In my old but good elna sewing machine thread got stuck in bobbin case and it is jammed

The following is an excellent video on how to check tension. She shows how to check bobbin tension and top thread tension (presser foot down). Then how to adjust top thread tension to get the stich to sit at mid fabric.

https://youtu.be/YF7EOCvpHLg


Thread nesting on the bottom side is almost always top tension too low or top thread is not in the tension discs (when foot down) but can also be top thread not feeding evenly, or a burr on either the bobbin case or hook race or debris in the hook race, or needle hook interference.

Always be sure to thread your machine with presser foot up. With foot up, the tension discs are separated to accept the thread.

Dont know what type of fabric your curtains are. You might try test sewing with a different tight weave cotton fabric just to see if it you can get it to work.

The tension dial is on the front of the machine right above the needle. The higher the number, the tighter the tension is applied to the top thread. HINT: When threading the top thread, ALWAYS RAISE the presser foot throughout the entire threading process. This allows the thread to seat completely in the tension disk. Tension is correct when the top and bobbin threads meet in the middle of the fabric.

Elna Lotus Instruction Booklet

Elna Lotus SP bobbin case repair Removing cotton jam from Elna Sewing...

Elna SP ST SU sewing machine repair tips (I use liquid Tri-Flow Synthetic Lube or silicone spray to loosen parts seized from old oil. Do NOT use WD-40, cooking oil, motor oil...)

Probably unevenly wound on the bobbin. Or tension needing adjusting, be careful to note where tension spring started before adjusting and do in very small increments.
Perhaps the spring has some fluff stuck on it and thread cannot slide through easily. Take out bobbin and clean

try changing the needle, make sure the machine is threaded correctly,using every thread guide. make sure the top thread hasn't come off the thread take up lever, try a different bobbin, could also be a timing problem

Had a similar problem with our Pfaff. Needed a replacement cam. broken gear.

If the bobbin case rotates freely it is probable that the pinion gear is broken. Replace the gear, retime machine and voila it works again. Trouble is tough to find the gear anymore good luck it is a good machine and worth the effort

try changing the needle
try rethreading the machine top & bobbin
make sure the bobbin is inserted correctly into the bobbin case, some
bobbins only go one way.
clean under the needle plate & in the bobbin area for lint & threads

have you got the manual for this machine? You can download one from here: http://www.ps2netdrivers.net/manual/elna.2005/

It is a front loading bobbin and removabel bobbin case so I think the bobbin is a plastic "Class 15" - like this http://www.sewingpartsonline.com/bobbins-class-15-plastic-10-pack.aspx

Thread breaking could be caused several factors such as the thread actually being old and brittle, the needle or needle plate having burrs and catching the thread, incorrect threading, bobbin in bobbin case the wrong way, or top tension discs playing up/too tight or thread catching somewhere in the threading path.

I'd suggest that you trouble shoot this and refer to the manual for cleaning instructions. If you can access them clean between the tension discs (turn tension down to zero and "floss" between the by running the selvage edge of a piece of clean cotton scrap through the discs to remove any lint/dye buildup). Wind a fresh bobbin and rethread referring to the manual to ensure the bobbin is turning the right way. New needle and rethread top thread refering to threading diagram. Tension on 5 and test sew again.

If you are still having problems, then the machine may benefit from a service by your local Elna dealer, a good service and clean at least every 2 years is recommended.

When you drop the bobbin into the case, there is a slot that the thread will slide in to. Gently continue pulling on it and it will feed itself through and come out at a small cut out. Then place the thread over the bobbin and rotate the handwheel to pick up the thread. If the bobbin case is not threaded properly, the threads will knot up and jam the machine. If you'll look closely at the bobbin case, you will see that there are actually two thin pieces of metal...the thread will actually slide between those two pieces.