Chainsaws

I have a Poulan Pro chainsaw pp4620avhd 802106. Starting was unsuccessful and I found the gas line in the tank had "fallen apart" Ethinol. I replaced the gas line and cleaned the fuel filter in the tank. No starting again. This carb on the saw does not have a primer bulb just a pull out choke. The carb only has one fuel line from the gas tank. So the tank is a closed system and for the gas to be drawn to the carb the tank has to be vented. My question is is there a vent for the gas tank and where is it located. Could it be a one way check valve in the gas cap??? Would appreciate any assistance!!