Need all castor.

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Sounds like its running out of prime and stopping...or glow plug is flaming out. Try blowing into a fuel line connected to the nva and turn the needle to make sure its metering. If using the flat needle it lets more fuel in on the flat side and the relation to the spray bar makes a big difference. The flat needle can actually be a different number of turns out and meter exactly the same as the flat changes its angle to the spray bar. 

It sounds like the prime is burning but the engine will not draw fuel from the tank.  If you have tried all the suggestions and you are sure that the fuel system has no clogs anywhere, then check the spray bar orientation.

The Fox spray bar has two fuel discharge holes.  Some say these holes are drilled 180 degrees apart, others say the holes should be at some angle less than 180 degrees.  I have no idea which is correct since the accuracy Fox was able (or willing) to produce was a bit sloppy at times.

A friend of mine once had the same problem you described.  When I looked at the engine, into the venturi, I could see that the spray bar was rotated such that I could see one of the holes.  Make sure the spray bar is rotated such that you can't see either of the two holes, or maybe you will be able to see only the edge of the holes if they are actually 180 degrees apart.

I think as said it is running out the prime. Fox needles need to be open a good 8 turns. They can run really rich and still run, but lean not at all.

As my 1960's memories kick in while reading all the comments I vaguely remember my flying buddy saying..."open the needle valve John...…...more John , more...….

I doubt that stock needle valves are all made exactly identical, especially Fox.  Nothing wrong with filing a needle to make it shorter, introducing more fuel.

        When I tried to run the Fox.35 in MIke Gretz's Fierce Arrow for the first time, I kept turning it out until it fell out and it was still not rich enough! So I filed the flat spot on the side of the needle, not the end of it. Just eyeballed it and was able to go back in three full turns. And I NEVER run a stock Fox needle valve without putting a short length of fuel line on the needle, just about the length of the threads, to seal off the threads so it won't suck air past the threads. Fox needle valve assemblies are not the pinnacle of accuracy but they can be made to work well enough for most uses.
  Type at you later,
   Dan McEntee

I just took an old Fox 35 that hasn't been run in many years. I cleaned it up and mounted it on a Twister. When I tried to get it running it would run for about two seconds and quit. I tried two different tanks and the fuel is 10% nitro and synthetic oil. I tried for an hour but it would just run for a second and quit.

   As noted by everyone above, not enough fuel it getting in.

    But, If it worked before, it should work again, particularly with the very thin fuel you are attempting to use.  Take out the needle and spraybar, and put them in a small container (like a 35mm film can), fill it up with lacquer thinner, shake it up, and then let it sit for an hour or so. Then get some wire (like 1/32 music wire) and poke it through, and get absolutely everything out of it. I also use a small bottle brush that comes with the Sullivan Fuel System Cleaning kit, or the ones you can get at the drugstore to clean between teeth.

   Also as noted above, if you are using typical RC fuel with all synthetic oil, you need to add a very large amount of castor oil. It may be lucky you couldn't get it going, because if you had, the fuel might have damaged the engine. I would not use any less than 25% oil, mostly castor, and up to 30% wouldn't hurt anything, except for raising your paper towel costs.

     Brett