If it's wavy, you need to adjust your differential feed. Check your manual for instructions.

Thanks for asking a serger question. Have you watched the videos on Sewing With Nancy's website?
They are really helpful, or have been for me. Sometimes it's good to get back to the basics. I believe she addressed this issue. Have a great day!*

Bikermom, Yes, differential feed is your answer. I also know that some machines do not have this. On my serger I have a lever. The lower settings are 0.6 and 0.8. My upper setting run 2 thru 1.3. "N" for normal is inbetween these. When I use my lever on 2 it will gather the material, so if my material is stretching I will do trials to see what setting I need to make the seam straight. I will start with 1.3 and work my way up toward 2. Hope this is clear and helpful, if you have any questions, please feel free to ask me. H&*.

Under "choose catagory", select "Techniques". You'll see Serging sessions 1, 2 and 3. :-)