Vacuume pump
The box marked ???? is a vacuum pump and is driven by the changing pressure from the hose marked ?????. The pressure changes as the engine turns over working the pump.
The pump requires sucking and blowing to pump the petrol from tank to the carb, vacuum fuel taps only require sucking to allow the petrol to run through.
I'd suspect your problem to be the float in the carb. Try setting it slightly lower than what you currently have and see if it is still leaking, or cleaning it out if you havent already done so as it could be some muck in the float valve seat.
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