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If your serger sews a straight seam but will not create an overlock stitch, improper threading is the most likely culprit. Sergers use four, eight or more cones of thread all at one time, and each thread must follow its own path through the machine. What some people fail to understand is that even if the machine looks to be threaded correctly, if each length of thread was not put through the machine in the proper order, the threads will not interact correctly, and the overlock function will not work.
In this case, it is best to start the threading process from scratch. Remove all the thread from the machine and begin again by threading the machine as instructed in your owner's manual. If you don't have a manual, look around on the machine for a color-coded threading diagram. It might be located inside an access door or around a section that can be removed to operate the serger in free-arm mode. -
If your fabric puckers while you are sewing, two common issues could be to blame. First, check that your differential feed is set correctly. The differential feed controls the serger's front and rear feed dogs. For normal sewing, the feed dogs should move at the same speed, but on delicate or closely woven fabrics, set the front feed dog motion to be shorter than that of the rear feed dog. Set it the opposite way for stretchy fabric, so the front feed dog motion is longer than the rear feed dog motion. This usually can be adjusted with the turn of a dial on the machine, but because models vary greatly, refer to your owner's manual for specific instructions.
Fabric puckering also can occur when thread tensions are too tight. Because each length of thread passes through its own tension gauge, you might have to loosen several tension dials before you correct the problem. Adjust only one at a time, and experiment on a piece of fabric identical to that of the garment you are sewing. As you experiment, remember to double the scrap of fabric, as you will be sewing two layers of fabric when you make the garment's seam. - One of the most frightening and frustrating aspects of using a serger is the possibility of having the serger's cutting blade accidentally slice through a section of fabric rather than just trimming the intended seam. This will happen if you pass the fabric under the presser foot when your fabric is bunched up or doubled. With a traditional sewing machine, you can simply remove the stitches, straighten the fabric and sew the seam again. With a serger, you will have created a large slice that can't be repaired. Because of this potential danger, always ensure that your fabric is lying flat and smooth as you sew.
