To both of you. Entirely closing the vent or mounting a container w/ a volume of at least 50x the volume of the space above the diaphragm makes a big difference! The diaphragm replaces only a super tiny amount of air, both ways. Small Tygon tubing and a sealed container works excellent. I've been running this setup on my DL50 and DA successfully for over a year.

With or without the cowling, on the ground or in the air, the engine will run exactly the same which makes it very easy to adjust.