When sewing, top stiches look good; however bottom stiches are uneven and stiches are loose. How do I make the bottom stiches look like the top ones? I suspect it may be a matter of adjusting the bobbin holder? I've had my Singer sewing machine for close to 30 years. Simple little machine that, overall, has done a fine job. Thank you for your help.

This is usually because the top tension is too low. Just raise the tension a little at a time and that will usually fix it.

Skipped stitches is usually indication that needle is blunt or burred.
Or if you are using a sharp needle on knit fabrics it may also skip a little.

Not sure what you mean by "change weight of material" - do you mean you are sewing applique and going from one fabric weight onto another? or different sewing projects?

It is a good idea to change your needle for each new project (4-6 hours sewing time max) and choose the right type of needle and size for your project.

However, the jamming part sounds more like thread tension issues, especially if the jamming is happening underneath from the bobbin. If stitching is loose underneath, then top tension is the problem and visa versa.

Recheck your threading path, top and bottom tensions and that you have good quality thread. Change the needle, rewind a fresh bobbin, thread up and try again.

There is really good info on www.sewing.about.com or I refer to my Singer Sewing Machine textbook at lot too. Good luck.

Think in opposites... the top stitch is formed by the bottom thread tension and the bottom of the stitch by the top. So either the bobbin thread is too tight or the top thread is too loose. Check the bobbin thread first as that is the one most overlook. It should come out of the bobbin without having to yank it hard but it should have a little resistance.

Totally rethread your machine, then check your needle, put in new one if you haven't changed it in awhile or not sure if it is bent. Usually it is in the top threading for problems underneath. Also, make sure that you are using the same weight of thread top and bottom or else you can have problems. Good luck

Usually it is your top threading is not right, double check that. Read you manual carefully, even if you think you know how to thread it, do it over again. Good luck

Meaning you have adjusted the tension for the top thread as well as the bobbin?

Use two distinct colors of thread - one for the top and one for the bottom.

Thread the machine with the presser foot up.

The two threads should meet in the middle of the stitch with neither obvious on the opposite side of the seam.

Set the top thread tension to 4 and adjust the bobbin as needed.

The manual can guide you through thread adjustment.

I found my book & was able to solve my problem with my irregular stiches with the proper adjustments.
Thanks anyways.

It should not affect the winding, the reason the needle is still going up and down is because the oil has gone dry on the shaft and unless you can get to the shaft where the balance wheel is then it would be wise to take the machine in for a service, loose stitching on the bottom is a tension problem, make sure the top tension is set about 4 or 5

First of all check your needle size and utility, for the tension machine it must be the same on top and bottom example pull the top thread and the lower thread they must be the same.
When sticthing the elastic you must devide the amount of elastic versus the fabric : dise the amount before stiching
Hope this will help
Good day