Tension/Pressure

TENSION:
Check the tension to see if it is correct.  Tension should always be adjusted with the presser foot down.  An easy way to check the tension is to use a different color thread on the bobbin than on the top.  You can then easily see if one of the top threads is pulling through to the opposite side.  The stitches should look the same on both sides of the fabric.  If the top thread shows on the bottom side, the top tension is too loose.  Turn the top tension knob to the right (clockwise).  If the bobbin thread shows on top of the fabric, the top tension is too tight.  Turn it to the left (counterclockwise).  Continue adjusting the top tension until the correct balance is achieved.  In most cases, adjusting the top tension is all that will be needed.  If this does not give you the correct tension, you may have to adjust the bobbin area only slightly-clockwise to tighten, counter-clockwise to loosen. 
Some machines have the bobbin tension adjusted at the factory and the manual tells you not to tamper with it.  In this case, you should be able to achieve a satisfactory adjustment from the top tension only.  If not, it is best to call for repairs.  When changing from straight sewing to zigzag, you may have to make some minor adjustment in the tension.  Sometimes a looser tension is required for zigzag stitching in order to prevent puckering.  Be sure to test first.

PRESSURE:
The pressure on the fabric must be right for the weight of the fabric you are using.  Correct pressure will allow the fabric to feed smoothly and evenly as you sew.  To check the pressure, take two scraps of fabric and pin them together at both ends.  Use pieces about six inches long that are cut on the straight grain.  Sew a 5/8" seam.  If pressure is correct, the pieces will line up evenly at the end.  If pressure is too heavy, the fabric will pucker in front of the pin at the end of the sewing.  If it is too loose, the fabric will drift and will not feed in a straight line.  On some machines the pressure is automatically controlled, but most machines have an adjustment screw.  It is usually found on top of the machine directly above the sewing area.  Turn screw to the right to increase the pressure, to the left to lighten it. 

The following problems can be caused by improper pressure.
If the pressure is too heavy, there will be one or more of the following signs:

1. There may be a shiny mark on the fabric along the stitching line.
2. The cloth may not move forward.  Stitches will bunch together.
3. Stitches will be shorter than intended.
4. Fabric may be cut by the teeth.
5. The lower thread may break.
6. The presser foot or teeth may be injured.

If the pressure is too light, there may be one or more of these signs:

1. The fabric may not feed correctly.
2. Stitches may be staggered., that is, uneven in length and out of line.
3. The needle may break in heavy cloth due to imperfect control.

Experiment until you find the pressure that is right for your fabric.  If two pieces of fabric feed unevenly and pressure adjustment does not correct the problem, place a piece of tissue paper under the bottom fabric, on top of the feed.  Stitch through fabric and paper.  Tear off the paper after sewing.  If this does not solve the problem.  It may be due to the fabric.  Pile fabrics and slippery fabrics are especially susceptible to this problem.  An even-feed  foot or a roller foot might be the solution.  Either of these will help the top layer to feed at an even rate with the bottom layer.  Pin-basting at intervals along the edge of the fabric should also be helpful. 
Most machines have a hinged presser foot that will sew over pins.  If your machine doe not have one, you can probably purchase one that will fit.  You may find it an advantage to leave the pins in as you sew. 
Start your sewing with the take-up lever in its highest position; this way you will know that the needle is at its highest point and you will have plenty of room to insert your fabric.  It will also prevent the thread from pulling out of the needle when you start to sew.  Never start sewing with the needle off the fabric; you will end up with a tangled mess.  The best way to start is by inserting the needle into the fabric a little past the beginning of the fabric.  Backstitch to the edge (do not go off the fabric).  Then sew forward straight down the seam.  If your seam needs reinforcing at the other end, sew to the end, backstitch two or three stitches, then sew forward to the end again.  Seams that will be crossed by other seams do not need to be reinforced.
If you are sewing several seams or darts at one time (as you should be), use the factory method of continuous sewing.  Feed each piece into the machine as the previous one is finished.  All pieces are "chained" together and then clipped apart afterwards-no need to raise the presser foot between each one.  Just leave about 1/2" between each piece and your machine will keep interlocking the top and bottom threads.  Feed the pieces continuously, one after the other, until you have finished them all. You can clip them apart as you sew or wait until you have finished sewing all of them.  Thread ends will remain twisted and will not ravel-no need to tie them.