Sounds like a blockage in the pump jet. With the air cleaner removed check if, when you snap the throttle open manually there is a squirt of petrol from the pump jet. You could also check the integrity of the vacuum advance and retard hose as these can perish and cause a leak.
As well as the hose, you could check the auto advance/retard is working at the distributor itself. A long time since I tried but I believe you can suck on the hose and check for movement at distributor.
