I have a Nelco L600 ,I need a carrying case where I can find another one exactly like my old case for my machine?

Sorry free I could not find free manual. Paid version is available at this link
Nelco Sierra 234F manual sewing machine instructions Enlarged Hard Copy - Sewing Manuals & Instruction (bonanza.com)

You probably can, however, if I had this nice Bernina, I wouldn't risk it. Instead, go find an old metal, mechanical sewing machine, like a Nelco, Singer, Kenmore. I have an old metal Nelco and a Pfaff 260 that I use to mend horse equipment.

1. http://www.sewusa.com/index.htm try finding it on this website. maybe you will get lucky.

As far as I am aware, Nelco hasn't been made for quite a few years--probably since sometime in the 1970's is my guess. I have an old Nelco sewing machine that is a tank of a machine and sews nearly anything I throw at it. But for finer work, I prefer my Bernina--probably because it has more stitch selections, sews a straighter stitch, and I know it better.

If you aren't able to have the machine demonstrated to you and you can test it to your satisfaction, I'd be reluctant (Actually, that goes for any used machine, not just Nelco!) The following may be helpful:

Thread Anybody heard of Nelco sewing machine Worth fixing up

In my experience, Nelco manuals are sometimes difficult to find. You might try:

http://www.sewusa.com/Sewing_Machine_Manuals/Nelco_Sewing_Machines.htm

Nelco manual Sewing Machines

nelco sewing pdf 1 Docs Engine com

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To find a user's manual, search the internet for:
Manufacturer + Model + the word "Manual"

Search YouTube videos for threading tutorials:

Nelco Zigzag Sewing Machine Final Test Demonstration Video

VINTAGE ALL METAL NELCO MODEL 400 2 SEWING MACHINE

NIFTYTHRIFTYGIRL Vintage Nelco model 250 sewing machine

With your Nelco, be sure that you install the needle properly. Frequently, Nelco machines take the needle with the flat side facing toward the throat of the machine--not toward the read like most other machines. The upper thread will thread through the needle's eye from the left toward the right. They also are a left needle position machine--meaning, the standard position of the needle when sewing straight stitch is at the left side of the presser foot, not center like most other machines.

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I saw Nelco on this website http://www.sewusa.com/Sewing_Machine_Manuals/Nelco_Sewing_Machines.htm

purchasing /obtaining a manual is always very helpful when working with a sewing machine.

However, simple stuff to check, is the needle in the right way, with the scarf (groove) to the back?

Is the needle new and the right type of needle for this machine? Sorry, I don't know a lot about Nelco however, most machines use 130/705H system needles, but there is always the exception, especially with something 40+ years old so it pays to check it out.

And lastly timing, the needle needs to be down at the lowest point as the rotary hook comes around to catch the thread. If this isn't happening, then it could be that the timing is out on the machine. It's never happened to me but it can happen. But I'd suggest you try changing needle, check threading path and bobbin firstly.

Good luck.

Try contacting www.tias.com, they carry a lot of old sewing machine manuals. They may be able to find one for you.