Fun fact, I once constructed a flamethrower using information gathered from an army field manual and plans for a potato gun I heavily modified. I was very proud of it, I had even taken the time to boil gasoline down as part of creating the napalm like ammo and was using CO2 cartridges and compressed air as propellant. I'm sure it would have worked but my dad found out what I was up to and halted attempts to test fire it. I'm pretty sure it was dismantled and the pieces scattered to the four winds. One of the reasons I'm proud of this accomplishment is that it was done pre-internet. I did intense research where now I'm sure you could look up how to build a flamethrower with little to no effort. Shockingly my parents did not drink themselves to death before I finally got kicked out but they still (charitably) refer to my teen years as "challenging." I think I was fifteen when I built the flamethrower, I know it was before I was old enough to drive but after I blew up a stump with home made C4. The stump demolition was more leniently regarded because I saved my dad a lot of work but I did get one of those, "so help me if you ever even think about doing that again" speeches.
Pre-internet? If this was done without the aid of the Anarchist Cookbook, I applaud you good sir.
I was researching homemade flamethrower so time ago but the simpler ones seem to be lacking in both range and safety measures (Richard Pryor joke here)
The friendly guys at Zombie Squad pointed out some nice alternatives for homemade weaponry that made me laugh http://zombiehunters.org/forum/viewtopic.php?f=33&t=106848&p=2410631&hilit=toilet+brush+slingshot#p2410631