Your fuel cap isn't venting properly. The tiny sintered bronze pellet cast into the center of the fuel cap is plugged, allow a vacuum to form in the tank as the engine runs, finally starving the engine. That's why you hear the hiss of in-rushing air when you open the fuel cap. The pellet is too small and quite useless.
Remove and completely disassemble the fuel cap, including the rubber breather nipple retainer. From the outside of the cap, drill through or punch out the breather pellet. Assemble the cap and install.
steve, August 2009
