I put a flame next to each inlet and they all seem to have some blow back.
So I took the head cover off and refit the cam sprocket just to make sure it was timed correctly. While the rockers were off I covered the top of the valve with wd40 and compressed the cylinder, and no bubbles were visible so given the excellent condition of the valves (plus lapped and leak test) when the engine was stripped I assume it's ok. Readjusted the tensioner, because the manual said that a loose one can change timing, and reassembled. She sounds better now with four distinct sucks and from what I can tell no audible blows, so hopefully that's shook the bugs loose. Might have been sticky valves or tight clearances, I don't know.
I'll put my carbs back on and try to sync again. By the way is she supposed to eat a lot, she went through liter quite fast at idle.
Thanks for all your advice