Pfaff 34 treadle keeps looping needle thread

[amateursewing]

I am using a size 16 leather needle and a size 35 high quality German polysterster leather thead for alterng bikers leathers, on

my Pfaff 34 on thicknesses of over 3 the thread in the needs seems to ravel up insteadof locking the underthread and making a good stitch. Then I do  the same

work on my Signer 20 electric machine and it mahince NEVER loops the needle thread, in fact it only gently has to be handwheeled over the 3-4 think seams and then

sews beautfilly. 

 

What is going on here? I supsoect it is somehing to do with the Pfaff 34 treadle, but this WAS a saddles machie.

 

Laurecne