Hi, thank you for contacting JustAnswer.com. My name is***** will do my best to provide the right answer to your question. I'm not a Chevy Expert - your question must have been miscategorized at first, it is now in the right category, for an Appliance problem with a sewing machine.
There might be two things wrong, at least:
1. make sure the cams aren't too loose, don't have too much 'play' in them. That might make the stitches sloppy.
2. more likely, though, the stitches may just be owing to the thread tension being too loose. So it might be well to turn up the upper thread tension one or two points, try again a bit, see if that improves the stitch quality. (If the thread starts to tangle on the top, you need to increase the bobbin thread tension adjustment also, to match the upper thread tension. Of course, if either thread breaks, you need to turn down the tension again.)
If the above doesn't help, you might want to photograph the defective stitching, and attach the photo to your Reply... and I will advise you further.