Between Buzzman72 and IDJ, I've had to look at this situation a whole 'nother way!How 'bout this! I've got a catalytic converter in the pipes. So, for brief moments, the exhaust acts like a closed system, until the pressure can bleed through the cat. In a closed system, all parts want to be at equal pressure. So, it's possible that the engine is using the exhaust crossover to equalize pressure between one side exhaust and the other until the pressure can bleed through the cat. The quickest path to equalize the pressure would be through the exhaust crossover. It would be faster than down the pipe to the Y and then back up to the opposite manifold. So, even with the crossover butterfly open all the way, exhaust would be pulsing back and forth under the intake all the time! This would boil any carburetor! With the old open pipes, there was no time to balance the pressure from one side to the other. It would just blow out the biggest opening.
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Buzzman72 made me think about the crossover, itself, a little more. I thought I had the exhaust crossover blocked, but I'll bet you dimes to donuts a "helpful" mechanic somewhere along the line re-opened the passage! This kinda confirms what IDJ said about if the vapor lock keeps going, even when I have a pump all the way in the back. This also makes sense, because all the mid-70's cars started having this problem when they started putting catalytic converters in the cars.
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