Vintage sewing machine not stitching - very generic tutorial
Does it need a new needle? That is usually the quickest easiest cheapest fix. Change the needle... some times we hit pins or sometimes people pull the material through the machine and the needle bends. Sometimes needles get dull? When was the last time it was changed?
Make sure the needle is in all the way
Try this first. Take apart the throat plate then try putting it all back together as seen in your owner's manual sometimes that helps a 301 - maybe your 319, 221, 401, what ever has to go a certain way - so make it go the way the manual shows
Make sure the hole lines up with the needle - nothing crooked...
(While you are in there clean out the lint and oil the machine - some times that can be the cause of your skipped stitches)
here is one I found and it is well said http://sewing-machine-troubleshootin...-Stitches.html
Look at your upper thread. Is it catching on the little notch on the spool? Turn it over and see if that helps...
completely unthread and rethread the machine
Is the thread in the tension in correct position between the disks?
check to see if the bobbin thread is correctly threaded through the bobbin holder
is the bobbin worn out? warped? the correct bobbin for the machine?
Is the needle is the wrong length? size matters - a few of the vintage machines take a different from normal length needle and the machine won't work.
Check the owner's manual and double check your needle size
Check to see if you have the correct needle diameter for the fabric you are using. Smaller needle number for light weight fabric
Larger needle number for heavy weight fabric.
Does the needle size match your thread size?
If the needle is put in backwards it won't work - the owner's manual should show how it goes
Make sure the needle is threaded in the correct direction. - it usually goes through the groove side first
The curve side of the needle goes toward the bobbin so the shuttle can pass and grab the thread
Is the tension is too tight? too loose? (See your manual for that)
If that doesn't work open up the throat plate and take off the feed dogs. It is easier to watch the thread and needle closely.
Does the needle, thread and shuttle engage? If so it is not timing. If it engages but does not complete the cycle it may be a burr. Burrs happen. You can smooth up the burr with some emery cloth.
You can feel a burr with your fingers. You can see a burr. You can take a piece of thread and 'floss' through there until it hangs up on the burr
I had a machine buggered up and the thread kept breaking - the tension was full of lint and dried oil
Do you have "help" When you aren't looking do kids/GKs mess with the machine?
If the hook misses the needle's eye it could be timing.
I had a machine that the needle hit the foot and broke every time. It plagued me for months trying to figure it out. Finally I swapped the needle bar for one from a donor machine. SO it was a bent needle shaft.
Sewing on really heavy material can bend your needle shaft. Just because the people selling a vintage machine say it can, doesn't mean your machine was intended to sew on that heavy stuff all the time.
This video can help you know if it is timing. I think the part where he tested it is where I would start to test to see if it is messed up timing.