RE: Engine cooling with cowl aircraft
3 to 4 clicks off of peak seems a little lean to me......I usually go 1/4 turn rich off of peak as a minimum......
How's your glow plugs holding up? One of the signs of a lean mixture is going through plugs....
You have an Extra...are both cooling holes open on both sides of the spinner? If so close the side away from the head......
You may have to build a baffle to port cooling air to the head.....Lastly take a good look at how cooling air would escape the cowl....and where the muffler is as heat rises.....
Let us know what you find