I have a singer 5040c an sometimes the tensioner

http://www.sewingpartsonline.com/singer-5040-sewing-machine-parts.aspx

SingerOnline com Apollo Bobbin case installation

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My Singer Stylist 9100 has constant problems Check the attached links,instruction and guides, Good luck
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Instruction Manuals

Basic Maintenance Tension Skipped Stitches

Singer 9100 Instruction Manual Thread Tension

Six Common Sewing Machine Problems Solved

Look for screws and snap in tab locations like getting into a laptop or an xbox. you'll need some sort of stiff thin tool to do this. Good luck

if you try to regulate the tension & it doesn't respond then the tension dial may need to be replaced

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Upper, it is your bobbin threading, lower bunching of thread it your top threading . Try this test, with the presser foot up, pull your top thread, with the presser foot down, pull thread again, it should barely move.
It is always the opposite of what you think, so rethread and do the test; Sometimes this little test will seat the thread in the tension discs better. I keep my tension on about 3 for most sewing, my other machines I use auto. I hope this helps you.

Do you have your manual? Because it is almost always the top threading, either tangled thread or a bent or dull needle causing the problem. You can download your manual at www.singerco.com for free and enlarge the detail of the diagrams for threading. I just hope that it is something simple and not your timing because that is not a do it yourself job, unfortunately. You can also call Singer customer service at 1-800-4singer. and get their advice. Good luck.

http://www.sewingonline.co.uk/instructions/singer185k/

We have those bobbins available here:

Singer Sewing Machine Bobbins

I have the same problem. Driving me nuts. The tension dial is broken, as far as I can tell. Just craps out from time to time. Sometimes turning it on and off helps. I personally think it's a problem with the electronic controls.