I had several of the early versions of the .91FX. You will find that the problem to be on the needle valve house, there is a silver hex bolt that bolts into it to plug a hole. The "O-rings" that OS used are too soft, and they need to be replaced. The glow fuel desolves these "O-rings". These "O-rings" allow air to leak into the needle valve control unit. There is also two of the fuel nipples that have the same "O-rings" I used to use Plumbers white teflon tape to wrap the threads, and this would solve the issue for 50 flights. On the "Hex nut" you will see one or two little holes drilled side ways into the threaded bolt. Make sure these are not covered with the teflon tape,or you will cut the fuel off. Of course, if you don't want to go through these hassles, buy a new needle valve assembly.