I bought an older singer I believe 301 but when i try to sew the thread on the underside is a big mess. The top stitching is fine. I've tried adjusting the top thread tension but its still a jumble mess!

good thorough cleaning- new needle- rewind bobbin- rethread (from spool all the way to needle-
this can be caused by many different things- but usually winds up being a piece of lint somewhere- a dull needle- or not threaded quite right...so those are the places to give attention to first- if after all those things are done you still have a problem - it may need to go to a shop for a check up=adjustment--all of the above should be done before doing anything with the tension.
also- make sure you remember to drop the foot- sometimes we get ahead of ourselves- and forget that- that can also cause the same mess

totally agree with ckcowl, I would think it would be as simple as going through the threading. Just one thread hook missed can cause grief.
Is your bobbin spinning correct clockwise when the thread is pulled.
And drop a bit of oil in it, it may be thirsty.
enjoy, I love these old machines.

Yup, I agree with both!! And I LOVE those machines :thumbup:

Now why won't you show us a pic???? :shock: :shock:
You know we LOVE eye candy!! :thumbup:

Is the bobbin the correct one for that machine? Even though 'they all look the same', they are different. I put a Singer bobbin in my old Kenmore (fit fine, so should work), well I did some unsewing because of the mess.

good luck