Pfaff coverlock 4.0 Owner's Manual page 40

Chainstitch
When the chainstitch needle thread tension is too loose
Coverstitch
When the coverstitch looper thread is too loose
When the coverstitch left needle thread tension is too
loose
When the coverstitch middle needle thread tension is too
loose
When the coverstitch right needle thread tension is too
loose
When the top surface of fabric bulges between needle
threads and the seams on the underside of the fabric
becomes tight;
40 Stitch adjustments
Increase the needle thread tension (blue), or decrease the
chainstitch looper thread tension (purple).
Increase the coverstitch looper thread tension (purple).
Increase the coverstitch left needle thread tension
(blue), or decrease the coverstitch looper thread tension
(purple).
Increase the coverstitch middle needle thread tension
(green), or decrease the coverstitch looper thread
tension (purple).
Increase the coverstitch right needle thread tension
(red), or decrease the coverstitch looper thread tension
(purple).
Decrease the coverstitch looper thread tension (purple).