I have an old Diamondback AScentex mtn bike that had a biopace crank on it when I bought it. I have tried to swap it out with a different crank, (a trutiv isoflex and a cheapo target bike crank) and they both had the same problem. The crank didnt seem to fit as far on the bb spindle as the biopace crank, and thus the f derailleur wouldnt shift the chain into the largest 2 sprockets. any ideas why this could be? is a bike equipped with a biopace from the factory doomed to be a biopace bike forever?

Obvious questions: Were the crank bolts torqued tight enough and the front derailleur limit screws and cable re-adjusted?

yes, tried all that. I can do a direct swap from the biopace which will work fine, and then put on a different crank and have totally different results