apologize in advance for the long post. 78 Monte, 305. not getting fuel to carb. replaced mechanical pump, and also went with an aftermarket electric pump. Can blow through thye supply line, not clogged. fuel will return through line, and after connecting supply to pump, priming carb, will start and run for about 5 minutes. then need to do the same again, run for another 5 minutes. Any help would be appreciated