A very nice lady gave me a Viking 6030 sewing machine (as well as this serger).
It had what is said to be a typical problem for this make and model: the lubricating oil had solidified into a wax- or plastic-like consistency, and the upper main shaft, many of the controls, and other mechanisms were frozen. The only way to repair this is to completely disassemble the machine, clean out the old oil, and reassemble it with new oil. After such a complete rebuild, all of the normal service adjustments must be made.
This post documents the rebuild process with photos and provides documentation. When done, your machine may be able to do this: