The full leght model name of my machine is 335-H3-734/01 6/01-921/01-BS
It has a second lever near the stich lenght lever to change the upper feed mechanism (foot + needle) speed than the under feed from 0 to 25%. In an old manual i read that this function is for "work extra fullness" for "setting sleeves in overcoats, sackcoats, ladies' suits and similar garmets".
I disassembly the second lever and set the speed difference to 0 stuck.
Moreover i have the limited stich lenght, i fix it with filing the plate under the stich lenght lever as i read in this post (
Now i have to change the pulley, to have less speed and more strenght. I think this model is originally designed for cloth, not leather.