On my old 1980 Fiat engined Plus 4 it was an electric fuel pump in the rear near the fuel tank.
On a vehicle of that age there is probably rust and sediment in the steel tank, which can get into the fuel feed and clog the filters - pump, fuel line and in the carb.
Going up a steep hill could have caused the sediment/rust to shift in the tank. You could have a look inside the tank with an endoscope type camera (not expensive on Amazon).
On my car I ended up replacing the old steel tank with an alloy one.
