fuel pressure dropping over time

Bewy said

Your return line might be your issue. It should ALWAYS be as large or larger than the feed line. Stated on p. 2 of the 4309 instruction sheet.
I also use the 4309 reg, bypass mode, with an elec pump. I use a 3/8" feed line, 1/2" return line. Rock steady fuel pressure.
A bypass system is basically a 'full circle' system. Beginning at the tank & ending at the tank. The engine [ via the regulator port ] takes what fuel it needs.

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That's just not true as a blanket statement. It totally depends on what pump you have, how many GPH it flows and at what PSI. Some manual pumps (which he's running) don't have enough flow to over come the loss from a large return, thereby starving the carburetor from sending too much back to the tank. That's exactly WHY the factory return filters used the metered orifice I spoke of earlier to regulate that return flow, so as not to make the pressure drop off too much going to the carburetor. There was just a guy the other day on the Facebook slant 6 page that was having a fuel starvation issue and that's exactly what happened to him. The solder in the return orifice fell apart and the return was totally unblocked and caused him fuel starvation issues at highway speed under load. He put another filter on and fixed the problem. Sure, for real performance mechanical pumps or electric pumps, you need a large return line. But even then, the correct way is to use the regulator in the return line for use as the return orifice, for the same reason. To regulate that return flow so the flow at the carburetor doesn't drop off.