Half way along the tube cut half of the tube away with a scalpel/Stanley knife to a length of approx. 2 cm ......this will allow air [atmospheric pressure] to enter the chamber above the fuel in the float bowl ..........if air cannot enter there [because the vents have been blocked by a piece of unvented tubing] at atmospheric pressure fuel cannot be sucked through the carb jets to supply the motor. Its a common mistake made by people who do not understand how a carburettor works.