There is a certain amount of pressure in the tank. Gas
will shoot out of the fuel line if you don't crack the fuel cap to equalize the tank pressure first. That actually indicates that the tank vent is probably working. It should hold pressure from the inside out but allow air in as the fuel level goes down...one way air valve. It's located on the top left of the tank as pictured below. There were several variations as illustrated.
If you haven't tuned a saw before I'd rely on someone who has to dial in the carb for you until you get the hang of it. It's not difficult, but there are very fine lines between a good and bad running saw from just making small adjustments. There's also a fine line between running great and burning up if the carb is set too lean on the "H" side. Small adjustments make big differences and there is a learning curve to understanding 2 cycle tuning IMO. Going by the book or the settings on the air filter cover is a start, but that doesn't always provide the desired results due to any number of factors not being in order..., a bad tank vent or air leak at an oil seal for example...., or a dirty air filter or dirty carb itself.
At least you got it running. Care to share what the problem was?
