Weird Starting

[71c104spd]

So my 71 with the 350 and the stock qjet is acting up on starting and has done this the entire time ive owned it i just haven't done anything about it yet except a carb rebuild. My issue is it cranks over well or what seems like normal to me and cranks and cranks for a loooong time without starting or doing any little hicups or anything. but if i let is sit for like 15 minutes it usually starts right up with a little bit of rough idle right at start up but not enough to kill it. I think the gas is draining out of the carb? how can i potentially fix it?
Thanks in advance.