However, the gap the spark needs to jump is close to 1/4" inside the ignition chamber.
Oh, I guess we're talking spudgun here?
If that's the case, here's a few car engine (CE) facts to compare with.
In a CE, the gas (gasoline+air mix) is compressed at the time of ignition, making it much harder to generate a spark and to ignite the gas - an ignition coil system that will make a spark of 10mm from a loose ignition wire to the motor block, may still be too weak to make a 0.8mm spark under the compression, in a spudgun it isn't compressed.
In a CE, the spark gap is usually between 0.5mm and 0.8mm, but still firing the gas correctly.
You'll need at least around 10kV to 12kV with a fast rise time for a 0.25" gap in a spudgun
Could the voltage generated by a camera flash be close to 15kv?
It would probably take some redesign to make it powerfull enough, but it should be pretty high in voltage - it's the output from the trigger transformer for the flash tube that you use, not the voltage generated for the tube itself (which is just around 200V to 400V).
You could use a smaller gap and two sharp needles pointing towards each other will help the spark jumping.
Now with that is just using that i all ready have; really i could use anything that makes a decent spark and make a smaller gap in the ignition chamber.
I'm not sure a piezo unit from a lighter will have much power spanning 1/4", but if you want to use it, you could remove the hammer and hit it with a small solenoid.
Another thing, how long does it take to charge the capacitor in the camera circuit?
That depends on how much current you supply.
The formula is: C = As/V
C = capacity in Farads
A = current in Ampere
V = voltage
s = time in seconds
This can be used to establish the capacity value of the cap as well, as usually, unmarked caps are used.
If possible, use a couple of caps, to keep the voltage high during sparks.
A xenon (flash) tube goes low impedance when triggered and unlike expensive flash units, disposables haven't got a turn off, so the tube drains the cap completely at each flash. When you just use it to make sparks from the HT transformer, you just use a small charge.
Instead of giving off a single spark, use an oscillator to generate a burst of say 10 to 30 pulses.
In a CE, you need around 1.0ms to 1.5ms of spark time for reliable ignition with an oldfashioned Kettering system, so if you generate a train of 10 kHz to 30kHz pulses, the gas will ignite if the mixture is good - no more misfires.