Find the fuel line that delivers fuel from the fuel pump to the carburetters under the hood. Now get a coffee can and a screwdriver that will fit that line. Also bring a length of the same diameter hose with you. Go for a ride, When the problem starts, disconnect the line and attatch the tubing you've brought, to the delivery line from the rear of the car.. Now turn on the ignition and see what kind of flow you've got. I have a suspicion that the fuel tank is serving up loads of crud and starving you out. A new tank may be in order. The clue is when you said the car was "abandoned". How long has it been sitting? The fuel tanks go bad pretty quickly on Bs
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon