When your sewing machine operates correctly, all the different parts work at very specific times. If one piece moves at an incorrect time, the fabric may catch or the thread may look jumbled. The "timing" of your machine refers to how the needle eye and the bobbin hook work together to form a stitch. Most sewing problems are not necessarily an issue with the timing, but with the user. Before adjusting the timing, ensure it's not another issue with the machine.
Ensure the needle is not bent. Twist the balance wheel up and down to view the entire needle. Replace the needle by turning the power off, unscrewing the screw with a coin, placing the new needle inside and retightening the clamp.
Check the thread for the proper path through the machine. Incorrectly placed thread may cause the sewing machine to not operate smoothly. Consult your user manual for the exact path of the thread, which may vary from machine to machine.
Remove the bobbin and look for tangled thread; adjust as necessary and replace. Keep the thread as smooth as possible to limit tangles.
Remove the plate from the machine, located just below the needle and foot. This part easily slides on and off most sewing machines.
Unscrew the hand wheel so the needle is somewhat loose, and align the mark on the needle bar with the mark on the needle bushing. Retighten the screw on the hand wheel.
Align the hook blade with the centre of the needle.
Turn the balance wheel to check the timing of the needle and bobbin. If correctly timed, the needle will dip into the bobbin with a clearance of about .004 to .01mm.
Replace the plate, and attempt to sew a sample swatch of fabric.
Adjusting the timing on a sewing machine is a somewhat complicated process. If you do not succeed in fixing the timing yourself, take the machine into a qualified repair shop.