Thanks Nick for that link. I know the owner of that site William Peters (also a dutchman but now doesn't own a cos anymore due to an accident), but haven't seen this page yet.

And Pete I do know this adjustment screw and have had the car on a CO machine to adjust it to the required 2.7%.

At idle it seems ok, but as soon as I tried to move the car it feels sluggish and only after setting the CO at about 4.5% or higher it wants to drive away.

With that higher CO setting I went to a rolling road session and I found a good power figure: 322bhp/485NM. (the usual greens, 3 bar, MSD chip etc).

But the operator showed me how lean it was from start till end. AFR started at 15,5(!) and the lowest figure at peak power showed an AFR of 13,9 (!!) (20 PSI boost).

Well, shocked from these AFR results but happy nothing went wrong I put the car in the garage and now checking everything to find out what this can cause before I will drive it again..