I think this is the key question:
Is there a proper spark at the plugs throughout the stall/run good cycle?
Pull a plug, reconnect it to the plug wire and ground the plug body to the block with a jumper wire. When you are all set up, start the motor, lean over the fender and monitor the plug for a robust spark. Best done at night or in a dark garage.
Watch carefully and attempt to determine if:
1. The engine stalls and then the spark is lost . Or:
2. The spark is lost and then the engine rolls to a stop.
It might be difficult to tell, but it might not. As an example:
On an impending close play at first, the umpire looks only at the bag. It isn't possible for him to watch the 1st baseman catch the ball and then look down to see if the runner got there before the ball. So, to determine whether the runner is safe or out, he watches only for the the runner's foot to hit the bag. At the same time, he is listening for the sound of the ball hitting the first baseman's glove. Which happens first may be separated by only a few hundredths of a second, but the mind can perceive even the slightest time differential.
The spark might continue to arc across the electrode gap even as you sense that the stall condition has started. With a fuel flow problem, the engine still has sufficient inertia to keep rolling along fast enough for the HEI system to produce a spark - at least for a short period of time. So, if the engine starts to quit but you still get a couple sparks before it grinds to a complete stop...think fuel flow problem.
If, while you are watching the spark, it is suddenly lost - gone and then you sense the stall has begun... that would point to an ignition issue.
A sure sign of an ignition problem would be: The engine is ticking over fine and then - even before you sense any sign of a stall coming on - you observe the spark slowly start to weaken, turn yellow and then finally die out. If it is only then that the engine stops rotating...ignition was lost.
Fuel flow problems associated with carbureted systems and mechanical fuel pumps generally don't ramp up to this level overnight. They usually develop slowly and are often intermittent in the beginning. Also, since fuel flow requirements are lowest at idle, that would make a fuel issue even less suspect. Since you have confirmed proper fuel pressure right to the point of stall, about the only thing that comes to mind as far as possible fuel causes would be air ingress into the fuel pump suction side.
If you still have a gut feeling that it is fuel related, do a flow test. Disconnect the fuel line from the carburetor inlet, connect a length of hose to it and run the other end of the hose into the bottom of a coffee can/glass jar, etc. Start the engine and keep it running on ether for 1 minute.
You should end up with about a quart of gas in the container. But that's about volume. To check for air, it's more important is to watch the end of the hose as the incoming gas is discharged under the level in the can. You shouldn't be able to notice any surface disturbance other than maybe a little swirling. If you note the presence of air bubbling up out of the hose...there is a leak in the suction side of the pump.
Oh yeah, if during the volume test, you find that you can keep the engine running as long as you keep it supplied with ether...it is a fuel issue.
