Every time after I have crammed the plastic chamber between the carbs and the airbox, I struggle with getting all of the "snorkels" fully seated, particularly #2 and #3. I just tried a different approach. I pushed an uninflated bicycle inner tube into the chamber from the airbox, and inflated the tube until everything was seated. I tightened the clamps and then deflated the tube.
If I had thought of this before the cramming the chamber into place, I would have planted the uninflated tube inside of the chamber first. Shoving it in from the airbox is not particularly easy.
Looking at the line drawing (view from above), the black is the chamber and its five snorkels. The red is a folded bicycle inner tube.
I crammed as much into the chamber as I could while leaving the stem outside.
After inflating the tube, the front side of the chamber was pushed toward the carbs while the back side was pushed toward the airbox, seating all five snorkels.
Bob
