My 648 has a broken drive gear so I'm unable to check it for you. Sorry.
Maybe you've already done this. You can take the needle plate off and turn the machine manually to see what's going on inside. Try it in bobbin winding mode and in sewing mode and see what it does. I'd also take the top and bottom covers off and check the condition of the gears. My daughter was sewing happily along when one of the gears just disintegrated. No warning, no nothing. If your gears failed, it could have thrown your timing off. that would cause your problem.
Rodney