Firstly the black pipe is simply a balance pipe and is not a requirement so you MAY run the carb without and it is quite legal.
It sounds to me that there is a tiny blockage somewhere and that with the pipe on the tiny blockage shows itself more...that aside a friend of mine Peter Palmer who's son races at Kimbolton had a 12.5 carb that they could never get to work, same type of problem as yours, in Seniors and couldn't run anything lower than a 160 jet....at a Whilton test day I stripped the carb, re-set it to factory spec and the carb is now their best carb and ended up running a 155 round Whilton....
Chris King
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