I think you are talking about the fuel pump and the return line that sends fuel back to the tank.
Look at the sight glass in the front of the carb.
The fuel level needs to be in the middle or better yet just below.
When you pinch the return hose, the fuel level in rising in the bowl and the stumble goes away.
Changing the pump did not solve the problem.
Sounds like you have a plugged accelerator pump nozzle.
1980 Toyota - 1997 3RZ-FE 2.7l Engine, Turbo R151F 4.31:1, Triple Turbo Marlin Crawler Billet (2.28x4.70x4.70) = 1,148:1 Crawl Ratio, Marlin Crawler Twin Stick and Short Throw Shift Kits, 30mm H/D Output Shaft, High Angle Drive Lines, 5.29:1, ARBs, High Pinion Front, 25mm HD Billet High Steer Kit, 6 Pin Locking Hub Bodies, 86+ Wide Rear End, V6 3rd member, Chromolly Axles all around, 37" IROKs with Beadlocks, York onboard air - Rollbar air tank, Premier Power Welder, Marlin Crawler 4" USA-made Leaf Springs, Bilstein Shocks, et cetera....