it sounds like you are burning fuel faster than the carb is refilling... and so you experience symptoms of erratic fuel flow.
check the vent in the gas cap. A clogged vent causes slow fuel flow and may even create a vacuum in the tank, slowing it to a slight trickle.
If that tank is or was rusty, the cap isn't immune to rusting (unless it's all plastic) and is probably rusty inside.
Fuel should flow in a thick, continuous stream from the end of the tank's fuel line in both the ON and RESERVE petcock positions.