Thanks for all of the suggestions...particularly the information that the vent on the back plate needs to be open to the atmosphere. It makes sense, but there is no mention of this fitting or its purpose in the instructions which came with the engine. The consensus seems to be that the engine will run all right once it is started, but it will not be happy when the plane is flying upside down. At this point in my career, flying upside down represents a mistake...

Thanks again...Mike