I looked at the photo posted by Paul Dye and have a suspected cause of the problem but was unable to post a reply there - new experience. A good test would be to light the area well and video the start, run and shutdown sequence. The air inlet is good distance from the center of rotation/thrust line of the crankshaft and there is the long forward arm from the vertical center of the engine air inlet to the location of the airbox inlet. All unit motion of the engine is magnified at the air inlet seal because of the two arms, vertical and horizontal. There are large rotary displacements of the engine around the crankshaft center line at startup and shutdown which cause really big side to side motions of the airbox while the cowl air inlet is rock solid. I suspect the problem could be solved by replacing the seal rubber with the relief split at the bottom rather than the side. A longer seal would not hurt either if will fit.
Bob Axsom
Bob Axsom