JUKI MO-634D trouble
Well, you might just need a good oiling. Make sure the machine is not threaded and there are no thread snarls where the loopers interact. It also pays to insert new needles and observe the loopers and needles interact. they should come withn .01-.5mm of touching eachother; there should be no hard positive contact between loopers and needles. So, if its tight to turn over by had, this is usually a cam or bushing that need oiling, or thread that got sucked into the machine and is binding the thing up. What I would do is remove the covers, slip the motor belt off, turn over by hand. If it frees up, the bind is in the motor, if its still tight, then id look for thread, and oil the machine with a light weight oil (sew mach oil, tri-flow... NOT 3in1 oil!). If you have a keen eye, you can wiggle the handwheel and try to observe what parts are moving slightly, and which are not. using that method you may be able to track the bind to a section of the machine or even a specific shaft/cam/component of the machine. Another common bind point in sergers is where the knives mate. Move the top knife to its idle (non cuttin) position and see if this relieves the bind.