I think I've found the problem.

[Frodo]

Have you checked the machine for correct needle length? 

My machine acts like yours when the needle is to short

 

[jimi]

7 hours ago, toxo said:

Do you find that the stitch length changes when going from say one layer to two layers?

Yes as Folker said, this is the pendulum effect, so the thicker you go the more you will notice the stitch length shortens. I would say this has nothing to do with the forward and backward motion of the feet because your outside foot holds the work while the inside foot and needle come into position.

Edited February 26, 2021 by jimi

[toxo]

1 hour ago, jimi said:

Yes as Folker said, this is the pendulum effect, so the thicker you go the more you will notice the stitch length shortens. I would say this has nothing to do with the forward and backward motion of the feet because your outside foot holds the work while the inside foot and needle come into position.

So does this happen with oscillating feed dogs also?

[jimi]

2 hours ago, toxo said:

So does this happen with oscillating feed dogs also?

No, this only happens on the top side of your Adler where your material is.

[Constabulary]

Open the face plate on your machine, the pivot point of your needle bar rock frame it on the top so the needle bar moves back and forth in an angle and the thicker the material the shorter the stitches. You have the same effect on patcher sewing machines. Or draw a triangle and move the ruler from the bottom further up. The distance between the legs becomes shorter the higher the ruler goes.

Maybe it´s of interest:

There are needle feed machines which have an even stitch length no matter what thickens you sew but there are very few. So far I only know two (I owned both). One is the BUSMC #6HM (and similar models) and one is the Singer 108w Class (and similar models). That is because they have no pivot points on the top and their needle bars are always straight (not in an angle) when feeding the material.

[toxo]

Thanks Folker. You said this before but fool that I am was hoping you might be wrong duh!

The problem I'm having is definitely the check spring. Maybe Frodos point about the needle bar is worth looking at because there's no way I can get that spring to let go of the thread early enough to get a loop on the way down or take up the tension on the way up. It was sewing ok in the past and I know it must be something I've done. Maybe I've asked too much of it at some point and moved something? 

Just looked at the book and I think I've got the presser feet to high. I'll have another go later. It's ok. I'm calm. Getting used to getting beat up by a sewing machine but I'm wondering why there isn't a "Fine tuning sewing machines for dummies"? Start here, you MUST do this first and then do that etc.

[jimi]

26 minutes ago, toxo said:

The problem I'm having is definitely the check spring

I would take off the check spring and have a good look and give it a clean, there might be a groove worn in there somewhere or you might see something catching?? On the 335 you have the small plate for adjusting the travel of the spring, adjusted by the small screw, and you also have the knob in the middle for the tension of the spring. I am sure you will find out what it is Toxo if the problem is there.

Edited February 27, 2021 by jimi

[amuckart]

On 2/25/2021 at 3:32 AM, Uwe said:

On most machines the check spring (you call it take-up spring) has two functions:

1. Keep the thread taught while the needle is descending, to prevent the needle piercing the thread on the way down. Once the needle is in the material the check spring should rest on the adjustable ledge and no longer apply tension, until ...

2. It puts tension on the thread again as it wraps around the far side of bobbin case. This makes the thread slip through the passageway under the throat plate quickly to avoid snags.

Make sure the adjustable ledge under the check spring is in the correct position to apply and remove thread tension as described above.

The only time the thread should overpower the check spring is when the take-up lever is near the very top of its movement as it pulls the knot into the material and more thread from the spool.

I reckon this bit should be made a pinned post on this forum. I’ve not seen such a succinct and useful explanation of check spring function and setting anywhere.

[toxo]

The spring on my 69 wasn't in the best of condition and college didn't have one. Folker suggested the spring on my Durkopp 239 would psobably fit and if so I could order a Singer one but I couldn't get the damn spring out of the Durkopp I was pulling as hard as I dared but I didn't want to ruin it. The 69 worked fine until it had done around 3/4 inches of sewing and then it would miss a stitch. I reasoned that the spring was catching somewhere so I got the needle nose pliers on the job to ensure it wasn't touching anywhere. Job done. No more missed stitches.