It's a nice low-cost flamethrower design, but there are a few problems I see.
The use of a blowgun has already been discussed - just put a ball valve behind it for safety/main on-off valve, and use the blowgun as a trigger only.
Use a regulator or a separate propane tank for the pilot light, or you won't get much pressure in the main tank.
The entire tire tube design isn't necessary; if you flush the chamber of air (ensuring the atmosphere in the chamber is over 90% propane...) there's zero chance of combustion inside. The upper explosive limit of propane is 10.1%. As long as there's significantly more than that inside, no combustion can or will occur. The tire tube just complicates things.
I don't like the idea of tapping your fill valve into that small of a fitting, through just one layer of pipe. I also don't like the idea of using threaded PVC fittings on a flamethrower tank...
Check out this flamethrower design, it's pretty complete.... even has a fuel gauge. http://www.spudfiles.com/forums/flameth ... t8457.html
Good luck, it's a cool project.
Just be extremely careful about where you use it.