you are correct about the tensions. My error.

Thread being pulled to the top can EITHER be bottom tension too loose OR top tension too tight. If top thread is breaking, I'd definitely start with the latter and adjust the top tension.

Which needle to use depends on the thread used AND which sandwich is used. Different fabrics such as batiks or thicker batting will dictate that you use a larger needle. Definitely if the thread calls for a 90/14 needle, use a 90/14 needle.

I only use topstitch needles for all forms of sewing. Several different sizes but always a topstitch. For embroidery I use a 40 wt. poly thread on top. I am wondering if you need a heavier thread on top. I use Bottom Line 60 wt. in the bobbin unless I want the top and bottom to be the same. Then I use 40 wt. top and bottom - this is for embroidery. For piecing I use So Fine top and bottom. For quilting it depends on the look I want. Sometimes a 30 wt. like King Tut. If you are having a tension problem I suggest you experiment with several settings using a sample of the quilt you are making. Dont try on your quilt until you get things the way you want.

janome horizon series machines work best with Organ brand needles.
(organ also makes the needles that are branded "janome.")

a friend and i were both having problems with thread shred and breaking while doing free motion.
after we did some research i switched to an Organ brand needle and all the problems disappeared.
rather annoying, since i have a huge collection of types & sizes of a different brand - which perform nicely in all my other machines. LOL
luckily, the organs are less expensive so it hasn't broken the piggy bank to start a second collection.

your choice of 60wt for the bobbin is a good one.
so far, no matter what i use on top, i get the best results with 60wt in the bobbin.

Organ makes the Superior Topstitch needles. Organ is the highest rated brand of needles in the sewing industry. Maybe the posters that don't recommend topstitch needles haven't tried them. Many think they are just for decorative topstitching.

So you are embroideing the quilting design? You are going through all those layers? Have you tried an embroidery needle 14/90? Does the thread shread when you just do normal embroidery?

I would set my quilt aside, hoop a test piece of fabric, with stabilizer, and try stitching out the design. I always loosen my top tension just a bit when doing machine embroidery.

One more thought. How old is your thread? You might want to put a tiny amount of Sewers Aid on the thread.

Please keep us posted.

Except she's not embroidering, she's quilting an embroidery design. For quilting, I think she'd still want a topstitch needle. Just my opinion, which is worth exactly what you paid for it. ;)