Settings on the carb are generally 2 turns out on the "H" and one turn out on the "L" for a start. Adjust from there. Bag tanks do not need a vent as they conform to the fuel level as it changes when filling or draining. They do need to be "burped" every so often so that as musch air as possible is removed from the bag. They can also pinch off the fuel supply if the bag folds just at the outlet. Make sure your bag isn't sliding back and causing your problems. Also make sure you have enough fuel so the bag is "puffed up" (without air of course) when full.