On first flights with a new engine (assuming it passes the nose high test) I'll always try to use no muffler pressure. Some engines are better than others at this. But from the symptoms you've got with the 1.55 it sounds like it needs the pressure because the carb can't generate enough fuel draw. Any RC engine will run just fine on the bench with no muffler pressure although transition could be affected if the carb is designed around muffler pressure.