Low speed.
Most of the fuel is coming out of the transfer slot because the throttle is open only a few degrees.
The three restrictions of importance at low speed cruise are idle feed, idle air bleed, and transfer slot.
One evening when I was still green I drove to my coworkers house to tweak the carb. He had been involved in stock elim racing so was my mentor of sorts on the car.
We flattened the base and drilled the throttle plates to improve the idle. I left about 10PM and on the first slight hill at 30 to 35 mph the engine was dying. Oh ****. Turned around, went back to his place and rang the bell. His wife looked thrilled (not!). he thought for second and then went and got some wire and stripped the insulation. We stuck the wire in the air bleeds until we had the right combination to restrict them so the engine would get the proper fuel in the off idle range. Drove home, and actually drove with the wires in that carb for quite a few months until we got some restrictors made by the machine shop where we worked.