How do you change the needle position

A hem gauge foot is adjustable and can aid with small seams

If you mean you want to close up the density of the stitching then you change the setting on the stitch length knob which would be on the right side of the machine near to the flywheel. However, if you mean you want to make a narrower stitch width wise, then this is achieved by several adjustments. Firstly, if it s a 4 thread, then unthread the left hand needle so it is only making a seam with three threads, the upper and lower loopers and right needle. Test this seam and see if it is narrow enough.

If you want it narrower, then you need to move the position of the lower cutting blade to the left so the machine is trimming closer to the needle. Test again. If the stitches are now hanging off the cut edge a bit, then tighten up the upper and lower looper thread tensions by a half number and test again. You want the threads to meet right on the cut edge so if the top looper is rolling under to the bottom then tighten it up some more (higher number) in small increments, test stitching after every adjustment.

I hope this makes sense to you. You should be able to download a manual from the Singer website, link is http://www.singerco.com/accessories/instruction-manuals then type in just the number of your model and click "search" give the page time to build, then click on the "free download" button to bring up the pdf file, again give it time to load, its a large file. Then save to your PC.

I can help you with your 334D as I have the same machine; there is several ways to adjust the stitching width but firstly check the stitching finger position is forward not back for rolled hemming. There is a little lever that is on the side of the cutting plate just in front of the foot and feed dogs, you need to push this forward; this inserts a stitching finger into the needle plate and the overlock seam forms around it. When you pull it back the finger retracts to form rolled hemming where you want the fabric to roll under. So hopefully this is all that is happening now with yours but I'll go through the whole process for clarity and others.

This is how I set up for a new fabric project. Turn the tension dials for all four threads to 5. Your needle tensions should not need to be changed often; perhaps just if from the right side of a seam you can see the needle stitch, you may tighten it fractionally. Stitch length knob on the side by the flywheel should be on about 2 and a half and the differential knob on zero also.

Now you want to adjust the cutting blade position firstly to get the width of seam best suited to the weight and type of fabric you are going to seam. If you are going to use the overlocker to join two pieces of fabric in a 4 thread seam to assemble a garment, then always test with two layers too. And if you are going to just neaten a single layer seam allowance, then test on just a single layer and set up for that.

I'm assuming you are seaming 4 thread but the comments are the same for the 3 thread, you just take out the left or right hand needle first or just cut the thread to that needle and leave it in place to get the 3 thread stitch. (Obviously if you are seaming something delicate like silk, take the unused needle out as it will leave holes in your fabric. Be careful when changing needles, use tweezers to hold the needle right up into the housing and never unscrew the screw too far, just enough to get the needles in and out. And always manually turn the flywheel towards you through a stitch sequence afterwards to make sure you've got the needles clearing the loopers, if you hear them touching, recheck needles are fully into housing.)

If you are seaming something light then you'll want a narrower seam. Have you moved your blade before? If not, you need to turn the needle to the highest point, open the front cover and raise the pressure foot and swing it to the side so you can see better. Now put pressure on the upper cutting blade towards the right to release the spring on it, then swing it upwards out of the cutting position. Adjust the position of the lower fixed blade by turning the knob that is nearby to it. You'll see the blade moving over. Once you get used to making this change, you can just put pressure onto the upper blade to release it with one hand, and turn the knob to move the bottom blade with the other.

If you move it towards the right it will trim further away from your needles giving you a wider seam and if you move towards the left you'll get a narrower seam, ie it cuts closer to the needles. So set it to the position you want depending on what you are seaming. Now relower the upper blade into position, swing pressure foot back into place and test stitch again on fabric offcuts. Take a look at the seam. Is the fabric "tunnelling" inside the seam? if so, release the tensions on the upper and lower loopers by a half increment only. Test again. Repeat process until tunnelling is gone and fabric is sitting flat inside the threads.

If the threads are looping off the edge of the fabric, then increase the tensions on the upper and lower loopers by a half increment only. Test again. Repeat process until the threads are sitting right up against the cut edge, and meeting right on the edge.

That is basically it - please feel free to ask me again for more help is this hasn't been clear or you still struggle with this machine. I presume that you don't have the instruction manual for it so I could scan a page or two if this will help.

This is a really good quality overlocker so don't give up on it; it sounds like you have just never been shown how to use it fully or never had the manual; consider purchasing one from Bernina or online as it is very instructive. If you find that you still struggle with getting the stitch tensions right, its a good idea to spend an hour or two with some calico and thread it up with four different coloured threads like the diagram inside the looper cover, then make stitching samples and adjust until its right. You can make a four thread, three thread wide with left needle, three thread narrow with right needle, 3 thread rolled hem and flat lock. Write down the tension numbers, stitch length and blade position onto the fabric sample and keep these in a note book for future reference. This will help you to remember what you need to change when you want to use that stitch type again.

And my last suggestion, if you can find a good sewing machine dealership or fabric store that runs classes, see if they run one for "introduction to overlockers" - I took a class from the Bernina dealer I bought my machine from back in 1992 and it was fantastic. I've used mine for bridal veils with fishing line, made table cloths, curtains, cushions, sewed dozens of knit garments for the kids and made most of my own clothes with my trusty Elna and this overlocker; its a very strong machine and better quality than many others on the market.

