Serger won't catch upper looper thread in left needle

Sergers take a little more learning to get good at using all of the various stitches, but sometimes, no matter how you adjust the tensions, put in new needles, make sure they are pushed all the way up, and properly installed, sometimes, some sergers just will NOT CATCH the upper looper thread, and lock it in with the left needle. 

CORRECT TIMING UPPER LOOPER POSITION FOR Singer 14U64A sewing a
 4 THREAD OVERLOCK STITCH, 
at it's extreme left position, right before the needles come down, and catch the upper looper thread, locking it in the left needle thread, and with the lower looper thread.

Basically, the "timing" on a serger, means that each changeable component (left needle, right needle, upper looper, lower looper) are each locked into the correct position on their own rotation through the stitch cycle, so that each of them are correctly positioned to catch each thread, and lock them into position, at the same time, during stitch formation. If they don't meet "in time" to form the serger and overlock stitches correctly, then your serger is out of timing. This doesn't always mean your serger is useless, for example, if the upper looper will allow the right needle to lock with the upper looper thread, and your lower looper locks with your left needle thread, you still have a perfectly usable, 4 thread MOCK SAFETY STITCH (also referred to as 3/4 Mock Safety Stitch). However, your serger won't be able to sew any stitches which require the upper looper thread to lock with the left needle, unless you get the upper looper retimed to reach far enough to the left, to allow it to lock with the left needle thread. 

It is possible to retime some sergers yourself, but unless you are mechanically inclined, I would not recommend attempting to time your serger yourself, if it is still under warranty, unless you really are desperate, but if you need to go ahead, please research, and educate yourself, beforehand, so you won't regret having tried. If your serger is sewing most stitches properly, and can sew the 4 thread mock safety stitch, you have to decide if it is worth risking knocking everything out of timing, just to get the 4 thread overlock stitch, instead of the 4 thread mock safety stitch. You WILL most likely void your warranty, if you do work on your serger or sewing machine mechanics, before the warranty expires, unless you are a professional sewing machine and serger mechanic. That being said, with vintage machines, fixing them yourself, is often your only option, especially in some areas where there aren't sewing machine and serger stores and dealerships.

There are videos online of people retiming sergers, especially on YouTube, and there are many vintage sewing machine and serger groups on Facebook, and elsewhere online, where people, sometimes including sewing machine technicians, actually help each other fix up vintage machines, and get them working correctly. 

I have taken photos of 2 different Singer 14U64A sergers, to show how this happens. These sergers should BOTH sew exactly the same stitches, but one of them sews a 4 thread MOCK OVERLOCK stitch, instead of a 4 thread OVERLOCK stitch. This is caused by the upper looper not travelling quite far enough to the left, for the left needle to catch the upper looper thread, as the needle goes down.

Singer 14U64A serger which is NOT correctly timed (upper looper threading eye does not move far enough to the left of the left needle, for the upper looper thread to be caught by the left needle as it comes down, so this serger in this photo, does NOT sew the 4 thread overlock stitch, it sews the 4 THREAD MOCK SAFETY STITCH, instead. This is a legitimate serger stitch, but is not included on most modern sergers, only vintage sergers, Singer Sewing Reference Library book 'SEWING WITH AN OVERLOCK" shows this stitch, both sides, and explains how to adjust your stitches to get it perfect. It says it is a good stitch to sew stretch fabrics, including swimsuits, and woven fabrics. 

Same serger photo as above, but with flash to help show details-
Singer 14U64A serger which is NOT correctly timed (upper looper threading eye does not move far enough to the left of the left needle, for the upper looper thread to be caught by the left needle as it comes down, so this serger in this photo, does NOT sew the 4 thread overlock stitch, it sews the 4 THREAD MOCK SAFETY STITCH, instead. This is a legitimate serger stitch, but is not included on most modern sergers, only vintage sergers, Singer Sewing Reference Library book 'SEWING WITH AN OVERLOCK" shows this stitch, both sides, and explains how to adjust your stitches to get it perfect. It says it is a good stitch to sew stretch fabrics, including swimsuits, and woven fabrics.

This next photo is the CORRECT TIMING FOR SINGER 14U64A serger, to sew 4 THREAD OVERLOCK STITCH. 

Singer 14U64A serger sewing 4 thread Overlock stitch, proper positions for loopers when upper looper is in the furthest left position, in relation to the left needle. Correct threading, as well.

Singer 14U64A CORRECT UPPER LOOPER POSITION at furthest left position, right before needles come down, which allows left needle to catch the thread behind the eye of the upper looper, goes down through this thread, and stitches it to the thread chain, or the fabric. If your upper looper eye does not go far enough to the left of the left needle, before the needles come down, your serger will NOT sew a 4 thread overlock stitch. The serger in this photo DOES sew the 4 thread overlock stitch correctly, it is correctly timed.

Singer Sewing Reference Library book (Singer has the copyright) "SEWING WITH AN OVERLOCK"
third photo down is the 4 thread MOCK SAFETY STITCH, notice the gap between 
the overlock stitch and the left needle straight stitch, this is what gives it the look of a 
safety stitch, but because the bottom looks like a standard overlock stitch, it is called a 
4 thread MOCK SAFETY STITCH 
Read the text, to see descriptions & details

Singer Sewing Reference Library book "SEWING WITH AN OVERLOCK"
third photo down is the 4 thread MOCK SAFETY STITCH, notice the gap between 
the overlock stitch and the left needle straight stitch, this is what gives it the look of a 
safety stitch, but because the bottom looks like a standard overlock stitch, it is called a 
4 thread MOCK SAFETY STITCH 
Read the text, to see descriptions & details

    Singer Sewing Reference Library book "SEWING WITH AN OVERLOCK"
these are available inexpensively online, they have step by step color photos of most 
serger stitches, and how to adjust them, as well as how to deal with thread jams, and other stitching
problems, and show tools and notions which are helpful. I highly recommend this book, but it does NOT have coverstitch serger information, it was written in the 1980's, before home coverstitchers became available.