1980 Dodge Colt fuel return line mikuni carb, no power load

What does the fuel return line do on this car? It looks like it comes off the accelerator pump on the carb. The carb has a bowl vented to the throat (not to a emissions canister.) The service manual does not discuss this line, but one diagram shows a 'return valve' in this line that is not present on my car. SOme diagrams of the carb show this line, some don't.

The real problem is lack of power at highway speeds and high load, ie a hill. I've checked a bunch of stuff: exhaust constriction, new plugs, new fuel filter and pump, timing, valve adustment, etc. Symptom is a shudder, not knocking and doesn't seem like just one cylinder missing. Either 3rd or 4th gear at 55 mph, a hill displays these symptoms.

The new (and old) fuel pump ony puts out 2 psi, it should be 3-5 psi. I want to know whether the fuel return line needs to be plugged during a fuel pressure test. Also, is there a screen in the fuel tank that could be clogged? Next step is methanol in the tank and blow the line out. Also, do a pressure test while not sucking from the tank.

Also going to test premium gas.

Also, the carb is new rebuilt (not by me, by a vendor.) I am loathe to tear it down to look at the secondary jets. The secondary vacuum diaphragm seems to be working, and sucking on the line while driving (to open the secondary) did not help.

Also, the compression is uniformly 125 psi, should be 150. But its an old car, I don't see that as the problem.