It would depend how transparent you were to begin with. If you were honest with your co-founder in fully advising that you'd take off to pursue your own projec

It would depend how transparent you were to begin with. If you were honest with your co-founder in fully advising that you'd take off to pursue your own project if it started going well, then he could make an informed decision about whether to co-found with you to begin with. Given the stakes involved, most co-founders would expect more of a solid commitment than that.