Well, it sort of depends on your experience with tuning and "listening" to the engine. It's one of those things that you pick up from doing it a bunch of times. If it quits without the cowl, well, you can just about eliminate heat as your problem. Then it may come down to closely looking over your fuel system to make sure you don't have a small leak or any other problems, and then asking someone with experience to tune your engine for you to see if it quits again. If not, then, well, you will need to work on the tuning process.

Regarding the hole in the cowl, I think the exit hole should be about twice as large as the inlet hole (for air circulation over the engine).

Hey, I have been flying for several years now and I still have an occasional dead-stick that I can't attribute to anything but my tuning process.

CGr.