I’ve had a Juki MO-634DE serger for 20 years and I’m having troubles all at once as it won’t make a chain stitch. I have threaded and re-threaded and I am thinking now it’s the needles. I had it serviced 1 year ago and all was well. I am now thinking it’s my needles, perhaps they are the not seated right or I inserted the wrong ones. Off to the fabric store to buy more today. But the directions say 130/705H and 75/11 thickness. I understand the 130/705H but am wondering if I have to use a 75/11? Wondering why I can’t use a 90/14 for thicker fabrics. I can’t imagine a 75/11 will hold up for thicker fabrics. But since it’s an older serger maybe I need to use a 75/11? I’ve never given much thought to my needles before, always inserting the two of the same needles when I replace them. But before I haul my machine in for servicing again I figure inserting new needles would be a good step. Thoughts?