Well, I seem to have this under control now! I switched to a #24 round point regular ole needle as advised by you and Steve. When I removed the #25 diamond point I noticed it wasn't squarely in...it was turned a little cock-eyed. I think I need new glasses! That twist must have been binding the thread enough to throw everything off. But with the new needle in (correctly!!) and the bobbin tension set back to 8 ounces everything is looking really good. After a little fiddling with the top tension I'm getting a nicely centred lock-stitch. The #138 thread is tight and buried just a little into the urethane coating without cutting it...top and bottom. I don't even need to change tension when switching to heavier material. Life just got easier and sweeter! And no lubrication. This is an excellent machine. You have to keep a close eye on tension at the start of each job with biothane. I just sent back a couple halters from another sewist doing sub-contract work for me. The design had a black underlay with a lime-green overlay. He had black bobbin thread and green top thread which should have looked good...except...the top tension was too high and the bobbin thread was poking through here and there. Looked like it was covered in fly dung or dirt. So back they went. This guy will learn just as I have but sometimes it's a costly lesson. Thanks for the advice Leatherworker.
This is my first post. Thanks for a having this great forum. I just installed a Cobra Class 4 (factory setup with #277 nylon). Great machine Steve! It sews thick leather like nobodies business. Problem is, I need to sew mostly BioThane with #138 braided nylon thread since I'm making endurance horse tack. I filled a bobbin and tried a test piece with the factory tension. The tension was so high the top thread puckered a lot and sometimes cut right through the urethane coating. So I started backing off the tension. This turned into quite the exercise. The bobbin tension was a big part of the problem. The factory setting seemed to be about 8 ounces on the bobbin and I have backed it down to 6.5 ounces. I may go down even further since this is starting to give me good results. I also removed ALL the the top tension and gradually dialed it in until I had a lock stitch centered in the work piece. I'm doing my test trials with a black bobbin thread and white top thread so I can see what's going on. It is feeding well and I am getting a very tight stitch but it isn't puckering or cutting through anymore. It improved again when I switched out the #24 sharp needle for a #25 diamond point. I thought the #24 would be plenty big enough...live and learn. Does anyone else have expereince or tips for working biothane on this machine? Threads? Needles?