that manifold is crap the carby should seal perfectly, okay it may work but over time with heat the o/ring will shrink and eventually break down and not seal , you seem to find with those cheap chinese carbys the bottom part where the carby bolts on seems to be longer at the bottom so the mounting flange on manifold should have been made wider to eliminate this problem, either take back to parts shop and ask for refund as part not made correctly or get a mate to make a new mounting plate that accepts carby and weld on manifold in place of the shit plate on manifold , talking from experience an ex mechanic and i have made numerous manifolds for bikes over the years, don't accept the that's how it is attitude