My aunt's does that occasionally. Normally, its something that builds up around the fuel pickup, and i just hit the fuel tank with about 4 psi of compressed air then vacuum the bottom of the tank. Last time, tho, it was the fuel pump line from the engine to the pump. It had gotten across the drive pulley and worn a hole in it. No vacuum, no fuel pump. I took out about 6" of rubber hose where it was damaged and replace it with copper tubing, then secured it up out of the way so it couldn't get into the pulleys again.