Have you used an overlocker before? If not, I'd suggest that you obtain a user manual for it as they are completely different to a sewing machine and at some point you will need to rethread it. You can purchase one from here
http://pages.sewing-machine-manuals.com/173/PictPage/3923709707.html

This machine makes three and four thread seams, which is a combination of two needles and two loopers. For the three thread seam you can use either the left or right needle which gives you different sized seams. For the four thread you use both needles, one goes through the middle of the seam and the other is on the left side securing the seam. On most overlockers you are able to move the cutting blade too and this will reduce or increase the amount of fabric being trimmed from the right side.

This page http://sewing.about.com/od/sergersoverlock/ss/serger.htm will give you some general information on sergers/overlockers too.

So adjusting tension is a matter of changing the tension on the top and lower looper threads to close up against the cut edge, or moving the blade over to make the seam a little wider or narrower. You may also need to finess the left needle tension so that the seam doesn't pull apart, I always test serge two layers and then pull open from the right side and see if the needle thread is showing (it will show a little bit but you dont want it so loose that it pulls open. Only tighten the needles in very small increments though, as you'll break threads and needles if you turn them up too tight.

Hard to explain without seeing what your machine is seaming now. But as a rule of thumb, start with all four tension dials (or 3 if doing a three thread seam) on 5. If the tension dials are correctly calibrated then this should give you a pretty good seam. You may wish to lengthen the stitches, I usually work with stitch length of 2.5 (same as your sewing machine stitch length) Stitch length will be a dial on the right side beside the flywheel or in front of the blade area. The blade adjustment should be a little knob near the blade area too. There will be a lower fixed blade and a moving upper blade which sits against the lower one, they work like scissors to trim the fabric. So to adjust the blade, you will need to take the pressure off the upper blade, then wind the knob to move it left or right.

Different weights of fabric will behave differently on the overlocker so you do need to adjust tension for each new project. For example, if seaming a jersey knit you'd use a four thread seam, this gives elasticity and strength and you can join two garment pieces with this seam. You'd probably make a 6mm or 7mm wide seam, the left needle will secure the fabric and the loopers will encase the fabric smoothly while the right needle secures them and gives extra strenth to the seam.

But on organza for example, I would make a very narrow three thread seam using the right needle as the organza will roll inside the overlocking if you cut the fabric too wide.

And on something like curtains I would neaten the edge with a wide 3 thread seam using the left needle to get a very wide seam, the fabric wont roll so you'd need to loosen the two loopers a little to smoothly encase the cut edge.
For example, on this image above you can see that the looper threads are laying a little off the edge of the fabric in places, particularly the lower looper, (the side that looks like Y's where you can't really see the middle needle thread), so I would tighten the lower looper about .5 on its dial, then test again.

I hope that this makes sense to you, you will need to test serge, and adjust one dial at a time, test again and look at the result, then maybe adjust another thread until you are getting a smooth looking seam.

You can download a manual from this link
http://www.singerco.com/uploads/download/664_14u34b,234b.pdf

Check page 16, changing the seam width is done by moving the cutting blade left or right and using only one needle. So if you want the narrowest seam, use the right needle only, remove the left one, and move the blade as far left as you can. Your manual says to turn machine off at the wall for safety, many overlockers will have a motor cutout when you open the front covers, but its good practice to turn off too. Then you put pressure against the upper moveable blade and swing it up out of the way and make adjustment to the lower fixed blade with a screwdriver in your kit (some overlockers it is a knob you move to move the blade position).

And reverse for a wide stitch, use left needle for 3 thread wide seam, and move the blade far right.

You can also use the 4 thread seam where you use both needles and trim up narrow but the seam will be heavier than a three thread.

Each time you make changes to the amount of fabric being trimmed, you may need to finess the upper and lower looper tensions to ensure the seam is tensioned enough to just meet on the cut edge of the fabric. This is the norm for overlockers, each fabric will behave a little differently so you need to test stitch and adjust a little. Your manual will explain this process too.

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.

narrow 3thread seam, so remove left needle. loosen top looper tension to 3 or 2, tighten bottom looper tension to 7 or 8. thread top looper with woolly overlock thread and reduce stitch length. there is also a thread finger u need to flick back on throat or needle plate usually to make rolled hem. also move blade to right to trim less fabric as you want the fabtic to roll under inside the seam.

left one, rolled hem is usually a narrow seam so you remove the left needle, change the needle plate or flick a switch that changes the position of the thread finger under the foot and tighten up the looper tensions. You want the top looper thread to go around to underneath and the bottom looper thread to disappear against the needle stitching. You also need to close up the stitching density, by decreasing the stitch length.

There are 3 things that come to mind for me.  I should say that I have a Bernina 1300DC, so your machine may not have the same parts, but check these things.  First, are you using 3 threads?  If so, is your needle in the narrowest setting, in the right hand hole closest to the body of the serger? Secondly, is your rolled edge lever retracted?  This lever is attached to a wide tooth that holds the width of the seam in place.  It is under the presser foot, between the loopers, and should retracted so that the stitches are allowed to become narrow.  If your machine doesn't have this lever, don't worry, you can also try a third thing.  Adjust your cutting width to a narrow setting.  There should be a knob that moves the upper cutting knife from right to left-- right being a wider seam, leaving more of the fabric intact, and left making a narrower seam that trims off more of the fabric as you sew. 
Hope this helps! Elsje

When you turn your machine on it automatically sets up for a straight stitch with the needle centered. Just guide the right edge of your fabric along the line marked 15 on the throat plate and you'll have very close to a 5/8" seam. If you need more precision you can adjust your needle position to the left or the right by using the stitch width and mirror setting buttons.