Trike: This is far, far too much overkill for an ignitor. A potato gun ignitor is supposed to provide the necessary electricity to merely ignite/spark a flamable material. To see all the dangers, you would really have to kill yourself 10x with the thing, but I can come up with a couple out of my head.
Potential problems I see:
1.High voltage=dangerous, you idiot
2.certain flamable liquids you might use are conductive, and if you contact the liquid during ignition in any way(through the vapor exaust, for example, while the current is still on), watch the fsck out.
3.Battery. Stun guns are only meant to function for a very limited time(enough to pop someone and run) and using it as an ignitor would suck juice.
4.Durability. You do not want to see what happens to a short circuit at 100,000 volts.
5.This much voltage could potentially ignite the PVC.
6.Exploding potatos. This is enough current, when hooked up to a potato, to fry it and make it explode long before the fuel explosion hits it.