Thread skipping on serger - Juki MO-644D Mechanical Sewing Machine

First thing I would try is installing brand new needles. Also, has it been cleaned and oiled recently?

Serger Stitches 101 Cheat Sheet Never Ever Without It Your New Must Have...

Serger is Skipping Stitches Threads Magazine Gatherings Forum Sewing...

https://community.babycenter.com/post/a44997916/help_serger_skipping_stitches

SKIPPED SERGER STITCHES

Overlock Serger Troubleshooter

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probably need a adjustment, you may have to have the looper checked

SKIPPED STITCHES ON A SERGER
1. Is the needle bent or worn--replace
2. Is the needle inserted correctly into the needle holder--check needle
3. Is the Serger threaded correctly--rethread
4. Is the presser foot pressure right for the fabric--adjust
5. Possible timing problem--need to be checked at a sewing machine shop

Many problems can cause a serger to skip. Start with the correct new needles and good quality thread. Skips can be tension a clearance or timing issue or the amount of thread pulled out by the thread guides. This is not something most consumers can repair. Even with the service manuals they can be difficult. I have spent thousands of hours working on sergers.and skipping at high speed can be difficult to solve. Some brands are more user friendly to service. This like will show some of the common settings and tools needed for a serger service.

SKIPPED STITCHES ON A SERGER

1. Is the needle bent or worn--replace

2. Is the needle inserted correctly into the needle holder--check needle

3. Is the Serger threaded correctly--rethread

4. Is the presser foot pressure right for the fabric--adjust
5. are the needles the correct ones for the serger
6. Possible timing problem--need to be checked at a sewing machine shop

SKIPPED STITCHES ON A SERGER

1. Is the needle bent or worn--replace

2. Is the needle inserted correctly into the needle holder--check needle

3. Is the Serger threaded correctly--rethread

4. Is the presser foot pressure right for the fabric--adjust

SKIPPED STITCHES ON A SERGER
1. Is the needle bent or worn--replace
2. Is the needle inserted correctly into the needle holder--check needle
3. Is the Serger threaded correctly--rethread
4. Is the presser foot pressure right for the fabric--adjust

Are you using the corfect serger system needles in it? Most domestic sergers will take the regular domestic needle 130/705X but work best with a serger needle with has a groove down the front as well as the regular back scarf. This helps with the thread pickup from the looper by the needles. Sergers also run much faster than domestic sewing machines so the tolerances are very fine.

My Bernina came with ELX705 needles and it does run better with these in it. Also you need to use mid size needles, don't go too small, probably a size 80 or 90 is best.

Check out this page for some good info as the different serger manufacturers do have their own recommended needle sometimes. http://sewingmachine221sale.bizland.com/store/page78.html

Always refer to your manual (or ring the manufacturer if lost) to double check what needle they recommend be used in your serger, it does make a difference.

Has it been serviced at all during your ownership of the serger?

The loopers can be knocked a little out of timing resulting in the skipped stiches on the heavy weight fleece occassionally. It sounds like you have tried all the usual fixes and know what you are doing.

Only other thing that might cause skipped stitches could be using a sharp needle rather than a ball point on the fleece. Otherwise maybe time for a trip to the shop??