Study up on how fuel pumps work. The higher the pressure they are pumping against, the less overall they will flow. Restrictions in the fuel system can lead to this.
Go back in my project thread about a year and you'll see all of my W400 shenanigans including fuel rail and siphon drilling. I actually bent my stock FPR to reduce base fuel pressure while installing a boomba fuel rail. Lots of fuel capacity on my setup now
Drilling the siphon is pretty easy, it takes about 20 minutes to get the fuel pump out, drill it, and get it back in....but having a gauge is pretty key so you know what's actually happening instead of guessing
