I feel for you mate if you have to strip the bloody carbs... particularly after a careful rebuild.
My 750 K2 used to be a bitch to start ... countless cardiac events kicking it over, followed by countless flat battery's after that. And in my case it's simply technique, ie couple of dry cranks, choke full open, throttle full open... works 1st time & every time, from cold & on the starter. Thence a bit of nursing / closing the choke whilst she warms...
Possibly not your answer... just trying to offer a (last?) alternative to a bloody strip-down....
Good luck!