All I posted this on several boat/engine forums.
I will put the answer/solution on all of them at the end.
So don’t be surprise and don’t « waste » your time to answer on all of them :-)

I have a quite old Suzuki DF 140 more than 10 years, 1000 h.
I changed the IAC valve recently.
I tuned the idle speed with the screw (not the « electronic » way..it may be important). Idle speed around 800 rpm. Just ok.
All seems ok when the cover is out.
With the cover after 1 or 2 secondes, it is slowing down, and may stall…

So it is like if the air was not flowing inside with cover…but what , how ? And especially at low speed when it is not needing much air.

After that it runs perfect, but I had to put in gear, then put the cover and all was ok from slow gear to full throttle.

Thanks a lot for your advises.

Regards Gildas