Hello Elliot,
I had, and continue to have, this same problem. It is probably not the ignition system to blame but the air-fuel mixture. If you spray in too little gas it either doesn't fire or else makes only a tiny "pop" instead of a powerful "bang". But if you spray in too much it won't fire at all. A friend of mine who was a career fireman told me this interesting bit of information: when a fire crew enters a building with a flammable gas leak, they usually don't worry about it igniting -- the chances of it happening that the air-fuel mixture is correct for combustion is small. (He said the greater danger to their own safety is when they EXIT the building, into open air, with gas absorbed by their clothing). If this is true
it tells you a lot about how exact the mixture must be for ideal ignition of your gun. I've read that using propane with a propane regulator is the solution. But I've also read that
this way doesn't give absolute assurance of repeated firing, because any amount of the propane that was not ignited remains in the chamber and changes the mixture that's introduced.
For the cost of a propane regulator you can build a pneumonic cannon instead. That's what I did, and have never regretted this decision. It works every single time, shoots the potato as far (or farther) than by chemical explosion, and unlike a combustion cannon I can take it to school picnics for the kids to shoot without objections by the parents or teachers.
Sissy (yes, I'm one of the few girls you'll find in this forum).
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