I don't have a DLE 20, but what I see makes me suspect a heat problem. There are 2 things *I* would do differently.

The first is to get something to block all air not going through the engine fins. In fact, I normally use a baffle just in front of the engine with the hole between 1/8 and 1/4 inch smaller than the fins. This means the last 1/16 of an inch or so of the fin is not getting fast air on it. This also makes it easier to insure that there is at least 2 times the exit as entrance, which I don't think you have right now.

The second is to build an exit ramp at the rear of the jug to help the cooling air turn and leave the cowl. Again, *I* do that by 'footing' the ramp at or near the base of the jug and angling it to the rear edge of the exit area. It sometimes helps to put a slight lip on the front side of the exit hole to help redirect oncoming air out of the way of the exiting air.

However, as I said those are things *I* would do. Oh, I think you can get an inexpensive IR gage at Radio Shack to check and see the heat with and without the cowl to help you decide if that is the problem.

Please keep us posted as per your results.