The feed dogs may be worn out (look at them, are they sharp or rounded?)
OR the presser foot needs a pressure adjustment. It's not holding the fabric on the dogs, so it's not feeding
OR you have both upper and bobbin tensions too tight.

Test the pressure adjustment first - it's more likely to be the problem.

Try adjusting the knob at the top, the one labelled "pressure regulating thumb screw" at the upper right of this picture. Be VERY careful to not unscrew it all the way - it's a hassle to get back on. I don't remember which way is tighter ... probably clockwise as you look down on it.

Use a medium weight cloth (a single layer of denim works well) and no thread or bobbin. Adjust in small increments until the test fabric feeds smoothly. Then thread the machine and see how it flows. Adjust your bobbin tension and upper tension next, then you should be good to go.

Pressure seldom needs adjusting after that unless you are using really THICK (loosen it) or flimsy fabric (tighten it).

http://www.singersewinginfo.co.uk/66/

How are you doing?

Have you unscrewed the feed plate and checked the feed dogs? Some times a lot of lent or treads can gather there and mess up. Some dismanteling is needed as the years go by. It's an easy disassembly and assembly luckily.

The 66 model handles most fabrics from denim to fine silk. I'll bet it has to do with correct threading or other setup of the machine, these models are very often hardly worn in the mechanical parts, but needs a lot of cleaning and oiling. If the paint looks nice, it will most likely all fine mechanically. Even worn machines is usually easy to get going again. Is it a back clamper or a side clamper?

As lazygardens also mentions, I would first check correct threading and thread tension.

This post was edited by taoseeker on Sat, Oct 11, 14 at 19:04