I am a newbie trying to stitch straight lines on my quilt with my Japanese 15 class machine. I have the walking foot and my stitch length is still messed up. It is making ultra small stitches. I have my stitch length set to the largest setting, and I am using the feeddogs. What can be doing this? Help!!
:(

Quick guess is: The machine cannot handle the weight of the quilt to feed it under your foot.
Or: You are holding too tight, causing a drag on the quilt that is crippling the machine's ability to feed the fabric through.
A possible solution would be to buy a walking foot for your machine ... a walking foot has a double "foot" on it, with two feet alternately lifting and setting down on the fabric to stabilize the fabric as the quilt is fed through the machine.
I have had problems sometimes with my quilt being so bunched up in my lap, that a crinkle got caught on the table and, needless to say, quilting slowed to a crawl, as my needle is going sixty miles an hour.
To check your control issues (to see if you are being too heavy handed) take a small sample of batting, backing, top fabric and see what your machine does next. Start without your touching it ... then, see what happens when you try to control it in your normal manner. (As a heavy handed machine operator <g> - - us folks with control issues, really do need to lighten up a bit!)
Hope this helps ...
oh! and, if the problem is your machine can't handle the weight, and you don't want to get the added help of a walking foot ... try to figure out some way to make your quilt almost level with the bed of your machine ... keep the quilt as unfolded as possible ...

I have the walking foot on my machine. I have been trying a scrap (no weight) and it is still doing the same thing. I can't figure it out. The needle is new, and my stitch length is set at the longest stitch, the tension is ok...

Time to take him to the repairman ... unless, he is working on regular fabric ... some machines just can't handle the weight.
Sometimes, the machine just gets out of timing and I don't know how to fix it, so I have to take it to a repairman.
The other thing to do is back it off of the full stitch length a bit ... maybe the little lever, if pushed too far goofs something up ...
Are your feed dogs up? (sometimes, the button get flipped and I didn't know it) ...
And, now I am at the end of my abilities to psychoanalyze the machine <g>. But do let us know what you find out, as soon as you find it out.
OH ... one last thing ... when is the last time you cleaned your machine?
Remember that machine I took to the repairman because I had got it out of timing?
Between that time and the next time my machine started doing that, I was working with a gal, and she taught me about cleaning my machine, completely down to removing the plate over the feed dogs. I kept putting it off and then my machine started acting up ... I almost forgot about cleaning out under the foot plate .... at first, I didn't see it, but I happened to tip my machine back a bit, and between the feed dogs ... under the plate ... there was absolutely FELTED fuzz ... so tightly packed that I could have put it on the piano hammers for felt, no less! It took the tip of my seam ripper to go between the feet and lift that stuff out of there ... front and back on both sides. The feed dogs can't work if they can't lift high enough to get above the throat plate (is that what that's called?) ... check it out! Just to be sure ... I am really glad I remembered that one! PHEW!

Are you absolutely sure you have the walking foot installed correctly? It's very easy to get this wrong. The fork has to "embrace" the needle screw in order to make the walking foot move up and down with the needle.

Try taking your quilt out and sewing just two pieces of fabric together. Are the stitches still small?

Ok, got the answer to my problem. A bumm walking foot. I took my machine and foot back to the place I bought the foot. They couldn't get it to work either. Seems they sold me a junk foot. It works on my practice piece, scared to try the quilt. :D

Can you take off the plate over your feed dogs? YOu may have a build up of lint under the feed dogs and they don't move properly. Hope this helps..

PHEW! I am glad that got fixed! It has been a fun exercise coming up with some kind of solution. I would have never thought about the walking foot not working ... imagine that! And, I was pretty sure I had that piece of equipment down cold! Sneaky little thing, eh?
I am happy that you will be able to get on with what you want to do!