I have a Fab-U-Motion with stitch regulator that I use with a Husqvarna embroidery machine. It works fine for regulating the stitches when I move horizontally, but when I try to do vertical motion stitching the regulator does not work at all and the stitches are really long. It used to work fine. What happened??

Sounds like something is amiss with the feed dogs or the stitch regulator itself. If it is a mechanical machine, it could be seizing due to lack of oil. However, if you are afraid to mess with the machine (understandable), you may want to take it for service. Could be something simple but could also be more complicated that would require a knowledgeable technician.

You should check with a Janome dealer if your machine can use a stitch regulator. Machines that take a stitch regulator need the ability to plug into the machine, which is not available on a lot of machines unless they have the stitch regulator function. A stitch regulator helps some people. It still takes a lot of practice moving the fabric smoothly.

On my BSR, it has an "eye" that measures the distance the fabric moves under the presser foot so it can stitch at the specified distance. The faster the fabric moves, the faster it stitches, but the stitches tend to more uniform in length.

FWIW, I read a summary of the Janome 8200 QC Horizon and there was no mention of a stitch regulator.

http://freemotionquilting.blogspot.com/2011/05/piecing-quilting-on-janome-horizon-7700.html

Is it an embroidery only machine. Typically embroidery is done with the needle in a fixed position while the hoop is moved around.

If the machine has the capability for a stitch regulator, then it is most likely possible. However, stitch regulators need to be attached (electronically plugged in) to the machine. If your machine does not have that, then you are limited to manual stitch regulation. (However, that may not be a bad thing. There are still a lot of professional sewists who continue to perform manual stitch regulation (such as free motion quilting) on mechanical machines. Their work is beautiful, but they have had a lot of practice. Even if they were offered a stitch regulator, most probably wouldn't want it anyway. Harriet Hargrave is probably one of them.
Harriet Hargrave Professional advice and the highest quality supplies for... The last time I saw her working on a quilt, it was a whole cloth silk quilt that she was quilting using a mechanical Bernina.)

You can contact a Janome dealer or search online. It should also be listed as an optional accessory in your paperwork.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwj-lejWiKbQAhUH6WMKHYLXBPcQFggjMAA&url=http%3A%2F%2Fjanome.com%2Fsiteassets%2Fsupport%2Fmanuals%2Fretired%2Fcomputer%2Finst-book-mc4800.pdf&usg=AFQjCNGCPQc_wJyl_Ts_4bFZ-7M3hX1ZnA&cad=rja

....

You should probably verify with either the vendor selling the stitch regulator unit or your Pfaff dealer. It would be best to get a guarantee before purchasing.

http://www.allbrands.com/products/21646-free-motion-assistant-7x9-frame-with-quilters-crui#tab_product_description

http://www.quiltingboard.com/main-f1/free-motion-assistant-t98619.html

http://www.allbrands.com/products/21647-free-motion-assistant-7x9-hoop-frame-qcc-w-o-stitc#tab_product_description

...

Hello

The machine is indicating that it "wants" you to move the hoop into position P2 or P3 according to what is blinking. This is normal.

Once that colour is completed stitching, the machine will communicate again for the nest Position.

Happy Stitching
Sewingirl/

it would be a good idea to have the machine checked, the stitch
length regulator need to be checked to see if it's worn or a gear
that move the part is worn and may need to be replaced.

I agree to have the regulator check, & even having the whole machine checked wouldn't hurt.

I am having irregular stitches on the forward/backward motion. I am using a Janome 16000 P on a Gracie II frame. (I purchased this as a used productin a private sale) Do I need a darning plate to effectively use the Sure Stitch regulator in a free motion mode or with a template?

The Designer 1 usually has a very nice free motion stitch using the V1 stitch. It should pull only slightly to the rear so that no bobbin thread shows while doing free motion embroidery. Upper tension may be increased for balanced stitch when stippling.
If your machine doesn't sew with the regular stitches with the teeth up, and it's properly threaded, the thread must be dragging on the bobbin case somewhere. First get the machine sewing a straight line, then try V1 again.