Spaceclam...an engine will idle perfectly well without muffler pressure because that's what they all really do. Soon as you come back to idle there's almost no restriction to the exhaust and whatever pressure was in the tank then bleeds back through the pressure line into the muffler. They'll all also run perfectly well at full throttle without muffler pressure too but you'd have to open the needle considerably and then it'd be very sensitive to pointing the nose up so it can really only be done on a test stand.