The NS551 needle and seat assembly is a replacement for the OE diaphragm style assembly you can see in the image to the right.
Instructions for installing the assembly are as follows:
https://youtu.be/tgT66up8yWE?si=bWA3V0FYvcIm83K8
Some of the conventional seats have holes or ‘side windows’ whose purpose is to supplement fuel delivery past the float needle when it opens. On some of the seats the windows have been removed so that all of the fuel will discharge over the top of the needle/seat assembly.
Removal of the side windows improves operation of the float needle on some applications.
