Remember, you need at least twice the area for air to leave the cowl as you need for it to enter.
Also, engines do tend to lean out in the air, no matter where their carb is. However, it is due to the fact that they will gain around 300 or so RPM's when they unload, so sometimes they go a bit lean when this happens. That is why you want to richen it up by about 300 RPM's from peak before you fly it.