My first thought is that would use an awful lot of thread. It seems like a good point that it makes a lot of holes in the fabric. Besides making the fabric weak it could allow the batting to migrate through.

Read the manual that came with your machine. It will tell you how to set your machine.

I have a Janome 4800 as well and I have used some of the decorative stitches for quilting. I have to be very careful and let the feed dogs feed the quilt, the least bit of pulling will result in skipped stitches and different lengths between designs. You can't enlarge the design but you can lengthen or shorten it. I haven't done it in a while but if I recall correctly I get better results with a walking foot.

I'm glad you mentioned the serpentine stitch. I was trying to remember what that was called. I have it on both my old Elna and Bernina, so probably most machines would have it. I've used it on quilts, and it's special without making any closer perforations than a straight stitch would. There are probably some others that would work well.

Thankyou so much for all the comments and advice. I've now gone back to the manual and reminded myself how to adjust the pressure foot pressure -duh - however, doing so made the problem worse. I've tried different top tensions, topstitch and quilting needles 11 and 14, 2 bobbin cases with different tensions, regular and walking foot, and I still can't get the decorative stitch to come out right on the quilt, though on the practice piece nearly every combination is OK!

Three things occur to me:
I'm not at all sure how I know when the tension is correct when using these stitches: the straight stitches in the row of stitching look fine top and bottom most of the time. However, I think that my rows seem to be pulling the quilt quite tight, so maybe I need a much lower tension.
The thread - although my machine does usually cope quite happily with the 50wt cotton thread that I'm using, maybe for quilting with these stitches I'd be better with a finer polyester?
The simplest of the 3 stitches I've tried - an outline of a star with no infilling - IS working OK. I wonder if the problem with the more complex stitches is with it being a log cabin block, plus the tightness of the stitching mentioned above - maybe the number and density of the stitching pulls the fabric in and there isn't enough "give" in the narrow strip I'm sewing down to be ablee to do this successfully?
Just thinking aloud here, and would be interested to hear your comments.
I'm actually enjoying this, rather than getting frustrated -the wall hanging isn't precious to me, so I'm pretty relaxed about using it as a learning exercise!

Hi. Just last week I was stitching decorative stitches onto the rag quilt I'm making (I've not finished it yet) and I found out that if I used the walking foot, my stitches do not align properly e.g. the filled stitch star would not be a whole complete star but broken up and not aligned.
When I change my presser foot to the one recommended by the machine (when I choose a stitch, the LCD panel on my machine will show the presser foot to use), the stitches come out good.
I supposed in my case, I have to use the foot as dictated by my machine for the stitches.
I started with a walking foot because I thought it would help me feed the layers of flannel evenly

That's exactly what I've found - the stitches aren't right with the ordinary foot either, in my case, but the problems are worse with the walking foot.

I use decorative stitching all the time on my quilts. I think it adds another design element, especially when I used different color threads. It does use a lot of thread. You have to be careful which stitches you use..I avoid the dense stitches (hard to remove if there's a problem and it's SLOW). Also if you use different colors on top and bottom, watch your thread tension. I only use W&N batting which only needs quilting every 10". I have a Janome6600 with a walking foot which I find I cannot use for some decorative stitches, especially when there is back/forth stitching. All a matter of practice and experience. When I first started, I worried that my quilts weren't quite up to standard, but I have gotten over that. While I like overall quilting patterns, I also love my decorative stitches. So do what makes you happy! Would love to see your work.

Thanks for this. Do you find you need a different top tension if you're using a decorative stitch? Higher or lower? As I said above, I'm not actually sure what I'm aiming for with the tension for these stitches.

Honestly, I'm not sure! I keep small sandwich pieces (with fabric and batting) for sample pieces before starting on my quilt. I begin with a mid range tension and adjust up or down, depending on how the stitches look (make sure you are using the actual threads you will use on the quilt, especially if you use different colors on top/bottom). Bascially, you want to make sure that top thread color doesn't pull thru to the back (and visa versa). If you're not sure what that would look like, play around with your tension and it should be pretty obvious. Also be very conscious of "pushing" your quilt thru as well the drag on the quilt as I've found both will distort your stitches. Good luck!