Yeah, the idea that the fuel cap vent is blocked works for me, creating an increasing vacuum in the tank that overcomes the fuel pumps ability to suck any more fuel. The hint that a blocked vent might be the issue is the sound of inrush of air when the fuel cap is removed right after the engine fails.
Crud in the fuel line also works well given that some US fuels seem to create issues if the car is not used for some time and when the car is used on the road it stirs up the crud which then gets picked up and sucked in to the fuel line and restricts it again limiting the fuel pumps ability to suck, which if left in time the vacuum decreases allowing some fuel to then make it`s way through.
One other option might be water in the fuel which again lies in the bottom of the tank below the level of pick-up, until it becomes disturbed when out on the road, again in time it can settle back down out of the way.
I once had water in the tank of a non- Morgan, and started at the wrong end trouble shooting... at the carb.. no fuel.. Disconnected the fuel line and spun the car over no fuel, replaced the whole fuel line, still no fuel being supplied, only then did I think to check for vacuum build up, DUH ! Opened the fuel flap, no fuel cap and there had been torrential rain before a big freeze... Rolled back under the car and gave the bottom of the tank a thump.. it was solid. Ice in the tank and snow on the ground... It was interesting to see the look on folks faces as I poured kettles of boiling water into the fuel tank, before I again crawled under and removed the tank drain plug and let the water out until the smell of clean fuel let me know that all the water had been drained...No not my car, but that of my then girl friend who had removed the fuel cap at the petrol station and forgotten to replace it.. Yeah, she was even stupid enough to marry me..
