Duct tape has some uses...motorized bicycles is not one of them. Where is your air (vacuum) leak? The most common place is on the intake side of the carburetor, either the gasket at the cylinder or the area where the carb slides onto the intake manifold. The kit supplied gaskets are notorious for early failure and the carb, if not properly sealed onto the intake manifold, will also leak. Use the recommended gasket sealers and be done with it. Go to a good auto parts store and spend less than you'd spend on a roll of duct tape for a product made to seal gaskets.
Tom