Mock safety stitch

Your Singer is a sewing machine not a serger, so it does not do a true "overlock" stitch. However, you can simulate an overlock by simply stitching so the thread zig-zags over the raw edge of the fabric.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwi4hNr1v4rbAhXM5YMKHUY_A-AQFggnMAA&url=https%3A%2F%2Fwww.singer.com.tr%2Fassets%2Fimg%2Fuploads%2Fproduct-files%2Fb753376e9a5f9ebda5b0ecf6d748166d_en.pdf&usg=AOvVaw30nIM561-VBdzOKlzyhhn8

One needle does sit slightly higher than the other. The stitches not catching sounds like a tension or threading problem.

Rolled hemming on any overlocker is usually achived the following way. Right hand needle only.
Blade over to the right usually to cut wider than normal but you can vary this to suit the fabric and stitch width you want.
Thread in this needle and the upper and lower loopers. If you can source any, use woolly overlock thread in the upper looper, this fluffs out when not tensioned and covers the fabric to give the look of solid stitching. Change upper looper tension to about 2 and lower looper tension to about 6 or 7.

The lever by the plate is usually to control the little stitch finger where the stitches are formed, move it backwards and it should move back towards you?? If this happens, then you want it in the retracted (towards you) position for rolled hemming so the stitching is much smaller and the fabric can roll. Normally this stitch finger holds the fabric firm for the loopers to form the stitch over for your normal 3 or 4 thread overlock.

Now test stitch and see how it looks. Tighten the lower looper thread so it lays right beside the needle on the underside. You may need to then tighten or loosen more the upper looper, you want the upper thread to wrap all the way around to the underneath against the lower looper thread and needle.

The cut edge of fabric should roll to underneath inside your seam. Once you've got this happening, turn the stitch length down to 0.5 or so to close it right up tight. This uses heaps of thread so I usually test everything else, then close it up at the end to minimise waste.

Hope this makes sense, good luck.

A rolled hem on any overlocker is formed by using the right hand needle only, and the two loopers to form a narrow three thread seam. You also need to retract the seam width finger as Trial2962 said. This is a little finger that sits on the stitching plate and the loopers form the stitching over it. By retracting it, the looper threads can form a narrow seam and in fact, roll the fabric inside the seam. You need to move the cutting blade as far right as it will go so you are trimming as wide as possible from the needle, turn the tension on the top looper rigth down to 2 so it is very loose and this thread wraps around to the underside. And tighten the lower looper tension to about 7 so it hardly shows and sits right up against the needle thread. Adjust and test until you get the seam looking like this, then turn the stitch length dial (on the right side by flywheel) down to 0.5 to close the seam right up tight. Lastly, if you can source it in the right colour, buy woolly overlock thread and run this through the upper looper only, this thread is fluffy and when not under tension, relaxes and "fills" out so the seam appears like a continuous coverage over the fabric.

You will need to practice a few runs and go slowly on corners, a curve is obviously much easier to serge than a right hand cover so if you can, cut the fabric with curved ends, much easier to get a great finish. When you chain off at the end, you need to unravel the tail threads and pull them inside the seam for a few mm with a needle to get a smooth finish, then trim the tail and seal with a drop of Fray Stop.

I would suggest that you buy a new manual for this as your overlocker has 5 threads that need to be threaded up correctly to form a stitch and there are several different stitches that can be made with this model such as a rolled hem, chain stitch, three thread overlock, four thread overlock and safety stitch with three thread overlock, lots to learn.

You can buy a manual here for $USD14.99: http://www.sewingpartsonline.com/instruction-manual-elna-pro5dc.aspx
or this website has a copy for $USD10.
http://www.sewusa.com/Sewing_Machine_Manuals/Elna_Manuals/Pro_4DC_DE_Instruction_Manual.htm

Always well worth buying the manual and saving a lot of frustration trying to guess how things work and it adds value if you are going to sell the item.

You can get some general overlocker threading tips from http://sewing.about.com/od/sergersoverlock/ss/serger.htm which may help you too.

Yes, it can. What do you have your differential set at? Try it in the middle and see if it makes a difference.1 is usually for overedging, 2 is usually for easing in a sleeve, etc. I have my tensions set at 4,4,4,4, stitch length at 3 to 4. Do not be afraid to try gradually changing the tensions, but do them one at a time, that is what I did and finally got it.

Sergers thread upper looper first, lower looper second, right needle third and left needle last in that order. If your thread breaks, CUT ALL THREADS and start from scratch or you will not get a good stitch or your machine may jam. It's probably just a "user error."
Good luck.

I think that you need to go to their site, www.singerco.com and look under sewing resources, they have great diagrams for threading sergers, then you can enlarge the diagrams and it will make sense to you. At least, it did help me understand it better than looking at my manual. Good luck

possible causes. not threaded correctly, actual probelm with tension, burr on neelde plate presser foot or looper. check to see if after you thread that particular tension if there is any resistance on the thread after it passs thru tension, if not see if thread or lint is in tension disc. clean out it present then try again.

I have a Juki MO 2516N. It does not have the capability to serge 4 threads. The only options are 2, 3, and 5.