Ok, let me get this straight, your pulling the heads cause it wont idle, man this is good...lmfao!
Tell me this Bowhay, did it idle ok with the 650? I'm thinking it did.
I think you merfed up something in the making of that mis-matched 750 you dreamed up.
Man before you go yanking heads and all, put a known "good" carb on there first.
I dont doubt you may or may not have a valve guide sealing issue, but seriously, the valve guides would practically have to be non existent to leak enough vacuum past them to not idle.
Just my 2 cents, Cp