Electric choke on 400

[Hydroglen]

Could use some help / advice on this one.
The electric choke on my 400 seems a little temperamental lately. This is mounted on the frt carb and is operated by a switch on the panel. It does not appear on 401 cars so maybe just the early cars.
XK 120s had something similair.
I have power and have made sure it is clean and working. The electromagnet works and lifts the piston. I am not sure what adjustment I should make to the little piston and needle next to it.
Perhaps one of you chaps have been down this path before and can point me in the right direction. Pehaps a scan of a tech page?
It has been great for a number of years, so I have no complaints other than lately. Currently we are not happy with each other.
Thanks,

Dorien
PS oil leaks are fewer thanks to your good suggestions.