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If this is an older mechanical machine (ie, NOT computerized), sounds like it has not been cleaned and oiled in quite some time and the old oil has solidified. DO THIS ONLY IF IT IS A MECHANICAL MACHINE! Consult your owner's manual, get some good quality sewing machine oil (not your grandmother's oil tin, 3-in-1 oil, WD-40, or cooking oil). Start with 1-2 drops of oil each place as directed by the manual. Gently hand-rotate the wheel (do not try to run the motor until the internal mechanics of the machine are moving freely!). To speed things up, direct heated air from a handheld hairdryer into the interior mechanism. You may need to repeat the process every day for a few days before things loosen.
The machine should be oiled every 8 hours of constant sewing, every time a new bobbin is installed, before its use after it has not been used recently, or at least every 6 months if it is in storage.
try rethreading the machine--use EVERY thread guide
make sure the needle is inserted up into the needle holder and turned the right direction
make sure the bobbin is inserted correctly into the bobbin case and turned the right direction
clean under the needle plate and in the bobbin area for loose thread or lint
make sure thread has not come off the thread take up lever
also the jam could have caused the machine to jump time and needs a timing or a adjustment
either a jam up in the bobbin which is preventing the needle bar from coming up, internally siezed up, or the friction belt internally to the flywheel has broken/worn out.
There are a couple of different SU models, the Star Series and the Air Electronics both had an SU (supermatic) model so I'll be general in my description. Firstly remove the needle cover plate and check around the bobbin race for jam ups. Or if the machine has been packed away for a long time unused, then we could assume this is a sieze up. FIRST REMOVE THE POWER PLUG FROM THE MACHINE FOR SAFETY. You'll need to remove the top machine cover and delve into the insides with a bit of patience, good quality sewing machine oil (tri-flow is perfect if you can obtain some, bike shops have it or I got mine from Ebay) and Qtips or something similar.
There should be a couple of screws holding the top cover onto the machine so remove these and then lift and tilt the cover to the back around the lifting handle and making sure you clear the bobbin winder mechanism on the right. Really good idea to take photos with your mobile as you go so you can remember where things went. Now apply triflow to the metal moving parts you can access, in particular the top of the Elnagraph shaft which should have a red mark for oiling point, and the needle bar swing bearing. Keep oil away from the belt/s on the right side of the machine where the flywheel is driven by the motor.
You'll need to carefully clean, oil and gently move the flywheel back and forwards until things free up.
I was sewing a headliner for a car and this happened. It's jammed. Unplug the machine, remove the item being sewn. By hand, give less than quarter turns back and forth on nthe large wheel that holds the belt. Try to work it free. Check inside the bobbin compartment as well for anything that may have jammed in there, it's stuck on something that came off, you just have to find it and gently remove the remnant. Keep working that wheel forward and reverse by hand gently. I hope this helps! It did for me!
Ironically, often times when the bobbin thread is knotting up, it's the top thread that needs the adjustment. Loos top thread thesion will tangle up in the bobbin cassing...Think about it for a minuted. Try a quarter turn to the right on your top tension and then go fro there. There could be a problem with the bobbon casing tension adjust screw and plate. but I doubt it...Nine times out of ten when the bobbon thread is tangles the top thread is too loose.