.... brrrrbrrrrbrrrr instead of brrrrrrrrrrrrrrrr if you know what I mean.
Since yesterday morning the idle is sounding like 3 cylinders are firing. Obviously is is not the case as i tested and look at the spark plugs. The engine move quite a lot at idle. In normal driving condition there is nothing to declare.
The engine is idling at 850 rpm ish. this morning it even stay at 2000 rpm while cold for about 10 miles or 15 min when I was at a red light or a stop. It didn't do that since I put back the timing to normal.
I also can hear like an exhaust leak but very tiny and only on idle (I need to lift the car tonight or this week end to investigate)could that be the origin of the idling problem ?
Strangely enough the car pass the MOT yesterday morning while doing this. nobody mention any exhaust leak.
I answer my question myself : air leak on the supercharger tubing discover with a can of carb cleaner as I sprayed it a bit every where and the engine was increasing its speed. I had to tighten the hoses fixations.