high idle when engine cold

From Factory it was working like this, there is a mechanism to have around 2000 rpm under 60 degreee celsius

 

The injected cars with the bi-metallic thermostat control idle about 2000-2200 RPM when cold, then settle down to about 1000. The length of time depends on the ambient temperature.

 

This is the fast idle device and it sits in the middle between the carbs. A cam on a bi-metal spring turns as the engine warms up until it eventually disengages and you drop to a normal idle.

A lot of people disable it and it's strange that your started working all of a sudden. Mine was originally disabled, then re-enabled and set higher than I like. There is an adjustment screw to lower the fast idle, which is what I did. You can adjust without removing the airbox, but you should pull it to see why it started working

 

The Fast Idle Device @flyingti is referring to is this: https://www.eurospares.com/Ferrari/308/308_GTB_(1980)/PartDiagrams/011/Throttle_Control?ref=038
I'd been suffering from a sticky throttle linkage and eventually traced the primary cause (primary, as in there were secondary causes as well) to Item #23 ("Lever") in the above-referenced diagram...which just so happens to interface with the Fast Idle Device.
I'm currently in the process of stripping and rebuilding my carbs and given the fact that I've disabled the manual choke, I will most likely disable the Fast Idle Device as well to eliminate another potential future problem.
- Dave

 

I remember this being mentioned in a road test article when they were new. It was somewhat concerning to the author with a cold engine but they were told by Ferrari that it would cause no harm