Have you checked the cam belt timing AND the ignition timing? With it starting only with the throttle half open points towards it being too rich. Either the cobra has a problem and the pressure is too high or the vff has a internal leak and propane is being drawn into the manifold through the vacuum line that's connected to it. You can test the vff by starting and running the motor and then taking a pair of pliers and squeezing the vacuum hose. If you notice a differance, bad vff.