Hi Per,
There are washers that fit into the top of the air slide valve itself.
See Pic attached
They are adjusted to affect the cut off temperature of the air slide valve.
Usually there is a number of them under a black steel washer which is held in place by a internal C clip.
Before adding or removing washers..........
Have you checked that the valve is moving freely and not binding?
It is a common problem which I discovered last year. The valve was jammed because of lack of lubrication.
Sometimes coolant can get in there when the system is being setup. It then becomes concentrated and gums up the valve.
Bob (Brisbane,Australia)
RHD,1967 early 250 SL, auto
Download Attachment: AirSlideValve_2.JPG19.93 KB