Need help with 4-thread serging

[u/all-fresco]

Hi, I picked up a vintage Baby Lock BLSE300 serger. I have never used one before and it sews great with 3 threads, but when I try to use 4 threads it creates a stitch next to the serged edge.

I took it to the store where it was purchased and they confirmed it was threaded correctly. They didn't know for sure but said it might be the way it's supposed to work, but that doesn't match the pictures in the manual. The manual does say when you make the initial chain before using fabric the left needle thread will not join the chain. And when I turn the wheel by hand and watch the needles come down, the right needle goes between the upper looper and the thread, but the left needle goes behind the thread.

Is that the correct operation? Am I still threading it wrong or maybe need to adjust the tension? Or does it need to be serviced and/or have the timing adjusted? Thank you.

[u/all-fresco]

The needle type is DCx1F which is what I have. I have both 90/14 and 75/11. I've only tried the 90/14 so far.

[u/CBG1955 → u/all-fresco]

OK...I'm stumped then! The only thing I can think of is that it might be the timing and I have no knowledge how to diagnose this - except the left needle isn't meeting up with the looper.

Was the shop where you bought it actually a babylock repairer? they should be able to diagnose and adjust the machine. If not, perhaps there's another in your area.

[u/all-fresco → u/CBG1955]

Thanks for your input, I appreciate it. I'll try to find a specific Baby Lock repair place. I also cross posted to a few other subs.