I messed with a 15 clone today. That darn thing ran like the wind for about a half a second after I put a motor on it. Then it locked up. I could hear a clack in the bobbin area so I pulled it out. Then I oiled it everywhere again. Then I ran the machine... ran freely.... I replaced the bobbin works and it bound up again. Then I swapped the shuttle etc out of another machine and it ran a quieter. I didn't really have time to explore more. What I wonder about your machine is how it runs with out the bobbin case in it/out. Check with your service manual to be sure it is in right. Some times people 'adjust' things in that area and really mess them up. If the bobbin carrier isn't around that little hook just so it will bind up.