Starting a 412

[Oliver]

Hello,
I think I've finally found a reason for problems starting after scratching my head thinking of every possible fault.
The choke on starting opens completely - instead of slowly as temp. rises.
The carbs a Carter Thermoquad with a "divorced" choke.
Before starting the choke plate is closed and you can feel spring pressure opening it and it closes shut as it should.
Once I manage to fire engine up it falls wide open and have to rev very fast to keep engine going, if at all.
Once engine is stopped after about 5-10 minutes it somehow closes and all looks as normal.
I'm assuming it's a fault with "divorced" choke on inlet manifold.
Having a turbo it's pretty hard to get access and might be difficult to get a replacement as the connecting rod is longer than on a standard choke.
Anyway next step is to take it apart and see what needs doing.
If anyone's had this problem and knows what to do, advice would be welcome.
Thanks