I'm in the process of making a potato gun. Its built, and glued, but having ignition problems (i do get spark though)

I posted this on a few forums, so I'll copy and paste what I'd allready written... Yeah, its kinda long

As a disclaimer, id like to mention that this wont be fired in the neiborhood, this will be fired at my friends 40+acre farm. I'll wear safety glasses, and use good common sence... bla bla bla, that sorta stuff

I'm using a 3 foot long 1 1/2" sch40 pvc pipe for the barral, and a 18" long 3" sch 40 pvc pipe for the expancion chamber... The end is threaded female, with a male companion (so i can add the fuel). Theres also a 1 1/5"-3" adaptor in there. I used purple primer, and glue to join them together...

Thesch 40 has a working pressure of 330 psi at 73 degrees. The ignighter is a $10 coleman quickspark...

Before I glued it, I did a quick test fire. It was just a dry fit test fire, and it was pretty awesome... 3 sec of spray using pledge, and it blew the gun apart... went "WOOF", and shot flames... Pretty damn cool imho. That was before I glued it

The pledge worked before I glued it, but doesnt any more. Allthough the cans getting almost empty though, but i was still spraying pledge (not 100%propellant)

I got spark, but no ignition. I degassed the expansion chamber (with a fan) a few times... still nothing. With or without the end cap on

I tried pledge, hair spray, spray deoderant (suprizingly unflamabal), and spray paint (degassing&cleaning after each test) I did get some carb cleaner, and that went... KAPOOF, but .5 sec of spray was a bit scary... I wont use it again.

The ignitor is 6" from the end of the expantion chamber, and the the spark gaps the probes at <1/2" into the barrel. The gap is about 3/8", which is about as long as it can be, If i want a consistant spark.

Any ideas on what else I should use for propellant?
Any ideas on anything else?

Heres some pictures
http://photobucket.com/albums/v644/NathanEbert/Potato%20gun/

Tks alot, I appreciate it
~Nate

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95 Honda Shadow ACE 1100.
1999 Plymouth Neon Expresso. 2.4 swap, 10.5 to 1 comp, big cams. Autocross time attack vehicle!
2012 Escape, 'hunter" (5 sp 4cyl)