I have a Janome 6500 & I use either Connecting Threads cotton or Coats & Clark Dual duty, always the same in needle & bobbin at the same time.. I had to adjust the tension somewhat. I do occassionally have a thread breaking problem & it seems to happen if to much lint gets under the bobbin holder thingy. I am no expert. I will be watching for tips from others, also. I would say I mostly use a size 11 needle.