SpudFiles

just finished up making a basic launcher and i've only launched about 4 spuds so far. i'm using a replacement grill igniter and hairspray for propellant. if i dont have a potato in the barrel and i spray in a 2 second shot of spray,stick the cap back on and hit the igniter it'll go off. do the same thing with a potato in the barrel and it wont go off. if i take the cap off and blow in it and then try it again, it sometimes goes off. my spark gap is about 7mm. not sure how to get a better mixture... can i shake it?

You should definitely install a chamber fan, It makes firing alot more consistent (not range, just ignition) 7mm should be enough if your cannon is not massive, but have you tried a duel spark system, they are pretty simple to construct. post the specs of your cannon though, it would be helpful

7mm is a pretty big gap. 1 or 2 mm (1/16 inch) is generally enough and will spark more consistently.

Your most likely problem is too much fuel. Try using less. Or better yet, get a $2 syringe and a disposable butane lighter and make an inexpensive fuel meter.
Disposable Butane Lighter to Syringe: The tubing is the insulation from 20G wire.


Bernzomatic Propane Torch Head to Syringe: The torch head's mixer has been removed by unscrewing it.


(from http://www.inpharmix.com/jps/Hacking_lighter.html)

Waiting for ten or twenty seconds after fueling will help the fuel equilibrate and make the gun more consitent but too much fuel still won't ignite.

Read the spud wiki page on fuels so you understand exactly what you are trying to do.

It seems like you are putting too much fuel into the chamber. With a spray-n-pray, you pretty much have to experiment. There is no surefire way to meter these. Try just one second of fuel.

BUT, the ultimate goal is to make it better, right? Check out the links Jimmy provided. Now, check this one out. Get it right each and every time.

Once the fuel is placed into the chamber, though, you should mix it right. A chamber fan works the best. But there are other methods of doing it. Check out the mixure on my first combustion. It worked well for propane.





Of course you could just scrap it and start over.....

thanks guys! i really appreciate the help. i reduced the gap to about two mm and i tried a different kind of hairspray at the suggestion of a friend and i'm all set now. Aqua Net. in a purple can w/a white top.

my gun is made from a 3" y connector with two cleanout caps attached and a 3' long barrel of 1 1/2" sch40PVC the sparker is in the "pointing out" side of the y screwed into the cap so it can be removed. (i just used a universal replacement grill lighter from a hardware store) i load from the muzzle end and then spray fuel into the chamber, screw the cap on and go. very simple design. 20 bucks for pvc 7 bucks for primer and glue and 16 for the igniter.

materials:
1-3" y connector
2-3"cleanout caps
1-3" to 1 1/2" reducer
4" of 3" schedule 40 pvc pipe
1 1/2' to 3' of 1 1/2 sch 40 pvc for barrel. depends on preference.
igniter
pvc primer
pvc cement

A chamber fan will definitely improve results. Also, you should try a different fuel. Hairspray works, but it is one of the worst fuels you can use. At least get something like right guard, that is the best "household" fuel you can get. Even better, you could use starter fluid or butane, but those require precise measuring.

I have used propane in a "spray and pray" launcher. Just open the cap, stick in an unlit propane torch, and wait a few seconds. the aspirator in the torch should mix it fairly well, too.