I have never run across a carb that does not have some type of venting system built into to it. As far as air in the line, this should not make any difference. The small fuel line tend to trap air bubbles, normally happens when you run out of fuel. They should clear by themselves over time, it should not affect performance. On larger motor there is either a vacuum operator fuel pump or an electric one. If the unit will not prime itself when trying to start, the vacuum pump style probably needs a new diaphram kit. However the pump will normally still function if you prime it. You can prime it by slightly pressurizing the fuel tank with compressed air. You'll see the tank expand a little, which will indicate enough pressure. If you pressurize the tank to long, you can flood the engine.
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