May I introduce you to the book that never left my serger table the first year I had my machine: Ultimate Serger Answer Guide. Good instructions, and excellent pictures of what's going on with poor stitches, possible causes, and how to fix it. Yes, it's old. But your serger is very similar to the sergers of the mid 90s.
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Wild guess: the left needle thread isn't being tensioned properly... probably not sitting in the tension properly or a dirty tension or tension setting off.
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But let's try my basic troubleshooting algorithm, which fixes most machines without driving you nuts:
First rule of serger troubleshooting: The threading is off. Second rule of troubleshooting: It's dirty. Third rule of troubleshooting: The needles are dinged up.
So... I'm going to suggest that you stop right now. Take ALL the thread OFF of the machine.... yes, cut the threads and run out the chain. Take all the cones off. Clean the machine per your manual. You're looking not just for fluff and drippy oil and yuck, but evidence... like the end of a bit of thread hanging out under the thread stand, or out of a tension. Oil per your manual's recommendations.
New needles, right size, perfectly placed in the needleholders, all the way up before you tighten the screws. Yes, this may feel wasteful but you can always save those needles for later... right now, we're trying to get the machine happy again.
Next, make sure the thread tree is all the way up and properly aligned with the front of the machine.
Now, dig out the threading section of the manual, and proceed to thread the machine from scratch *in the order* specified in the manual. If it says "raise the presser foot", do so. As you place each cone on the machine, run the thread through the thread tree and around the first tensioning device (usually a bar above the tensions) and give the end a little tug. All four threads should feel at about the same tension.
Even though it'll make you feel like a fool, read each step of threading *out loud* before you do it. It's amazing how often you catch yourself doing something by rote that's different from what the manual says. My hypothesis is that reading the steps out loud give your fingers a chance to catch up.
Once again, check the tensions -- pull each thread a bit after it's been through the actual tension dial or slot, and make sure they're set to the default for the stitch you're doing on this machine.
Now try a sample. Still doing it? Or is it "cured"? Most of the time, the machine is cured except for maybe some tiny tension tweaks.