I have a 1981 528 that recently loses power when under load. It idles for the most part normally but if there is a hill or when you accelerate past a certain point it bogs down. I have replaced fuel filter, fuel pump, plugs, rotor and cap and there is little improvement. It;s worth noting at one point it would not start but when I replaced the fuel pump it now starts every time. I was told it might be a clogged injector but am hoping I can get more info before exploring that.
Well if u see a post of mine u will see i have the same problem has u. We recently resolved the problem. It was a pressure valve that wasnt bloking the fuel and and sending it always back to the fuel tank.