King Tut is 40 weight. I have 2 bobbin cases for my long arm. One I use for a 60 or 70 wt thin thread, and the other I have set for 40 weight. You will have to adjust your bobbin tension if you have it set for a light weight thread. make teeny tiny adjustments! If you don't have a bobbin tension gauge, this is a good way to check:
https://www.youtube.com/watch?v=Q1mRhcquZTM