My guess is the regulator can’t keep the pressure low enough (6psi), while maintaining a high flow rate to the carb, without still returning fuel to the tank....... fuel that really should be going to the carb.

I’d plug the return to the tank and redo the timed test with the valve and see how much extra flow you get from that.
BTW, what regulator are you using?

Your pump is supposed to be capable of 60-80psi right?

If plugging the return results in a significant increase in flow into the can while maintaining 6psi(you “should” have to open the ball valve farther to get the 6psi system pressure), then I would try running two regulators.

I’d have the first stage regulator(the return regulator) set to about 25-30psi, then run the outlet of that one into a dead head unit that can handle the 25-30psi inlet pressure, and set it to 6psi.
The MagnaFuel regulators are designed to operate at 25-30psi inlet pressure.

Basically, you want it so the return regulator shuts off the fuel going back to the tank when the demand is high, and I think you’ll have a better chance of accomplishing that if that part of the system remains fairly high pressure.

If plugging the return doesn’t result in a lot more flow into the can........ then it’s on to a new pump.

If the pump will keep up, you would basically be duplicating this(only it would have higher system pressure in the first stage than the one in the link).

https://www.aeromotiveinc.com/product/efi-2-carb-dual-fuel-pressure-regulator/