Have you watched the fuel line while the engine is running? Is it getting gas, bubbles in the fuel, etc. Have you checked to make sure that there are no cracks or holes in the fuel lines? Remove the fuel line and exhaust line; blow through one while holding your finger over the other to see if the lines and tank will hold the pressure. If you continuously have to blow in the line; you have a leak somewhere. If that is the case; replace your fuel lines and make sure that the tank is not leaking at the stopper or if there is a hole in the tank.