I have a generator with a Tecumseh engine. It keeps vapour locking on me. I purge the fuel line before starting, but slowly bubbles start to form & work their way up to the shutoff valve, but they won't go past it into the tank. Within 15 minutes, the fuel line is full of fuel vapour (or air) & the engine stalls. The gas cap works & it does it whether the cap is on or off. I have installed a piece of garden hose as a temporary heat shield, but that hasn't helped. Any ideas on where to go from here?