The nipple and fuel line show the diaphram a constant atmospheric pressure so it keeps on pumping. Without it you can get the engine in the wrong position (in our cases it was a right to left, left banking knife-edge) where the plate is in the back and it seems to read this as a vacuum and stops pumping gas. It's not just these engines but nearly all gassers. My buddy had so much trouble with his DA that he finally ran the end of the fuel line in the fuse into a hole in the top of a plactic medicine bottle so it was out of the sir completely. Just be sure that after you tap the plate and put the nipple in you grind off the inside portion of the nipple smooth so it doesn't cut the diaprham.