The rocker cover (black cap you are talking about) is one of the hottest parts of my 4 strokes along with the muffler. I have never been able to hold on for even 1 second. So do not touch these parts. You are most likely running way to rich. IC engines get hot, don't worry about the heat . Just make sure they are slightly richer than peak.

If you are really concerned about heat some of us heli guys have learned to richen until you can hold the engines BACKPLATE (one of the coolest parts of the engine) for 3-4 seconds.

But on my planes i have never bothered to check. I lean my plane engines to peak and back off about 200-400 rpms.