it could be a dirty fuel filter located in the gas tank or fuel linecould be clogged. it could also be a dirty carburetor. your cap is ventilated so when pressure builds up it doesnt explode.something has clogged up between the filter in the bottom of the gas tank to the airbreather' try removing the air breather first tnen let it run for awhile and see what happens.if this fixes it then get a new air filter or clean your old one.make sure where u start it that it is not in a real dusty area if this doesnt fix it then try fuel filter and so on. hoped this helped you