Tension setting I am trying to sew a new highchair cover using vinyl fabric with fuzzy backing and I'm using bias tape. The top stitch seems fine (I lengthened the stitch all the way) but the bottom stitch is looping. I have adjusted the tension on the bobbin and also the topstitch tension but neither have made a difference.

Top thread is "frayed"?

Could be tension is too high, problem with needle, needle wrong type, other.

Does it straight stitch ok using same fabric, thread, needle, bobbin, and tesnsion?

You really need to get a manual. Trying to figure out the problems is difficult if you are not an experienced sewer. There is a manual on eBay right now for your machine.

A little about tension. 95% of the time it is the top thread tension. It will be the thread on the bottom of the fabric affected. If the the thread on the top is loose it is the bobbin tension. Do not adjust both at the same time. Start with the top tension making small adjustments and then doing a test on scrap fabric.

Check to see if it is threaded correctly, without a manual it may not be obvious if it is or isn't. Check that the thread is between the tension discs correctly.

Vinyl is a very difficult fabric to sew because it is sticky and tends to grab the thread and hold on to it causing loops in the thread. It would help to use a roller foot or walking foot to help move the vinyl through the machine. Also, try using a very sharp leather needle or top stitch needle. Sew SLOWLY!

All About Needles

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try a new needle

try adjusting the top thread tension to see if that helps the bottom loopy stitch

try adjusting the top thread tension and see if that helps
also try changing the thread.
also change the needle--make sure it's the right needle for the fabric
being sewed.

It sounds like you have a tension problem. If you can test using a piece of scrap vinyl to set the tension, that would be the best. Let me give you instructions for adjusting the tension on anysewing machine. Be sure to have a scrap piece of the fabric you are using andthread the top of the machine with a different color of thread than thebobbin-it'll make it easier to see what's going on. Thread your machine with the presser foot UP. The disks that control the tension (threadgoes through them) are tightened up if the foot is down. Change the machine tension with the presser foot DOWN.Because the disks won't be engaged (see above) if the foot is up, the diskswon't move. Start at the middle setting, 3. Go up to tighten, decrease toloosen. Each machine is different, even 2 different machines of the same model.The best thing to do is start at 3 and use a different color in the top andbobbin thread so you can easily see what changes to the tension do to yourstitches. Now use a straight stitch a sew 3 or so inches. Take a lookat the top and bottom of the fabric. You don't want to see any (or at least notmuch) of the other color showing from the other side. Loops on the bottom of the fabric means you need to tighten(increase) your top tension. If you're stitches on the bottom are causingpuckering, then you need to loosen you're top tension. For the most part, youreally don't want to touch your bobbin tension-leave that to a repairtechnician. The factory setting shouldn't be touched except by someone whoreally knows what they're doing. Keep sewing a few inches at a time until you have the resultsyou want. Now you're tension is where you need it!

Try adjusting the top tension dial to tighten up the loose stitching on the underside. Generally if you have loose threads underneath, its the top tension and visa versa.

However, if you are using a thread heavier than normal sewing thread through the needle and bobbin (which you probably need for this canvas) then the machine may struggle. I've sewed vinyl seat covers and gave up with my domestic machine and purchased a second hand industrial Singer machine 20U to cope with this heavier weight textile. Generally for anything heavier than upholstery fabric a domestic machine may not be strong enough.

First thing is pin the heck out of it. I would not use a needle bigger than a 16. Set the tension midway and then just tweak it one way then the other to get a smooth stitch. And for your thread, just use polywrapped cotton thread and you should be fine.

You can try adjusting your stitch length this may solve the problem right away.
It could be the thread your using, I have had to switch threads before. Rewind your bobin, sometimes issues can occur by a poorly wound bobin.
You can also try to use a different foot (my machine comes with a satin stitch foot)and or slow the speed of your feed dog.
Sometimes a very fabric does need some sort of backing, a tear away, or wash away might be your only answer.
I hope this helped.

What size needle are you using?