centers itself in a pocket when tightened. In this way the linkage support is exactly and precisely attached in the exact same place as before. If standard nuts are used instead, The linkage may not return to its correct position , throwing the fuel mixture out of adjustment. Often times these are lost during repairs and replaced with standard nuts. In addition often times these special nuts are installed backwards (ball end out), also causing improper alignment. I'm sure someone has some good photos ?