What stitch should I use for a blanket stitch on my Janome 2160DC

Janome has several dual voltage models.
MC9900 861A MC500E 864AU MC350E 852A2 MC200E 856A MC8900QCP MC8900QCP SE MC8200QC 858G MC8200QC SE 858G2 JNH7700QCP 858A S7 863AU S5 863A S3 863AL MC6600P MC6500P 846A MC6300P 4120QDC 809A DC2015 3160QDC 2160DC JNH2030DC 808A

per the owner;s manual: adjust your bobbin tension

Please expand your question so we know what information you are requesting. Also give us the brand name of the machine.

You'll need to experiment with your machine tension on scraps of the same fabric and batting you intend to use for FMQ. Usually, the upper tension needs to be loosened some to prevent bobbin thread from being pulled to the top of the fabric, but that too is a matter of testing until it is where you like it. The stitch length is not important as long as you drop the feed dogs so it will not attempt to pull the fabric or create any interference with your manually moving the fabric. (A silicon Slider is nice to have--you can find them on the internet. It creates a slick surface so your fabric will move easily.) You will manually determine the stitch length because you will be manipulating the fabric. Also, be sure to use a darning foot or free motion quilting foot so there is space between the foot and the needle plate to move the fabric. Install a brand new sharp needle. Some people like to use Microtex, embroidery or top stitch needles. Beyond that, Practice, Practice, Practice! Practice making smooth movements. If you can't practice on the machine, get a cheap white board and practice drawing your designs on the board. You can erase and do it over and over, even while you're sitting watching your favorite TV show. If you like a particular design, take a photo of it with your cell phone so you can refer to it later.

...

if the feed dog lever will not come up after you have moved the lever the there could be something wrong with the lever, it needs to be checked to see if it's broken or loose

Your machine is a beginner model with only 8 stitches. Looking at the photo of the machine, it does not appear to have a blanket stitch.

Singer 1507 Sewing Machine

Usually, only more advanced (and expensive) machines with more stitches will have the blanket stitch.

...

Usually, your machine would need to have a blanket stitch selection in order to perform a blanket stitch. If your machine does not offer that option, then you are probably out of luck unless it is a highly advanced computerized model that you can create your own stitch designs.

...

Once upon a time, a blanket stitch used to be a long wide zigzag stitch. If you are referring to an electronic stitch version that would look like the inverse of a blind-hem stitch, then no, this is not possible on the 930 record machine.
What is nowadays called a blanket stitch used for quilting applications requires the machine to stop feeding during the stitching cycle to allow for the single stitch to the left and then back again. An electronic step motor machine, common to all newer Bernina machines can do this and or similar functions. That being said, don't be thinking about getting rid of your 930. Better to get one of the newer units for occasional blanket stitching and keep the Swiss made 930. The Bernina 930 is a lifetime machine. Computers machines are not lifetime class machines by virtue of being computers. Electronics are short lived.
SewTechnical Home

I would use a foot that can do a zig zag stitch or a blanket stitch

is it the computerised kind? Then you should be able to change the stitch by scrolling through the number. Mine says Utility 1-28