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Ok for a while now I have been running into a problem every once and a while.
my truck (1973 F-250 HB 360) would fall on its face like it ran out of gas, I thought this might have been a vapor lock issue so i pulled all the fuel line away from hot spots but it is still happning. today I took a few minutes and looked under the hood and found the supply line (to Carb) fitting was real loose not leaking but very loose, i put a wrench on it and one turn it started to leak...
I would go let it sit and run it and see if it still does it since you fixed what you think the problem might be. But if the issue still arises maybe it could be sediments in your gas tank plugging the hole for gas or possibly a carb issue.
I have a 76 f100 with a 360 and it did the same thing,it would do it the first couple minuets of driving it than it was fine,didn't matter warm or cold.After it did that once or twice it was fine.Never did find out what caused it.
I would think that if the loose fitting was before the pump, then it would suck air, after the pump, it shouldn't have any effect if not leaking under pressure.
I would check the rubber line between the tank and the frame. These sometimes dryrot and won't leak fuel, but will allow air to be sucked in.