I think in '72 they went to HEI. It could have been a mid year change. My 71 had a problem that sounds similar. My problem was the second tank still had some scum in it and needed to be cleaned better. It kept clogging my filters. My fuel pump diaphram was brittle too and needed to be replaced. Old fuel ines can look OK but be breaking apart from the inside. If they are getting hard and not too flexible, you might want to replace them now too.
